.. _chapel-evolution: Chapel Evolution ================ Like any language, Chapel has changed over time. This page is designed to describe significant language changes that have a high likelihood of breaking existing user codes or code samples from presentations or papers that predated the changes. Note that the compiler flag ``--warn-unstable`` is available and can be useful when migrating programs to the current version of the language. The purpose of this flag is to identify portions of a program that use a language or library feature has recently changed meaning or which is expected to change meaning in the future. version 2.0, March 2024 ------------------------ .. _readme-evolution.default-task-intent-arrays: Default task intents for arrays ******************************* In 2.0, the default task intent for an array is now determined by the outer variable. If the outer array is ``const`` then the default intent is ``const``, otherwise the default intent is ``ref``. Therefore, if an array is modifiable outside a parallel block, it is modifiable inside the parallel block. It is no longer necessary to use an explicit intent like ``with (ref myArray)`` to modify ``myArray`` in a parallel block. This change applies to ``forall``, ``coforall``, ``begin``, and ``cobegin``. Consider the following code which illustrates this. .. code-block:: chapel proc myFunction(ref A: []) { begin { A = 17; } } The default task intent for ``A`` is ``ref``, since the argument formal ``A`` is mutable. This simplifies parallel code, making it simpler and cleaner to write. Prior to 2.0, the above ``begin`` would have resulted in a deprecation warning. In 2.0, this is valid code again. .. _readme-evolution.assoc-dom-par-safe: Associative Domains default to ``parSafe=false`` ************************************************ Associative domains have been stabilized and prioritize performance by default; however, some diligence is is required for correct use.. Associative domains in Chapel have a ``parSafe`` setting that determines their behavior when operated on by concurrent tasks. ``parSafe`` stands for "parallel safety". Setting ``parSafe=true`` allows multiple tasks to modify an associative domain's index set concurrently without race conditions. It is important to note that ``parSafe=true`` does not protect the user against all race conditions. For example, iterating over an associative domain while another task modifies it represents a race condition and the behavior is undefined. See the `documentation `_ for more information. The default of ``parSafe=true`` added overhead to operations and made programs slower by default, even when such safety guarantees were not needed. This is because it uses locking on the underlying data structure each time the domain is modified. This overhead is unnecessary, for example, when the domain is operated upon by a single task. Motivated by this we have changed their default from ``parSafe=true`` to ``parSafe=false``. With this change associative domains have been stabilized, except for domains requesting ``parSafe=true``, which remain unstable. Here's a breakdown of the changes and how they might impact your programs: 1. New default for associative domains: * Previously, associative domains were "parSafe" by default. This has changed to ``parSafe=false``. For example: .. code-block:: chapel var dom: domain(int); used to imply that ``dom`` was set to ``parSafe=true`` but now it defaults to ``parSafe=false``. * This means that the checks to guarantee parallel safety are no longer inserted by default, thus improving performance. Therefore, it is now the user's responsibility to ensure parallel safety as needed. * A warning will be generated for domains without an explicit ``parSafe`` setting, to draw user attention to code that may need to be updated, unless compiled with ``-s noParSafeWarning``. .. code-block:: chapel var d1: domain(int); // warns var d2: domain(int, parSafe=false); // does not warn where the compilation output of the above program would look as follows: .. code-block:: console $ chpl foo.chpl foo.chpl:1: warning: The default parSafe mode for associative domains and arrays (like 'd1') is changing from 'true' to 'false'. foo.chpl:1: note: To suppress this warning you can make your domain const, use an explicit parSafe argument (ex: domain(int, parSafe=false)), or compile with '-snoParSafeWarning'. foo.chpl:1: note: To use the old default of parSafe=true, compile with '-sassocParSafeDefault=true'. * Since ``const`` domains are never modified, they are exempt from these warnings as changing their default to ``parSafe=false`` does not have the potential to impact correctness. .. code-block:: chapel const dom: domain(int); // does not warn * In order to ease the transition, users can temporarily revert to the old behavior by compiling with ``-s assocParSafeDefault=true``. .. code-block:: console $ chpl defaultAssociativeDomain.chpl -s assocParSafeDefault=true If a program used associative domains and relied on ``parSafe=true``, it might be useful to try compiling with ``-s assocParSafeDefault=true`` and then add an explicit ``parSafe`` argument for each associative domain individually, to ensure no races are introduced into the program by forgoing the parallel safety guarantees. 2. ``parSafe=true`` domains are unstable: * Domains using ``parSafe=true`` are still considered unstable and continue to trigger unstable warnings when declared. For example: .. code-block:: chapel var dom: domain(int, parSafe=true); // generates unstable warning generates the following compilation output: .. code-block:: console $ chpl bar.chpl --warn-unstable bar.chpl:1: warning: parSafe=true is unstable for associative domains 3. Associative domain literals: * Associative domain literals also generate warnings by default. Use explicit type declarations like ``domain(int, parSafe=false)`` to avoid them. .. code-block:: chapel var d1 = {"Mon", "Tue", "Wed"}; // warns var d2: domain(string, parSafe=false) = {"Mon", "Tue", "Wed"}; // does not warn where the compilation output of the above program would look as follows: .. code-block:: console $ chpl baz.chpl baz.chpl:1: warning: The default parSafe mode for associative domains and arrays (like 'd1') is changing from 'true' to 'false'. baz.chpl:1: note: To suppress this warning you can make your domain const, use an explicit parSafe argument (ex: domain(int, parSafe=false)), or compile with '-snoParSafeWarning'. baz.chpl:1: note: To use the old default of parSafe=true, compile with '-sassocParSafeDefault=true'. version 1.32, September 2023 ---------------------------- .. _readme-evolution.c_string-deprecation: ``c_string`` deprecation ************************ Version 1.32 deprecates the ``c_string`` type in user interfaces. Please replace occurrences of ``c_string`` with ``c_ptrConst(c_char)``. Note that you need to ``use`` or ``import`` the ``CTypes`` module to have access to ``c_ptrConst`` and ``c_char`` types. Here are some cases where directly replacing ``c_string`` with ``c_ptrConst(c_char)`` may not work and what to do instead: ================================== ============================================ if your code is... update it to... ================================== ============================================ casting ``c_string`` to ``string`` use a ``string.create*Buffer()`` method casting ``c_string`` to ``bytes`` use a ``bytes.create*Buffer()`` method casting ``c_string`` to other type create a string and cast it to other type casting ``string`` to ``c_string`` replace cast with ``.c_str()`` casting ``bytes`` to ``c_string`` replace cast with ``.c_str()`` casting other type to ``c_string`` create a string and call ``.c_str()`` on it using ``param c_string`` use ``param string`` ================================== ============================================ Additionally, several ``c_string`` methods are deprecated without replacement: - ``.writeThis()`` - ``.serialize()`` - ``.readThis()`` - ``.indexOf()`` - ``.substring()`` - ``.size`` * An equivalent for ``.size`` is the unstable procedure ``strLen(x)`` in the ``CTypes`` module. .. _readme-evolution.ref-if-modified-deprecation: The default intent for arrays and records ***************************************** In version 1.32, arrays and records now always have a default intent of ``const``. This means that if arrays and records are modified inside of a function, a ``coforall``, a ``begin``, or ``cobegin``, they must use a ``ref`` intent. This also means that record methods which modify their implicit ``this`` argument must also use a ``ref`` intent. Previously, the compiler would treat these types as either ``const ref`` intent or ``ref`` intent, depending on if they were modified. This change was motivated by improving the consistency across types and making potential problems more apparent. Since there is a lot of user code relying on modifying an outer array, the corresponding change for ``forall`` is still under discussion. As a result, it will not warn by default, but modifying an outer array from a ``forall`` might not be allowed in the future in some or all cases. Consider the following code segment, which contains a ``coforall`` statement which modifies local variables. Prior to version 1.32, this code compiled and worked without warning. .. code-block:: chapel var myInt: int; const myDomain = {1..10}; var myArray: [myDomain] int; coforall i in 2..9 with (ref myInt) { myInt += i; myArray[i] = myArray[i-1] + 1; } Note that to modify ``myInt``, an explicit ``ref`` intent must be used whereas ``myArray`` can be modified freely. The changes to the default intent for arrays is an attempt to remove this inconsistency and make the treatment of types in Chapel more uniform. This code also modifies both ``myInt`` and ``myArray`` in a way that can produce race conditions. With ``myInt``, it is very apparent that there is something different than a simple serial iteration occurring and this can signal to users to more careful inspect their code for potential bugs. However ``myArray`` can be used without that same restriction, which can be a source of subtle bugs. In 1.32, the loop is written as: .. code-block:: chapel var myInt: int; const myDomain = {1..10}; var myArray: [myDomain] int; coforall i in 2..9 with (ref myInt, ref myArray) { myInt += i; myArray[i] = myArray[i-1] + 1; } This removes the inconsistency and calls greater attention to potential race conditions. This change also applies to procedures. Consider the following procedure: .. code-block:: chapel proc computeAndPrint(ref myInt: int, myArray: []) { ... } It is clear that ``myInt`` may be modified and a user of this function can save this value beforehand if they need the value later. But without knowing what is contained in this function, it is impossible to tell if ``myArray`` is going to be modified. Making the default intent for arrays ``const`` removes this ambiguity. This consistency is extended to records as well. Consider the following record definition: .. code-block:: chapel record myRecord { var x: int; proc doSomething() { ... } } Without knowing what the body of ``doSomething`` does, it is not clear whether ``x`` may be modified. In version 1.32, if ``x`` is modified the method must be marked as a modifying record using a this-intent. .. code-block:: chapel record myRecord { var x: int; proc ref doSomething() { ... } } Now it is clear that the method may modify ``x``. version 1.31, June 2023 ----------------------- Version 1.31 renames and adjusts two of range's parameters, formerly ``range.boundedType`` and ``range.stridable``, as well as the former domain parameter ``domain.stridable``. For details please see `Range Types` in the online documentation for `Version 1.30 `_ and `Version 1.31 `_. Range boundedType / bounds parameter ************************************ Prior to Version 1.31, the boundedness of a range ``r`` was determined by ``r.boundedType``. As of 1.31, it is determined by ``r.bounds``. At the same time, the type of this field changed from: .. code-block:: chapel enum BoundedRangeType { bounded, boundedLow, boundedHigh, boundedNone }; to: .. code-block:: chapel enum boundKind { both, low, high, neither }; This change helps make Chapel code shorter, improving its readability. When updating your code, simply update the names accordingly. For example, from: .. code-block:: chapel if myRange.boundedType == BoundedRangeType.boundedLow then ....; to: .. code-block:: chapel if myRange.bounds == boundKind.low then ....; Range and domain stridability / strides parameter ************************************************* Prior to Version 1.31, ranges and domains had the parameter ``stridable``, which was a boolean that indicated whether the given range or domain allowed non-unit strides. As of 1.31, this parameter is replaced with ``strides`` whose type is: .. code-block:: chapel enum strideKind { one, negOne, positive, negative, any }; This change creates additional opportunities for optimization, for example in the cases where the range's stride is known at compile time to be positive or to be -1. This also avoids a terminology problem where ``stridable=false`` implied, incorrectly, that a range could not be strided. The ``strides`` values are now self-explanatory instead of the non-specific values ``true`` and ``false``. When updating your code, update the field name and replace boolean values with enum values. For example: =============================== ============================================= change from... to... =============================== ============================================= ``myRange.stridable`` ``myRange.strides`` ``if myRange.stridable then`` ``if myRange.strides != strideKind.one then`` ``range(stridable=false)`` ``range(strides=strideKind.one)`` ``range(stridable=true)`` ``range(strides=strideKind.any)`` another potential replacement: ``range(strides=strideKind.positive)`` =============================== ============================================= When getting an error like "assigning to a range with boundKind.positive from a range with boundKind.any", insert a cast to the desired range type. Analogous updates are needed in code operating on domains. version 1.28, September 2022 ---------------------------- Version 1.28 included some significant changes to the overload resolution rules. In addition, it enabled implicit conversion from ``int(t)`` to ``uint(t)``. This section discusses some example programs that behave differently due to these changes. See also: * The `1.27 overload resolution rules `_ * The `1.28 overload resolution rules `_ Behavior Differences for Mixes of Signed and Unsigned ***************************************************** Prior to 1.28, numeric operations applied to a mix of signed and unsigned types could have surprising results by moving the computation from a particular bit width to another—or by moving it from an integral computation to a floating point one. For example: .. code-block:: chapel var myInt:int = 1; var myUint:uint = 2; var myIntPlusUint = myInt + myUint; // what is the type of `myIntPlusUint`? Before 1.28, this program would result in compilation error, due to an error overload of ``operator +`` in the standard library. Version 1.28 adds the ability for an ``int`` to implicitly convert to ``uint`` and removes the error overload. As a result, the ``uint`` version of ``operator +`` is chosen, which results in ``myIntPlusUint`` having type ``uint``. This behavior can also extend to user-defined functions. Consider a function ``plus`` defined for ``int``, ``uint``, and ``real``: .. code-block:: chapel proc plus(a: int, b: int) { return a + b; } proc plus(a: uint, b: uint) { return a + b; } proc plus(a: real, b: real) { return a + b; } var myInt:int = 1; var myUint:uint = 2; var myIntPlusUint = plus(myInt, myUint); In 1.27 the call to ``plus`` would resolve to the ``real`` version because ``int`` could not implicitly convert to ``uint``, but both ``int`` and ``uint`` could implicitly convert to ``real(64)``. As a result, ``myIntPlusUint`` had the type ``real``. This change from integral types to floating point types could be very surprising. In 1.28 the call to ``plus`` resolves to the ``uint`` version, and ``myIntPlusUint`` has type ``uint``. This behavior also applies to ``int`` and ``uint`` types with smaller widths: .. code-block:: chapel var myInt32:int(32) = 1; var myUint32:uint(32) = 2; var myInt32PlusUint32 = myInt32 + myUint32; In 1.27, the ``int(64)`` ``+`` operator is chosen (because both ``int(32)`` and ``uint(32)`` can implicitly convert to ``int(64)``), which results in ``myInt32PlusUint32`` having type ``int(64)``. This could be surprising when explicitly working with 32-bit numbers. In contrast, in 1.28, due to the ability for ``int(32)`` to implicitly convert to ``uint(32)``, the ``uint(32)`` version of the ``+`` operator is chosen and ``myInt32PlusUint32`` has type ``uint(32)``. Param Expression Behavior ************************* Some expressions consisting of mixed-type literal or ``param`` values now have different behavior. For example: .. code-block:: chapel var x = 1:int(8) + 2; // what is the type of `x` ? Note in this example that the literal ``2`` is a ``param`` with type ``int(64)`` and that ``1:int(8)`` is a ``param`` with type ``int(8)``. In 1.27, this program would output ``int(8)``, because the overload resolution rules would favor the ``+`` overload using the type of the non-default-sized ``param``. The result is that in 1.27, ``x`` had type ``int(8)``. In 1.28, the rules are simpler and a closer match to the corresponding case with regular variables (``myInt8 + myInt64``). There is no longer any special behavior for non-default-sized ``param``. As a result, the value ``x`` now has type ``int(64)``. For similar reasons, the type of ``nI`` in the following code is now ``int(64)`` where previously it was ``int(32)``: .. code-block:: chapel const nI = ((-2):int(32))**53; A similar change can also appear with range literals that use mixed type ``param`` lower and upper bounds. The following range construction also makes use of the new implicit conversion from ``int(t)`` to ``uint(t))``: .. code-block:: chapel var r8 = 1:int(8)..100:uint(8); writeln(r8.type:string); In 1.27, this would generate a range with index type ``int(16)``. In 1.28, it produces a range with index type ``uint(8)``. Speaking of range literal construction, a range like ``1:int(8)..10`` still produces an ``int(8)`` range in 1.28. However, as we have discussed, something like ``1:int(8) + 10`` would result in an ``int(64)``. For now, the range implementation has been adjusted to preserve the old behavior specifically for the ``..`` operator. However, this may change in a future release. Change for some mixed int/uint overloads **************************************** This example shows a change in behavior for two overloads where one is ``int`` and the other is ``uint``: .. code-block:: chapel proc dbm(a:int(8)) { writeln("dbm int8"); } proc dbm(a:uint(64)) { writeln("dbm uint64"); } dbm(42:int(64)); Previous to 1.28, this program would call the ``int(8)`` version of the function. It can do that because the compiler knows that the ``param`` value ``42`` will fit into an ``int(8)``. Such a conversion is called a ``param`` narrowing conversion. However, in 1.28, this function now calls the ``uint(64)`` version of the function. The main reason for this is that the 1.28 rules prefer to not do ``param`` narrowing conversion when another candidate does not need it. In this case, ``int`` to ``uint`` is not a ``param`` narrowing conversion so that is preferred. Change for function visibility / shadowing ****************************************** The new overload resolution rules in 1.28 consider function visibility or shadowing before considering how well the arguments match. Consider this example: .. code-block:: chapel proc f(arg: int) { writeln("f int"); } proc main() { proc f(arg) { writeln("f generic"); } f(1); // which `f` does this call? } Inside of ``proc main``, the call to ``f`` now resolves to the generic inner function. In contrast, in version 1.27, the outer ``proc f(arg: int)`` would be called. version 1.22, April 2020 ------------------------ 0- vs. 1-based Indexing *********************** Version 1.22 makes a major breaking change to Chapel with respect to indexing for cases that involve implicit indices. Historically, Chapel has used 1-based indexing for such cases, where it now uses 0-based indexing. The major types that are affected by this change are tuples, strings, ``bytes``, and lists. In addition, arrays that don't have a well-defined index set also start at 0. Such cases include array literals or inferred-type arrays formed by capturing a general iterator expression. This change also has a ripple-down effect to features and routines related to these types. For example, varargs arguments are equivalent to tuples in Chapel, so inherit their 0-based indexing. Similarly, queries on rectangular domains and arrays are based on tuples, so their dimensions are now numbered from 0 as well. Certain library routines such as ``find()`` on strings used to return 0 when no match was found, but now return -1 in order to avoid returning a legal string index. The following sections summarize the rationale for this change and then provide some tips for updating existing Chapel code. Rationale for 0- vs. 1-based Indexing ************************************* In the original design of Chapel, we hoped to make the language as neutral to 1- vs. 0-based indexing as possible, to avoid running afoul of the strong emotions that such choices evoke in users when it doesn't match their preference. As a result, Chapel's primary types for parallel computation on regular collections of data—namely, its ranges and rectangular domains, as well as rectangular arrays defined by ranges or domains—require users to specify both low and high bounds. Happily, these core features are not affected by this change in Chapel 1.22, so codes relying solely on such features will not require updates. However, for other types such as tuples and strings, we were forced to make a decision. At the time of Chapel's inception, the main languages from which we were trying to attract users were C/C++, Java, Fortran, and Matlab. Since half of these languages used 0-based indexing and the other half used 1-based, there didn't seem to be an obvious best answer. In the end, we decided to go with 1-based indexing on the argument that we were striving to create a productive language, and that counting from 1 is arguably most natural for most people. Over time, however, the vast majority of newer languages that we look to for users or inspiration—most notably Python, Swift, and Rust—have been almost exclusively 0-based. Meanwhile, very few notable new languages have used 1-based indexing. Furthermore, when polled, the vast majority of active Chapel users expressed a strong preference for 0-based programming, given the choice (though there were also notable outliers, particularly from the Fortran community). We also realized (a) that Chapel's design should be more concerned with lowering barriers for existing programmers than for non-programmers; and (b) that even though we had arguably biased the original design in favor of Fortran programmers, most of Chapel's early adopters have come from C/C++ and Python backgrounds. Based on this, we undertook an experiment to see what it would take to convert from 1-based to 0-based programming. Reviewing Chapel's ~10,000 tests and modules resulted in changes to ~1,000 of them. We also updated some significant applications such as Arkouda and Cray HPO. While the overall effort of making the change was not insignificant, it also wasn't particularly difficult for the most part. Overall, our finding was that in cases where the changes weren't simply neutral in their impact on style, it almost always benefitted the code in terms of clarity, because there tended to be fewer adjustments of +/- 1 in the code. For these reasons, we decided to bite the bullet and make the switch now, while we felt we still could, rather than later when it would clearly be too late to do so and cause more of a revolt among our users. Index-neutral Features ********************** This experience also led to a number of new programming features in Chapel 1.21 designed to help write code in more of an index-neutral style. Chief among these are new ``.indices`` queries on most of the relevant types as well as support for loops over heterogeneous tuples. We also introduced features that we found to be useful in updating code, such as support for open-interval ranges and ``.first`` and ``.last`` queries on enumerated types. To this end, even though Chapel still has cases that require making this 0- vs. 1-based indexing decision, we encourage code to be written in an index-neutral style whenever possible, and believe that most common code patterns can be. Tips for Updating Existing Chapel code ************************************** The following are some tips for updating codes based on our experiences: * First, updating code is easiest when it has some sort of testing infrastructure that can be used to validate that its behavior is unchanged. If you don't already have such testing for your code, it may be worthwhile to invest in creating some before attempting this upgrade. * Next, when transitioning code to Chapel 1.22, make sure to compile it with neither ``--fast`` nor ``--no-checks`` enabled so that bounds checks are turned on in the generated code. In cases where a program is accessing all of the elements of a collection (as is common for tuples) this will help identify data structures that require updates. When you do get an out-of-bounds error, don't simply update the specific access, but use it as a cue to look through the code for other references to that variable that will also need updating. * When possible, try rewriting your updated code to use an index-neutral style of programming. For example, given code like this: .. code-block:: chapel var t: 2*int = ...; var x = t(1), y = t(2); for i in 1..2 do writeln("t(", i, ") = ", t(i)); It would be reasonable to rewrite it like this: .. code-block:: chapel var t: 2*int = ...; var x = t(0), y = t(1); for i in 0..1 do writeln("t(", i, ") = ", t(i)); But arguably preferable to update it like this: .. code-block:: chapel var t: 2*int = ...; var (x, y) = t; for i in t.indices do writeln("t(", i, ") = ", t(i)); If you have a pattern that you're trying to write in an index-neutral style, but can't, don't hesitate to `ask for tips `_. * Some common pitfalls to check for in your code include: - Search for queries on the dimensions of rectangular domains and arrays. For example, ``myDomain.dim(1)``, ``myDomain.low(1)``, ``myDomain.high(1)``, or ``myDomain.stride(1)`` will need to be updated to reflect that array dimensions now count from 0 rather than 1. These will result in out-of-bounds errors in cases where you query all dimensions of an array, making them easy to find; but it can be worthwhile to grep your code for such patterns to make sure you don't miss any. - Also search for instances of ``find()`` or ``rfind()`` that are relying on comparisons to zero/nonzero values, and update them to compare against -1. For example, patterns like ``if mystring.find('z')`` need to be updated to ``if mystring.find('z') != -1``. - Search for instances of ``split()``. A common idiom is to write ``var substrs = mystring.split(5);`` and then to index into the result using ``substrs[1]``, ``substrs[2]``, etc. Since this is an instance of capturing an iterator expression, you'll either need to subtract one from the indices, or else declare `substrs` to have a specific type, like ``var substrs: [1..5] string = mystring.split(5);`` - Search for varargs functions and make sure they are updated to use 0-based indexing or index-neutral features. - Search for any calls to ``Reflection.getField*()`` and update those the cases that use integer indices to reflect 0-based numbering. - Look for any calls on lists that use explicit offsets, as these will likely need updates. For example ``mylist.pop(1);`` will need to become ``mylist.pop(0);`` - Some other common string patterns to look for in your code that `may` indicate something requiring an update include: - ``1..`` - ``[1]`` - ``(1)`` - ``[2]`` - ``(2)`` - Think about whether there are other places in your code that compute index values numerically yet which don't have obvious syntactic cues. Need Help? ********** If you are able to share your code with us and would like help updating it to Chapel 1.22, please don't hesitate to `ask for help `_. Given our experience in updating the Chapel code base itself, we have found it fairly easy to update most codes, even when we're unfamiliar with them. version 1.21, April 2020 ------------------------ Version 1.21 made several improvements related to record initialization, assignment, and deinitialization. In summary: * Some patterns of default initialization followed by assignment are now converted to initialization. See :ref:`readme-evolution.split-init`. * Some patterns of copy initialization followed by deinitialization are converted to move initialization. See :ref:`readme-evolution.copy-elision`. * The result of a nested call expression can now be deinitialized at the end of the containing statement. See :ref:`readme-evolution.statement-deinit`. .. _readme-evolution.split-init: split initialization ******************** Split initialization a new language feature in 1.21 that is described in the language specification - see :ref:`Split_Initialization`. Consider the following example: .. code-block:: chapel var x: myRecord; // default-initialization in 1.20 x = new myRecord(); // assignment in 1.20 -- initialization in 1.21 In 1.21, instead of default-initializing ``x`` and then assigning to it, ``x`` will be initialized on the second line. Note that split initialization also changes the copy and assignment behavior of ``out`` intent formal arguments. Occasionally programs that are written to test assignment (separately from copy initialization) need to avoid split initialization. One way to do so is to add a mention of the variable immediately after it is declared, as in the following code: .. code-block:: chapel var x: myRecord; x; // adding this mention prevents split-initialization // instead, x is default-initialized at its declaration point above x = new myRecord(); .. _readme-evolution.copy-elision: copy elision ************ Copy elision a new language feature in 1.21. When the last mention of a variable is the source of a copy-initialization, the copy-initialization is replaced by move-initialization. For example: .. code-block:: chapel class MyClass { var field; proc init(in arg) { this.field = arg; } } proc copyElisionExample() { var a = new myRecord(); var b = a; // now move-initializes `b` from `a` return new MyClass(b); // now move-initializes the field from `b` } .. _readme-evolution.statement-deinit: deinitialization point of nested call expressions ************************************************* In 1.20, all variables are deinitialized at the end of the enclosing block. That changed in 1.21. Compiler-introduced temporary variables storing the result of a nested call expression can now be deinitialized at the end of a statement. In particular, results of nested call expressions are now deinitialized at the end of the statement unless the statement is initializing a user variable. For example: .. code-block:: chapel proc makeRecord() { return new myRecord(); } proc f(arg) { return arg; } proc deinitExample() { f(makeRecord()); // Compiler converts the above statement into // var tmp = makeRecord(); // f(tmp); // In 1.20, tmp is destroyed at the end of the block. // In 1.21, tmp is destroyed at the end of the above statement. var x = f(makeRecord()); // In both 1.20 and 1.21, the temporary storing the result of // `makeRecord()` is deinitialized at the end of the block. } version 1.20, September 2019 ---------------------------- Version 1.20 made language changes that address problems with classes. In summary: * variables of class type can no longer store `nil` by default but can opt-in to possibly being `nil` with `?`. See :ref:`readme-evolution.nilability-changes` * certain casts have changed behavior to support nilability changes See :ref:`readme-evolution.nilability-and-casts` * un-decorated class types such as `MyClass` (as opposed to `borrowed MyClass`) now have generic management See :ref:`readme-evolution.undecorated-classes-generic-management` * arguments with `owned` or `shared` declared type now use `const ref` default intent rather than `in` intent. See :ref:`readme-evolution.new-default-intent-for-owned-and-shared` * ``new C`` now creates an `owned C` rather than a `borrowed C` See :ref:`readme-evolution.new-C-is-owned` .. _readme-evolution.nilability-changes: nilability changes ****************** Previous to 1.20, variables of class type could always store ``nil``. In 1.20, only nilable class types can store ``nil``. Non-nilable class types and nilable class types are different types. A class type expression such as ``borrowed C`` indicates a non-nilable class type. As an aid in migrating code to this change, the flag ``--legacy-classes`` will disable this new behavior. Consider the following example: .. code-block:: chapel class C { var x:int; } var a: borrowed C = (new owned C()).borrow(); In 1.19, variables of type ``borrowed C`` could store ``nil``: .. code-block:: chapel var b: borrowed C = nil; var c: borrowed C; a = nil; The 1.20 compiler will report errors for all 3 of these lines. To resolve the errors, it is necessary to use a nilable class type. Nilable class types are written with ``?`` at the end of the type. In this example: .. code-block:: chapel var a: borrowed C? = (new owned C()).borrow(); var b: borrowed C? = nil; var c: borrowed C?; a = nil; Implicit conversions are allowed from non-nilable class types to nilable class types. When converting variables to nilable types to migrate code, there will be situations in which it is known by the developer that a variable cannot be ``nil`` at a particular point in the code. For example: .. code-block:: chapel proc f(arg: borrowed C) { } proc C.method() { } config const choice = true; var a: owned C?; if choice then a = new owned C(1); else a = new owned C(2); f(a); a.method(); Errors on the last two lines can be resolved by writing .. code-block:: chapel f(a!); a!.method(); where here the ``!`` asserts that the value is not ``nil`` and it can halt if the value is ``nil``. Note that in ``prototype`` and implicit file-level modules, the compiler will automatically add ``!`` on method calls with nilable receivers (i.e. in the ``a.method()`` case above). In the above case, a cleaner way to write the conditional would be to create a function that always returns a value or throws if there is a problem. For example: .. code-block:: chapel proc makeC() throws { var a: owned C?; if choice then a = new owned C(1); else a = new owned C(2); return a:owned C; // this cast throws if a stores nil } proc main() throws { var a:owned C = makeC(); f(a); a.method(); } .. _readme-evolution.nilability-and-casts: nilability and casts ******************** Because casts to class types should necessarily return something of the requested type, and because many class types now cannot store ``nil``, certain patterns involving casts will need to change to work with 1.20. class downcasts ^^^^^^^^^^^^^^^ In a class downcast, a class is casted to a subtype. If the dynamic type of the variable does not match the requested subtype, the downcast fails. In 1.19, a failed downcast would result in ``nil``. In 1.20, a failed downcast will result in ``nil`` only if the target type is nilable and will throw an error otherwise. For example: .. code-block:: chapel class Parent { } class Child : Parent { } var p:borrowed Parent = (new owned Parent()).borrow(); var c:borrowed Parent = (new owned Child()).borrow(); writeln(c:Child?); // downcast succeeds writeln(c:Child); // downcast succeeds writeln(p:Child?); // this downcast fails and results in `nil` writeln(p:Child); // this downcast fails and will throw a ClassCastError casting C pointers to classes ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Casts from ``c_void_ptr`` to class types were previously allowed. However, since ``c_void_ptr`` can store ``NULL``, this case needs adjustment following the nilability changes. Additionally, since ``c_void_ptr`` refers to a C pointer, and C pointers are manually managed (i.e. you call ``free`` on them at the appropriate time), it makes the most sense for casts from ``c_void_ptr`` to end up with an unmanaged type. Consider the following example: .. code-block:: chapel class C { var x:int; } var myC = new owned C(); var ptr:c_void_ptr = myC.borrow(); // store the instance in a C ptr Now we can cast from ``ptr`` to the class type: .. code-block:: chapel var c = ptr:C; // cast from a C pointer to the borrowed type This example would work in 1.19. In 1.20, it needs to be updated to cast to ``unmanaged C?``: .. code-block:: chapel var c = ptr:unmanaged C?; As with other values of type ``unmanaged C?``, from there it can: * be borrowed, e.g. ``c.borrow()`` * have ``!`` applied to convert to a non-nilable value or halt, e.g. ``c!`` * be cast to a non-nilable type, throwing if it is ``nil``, e.g. ``c:borrowed C`` .. _readme-evolution.undecorated-classes-generic-management: undecorated classes have generic management ******************************************** Undecorated classes now have generic management. As an aid in migrating code to this change, the flag ``--legacy-classes`` will disable this new behavior. Supposing that we have a ``class C`` declaration as in the following: .. code-block:: chapel class C { var x:int; } Code using ``C`` might refer to the type ``C`` on its own or it might use a decorator to specify memory management strategy, as in ``borrowed C``. The type expression ``C`` was the same as ``borrowed C`` in 1.18 and 1.19 but now means generic management. For example, in the following code: .. code-block:: chapel var myC:C = new owned C(); ``myC`` previously had type ``borrowed C``, and was initialized using including an implicit conversion from ``owned C`` to ``borrowed C``. In 1.20, ``myC`` has type ``owned C``. Since the variable's type expression is generic management, it takes its management from the initializing expression. This change combines with the nilability changes described above to prevent compilation of existing code like the following: .. code-block:: chapel var x:C; Knowing that ``C`` now cannot store ``nil``, one might try to update this program to: .. code-block:: chapel var x:C?; However this does not work either. ``C?`` indicates a nilable class type with generic management, and a variable with generic type cannot be default-initialized. To update such a variable declaration to 1.20, it is necessary to include a memory management decorator as well as ``?``. For example: .. code-block:: chapel var x:borrowed C?; The resulting variable will initially store ``nil``. .. _readme-evolution.new-default-intent-for-owned-and-shared: new default intent for owned and shared *************************************** The default intent for `owned` and `shared` arguments is now `const ref` where it was previously `in`. Cases where such arguments will be interpreted differently can be reported with the ``--warn-unstable`` compilation flag. Consider the following example: .. code-block:: chapel class C { var x:int; } var global: owned C?; proc f(arg: owned C) { global = arg; } f(new owned C(1)); This program used to compile and run, performing ownership transfer once when passing the result of ``new`` to ``f`` and a second time in the assignment statement ``global = arg``. This program does not work in 1.20. The compiler will issue an error for the statement ``global = arg`` because the ownership transfer requires modifying ``arg`` but it is not modifiable because it was passed with ``const ref`` intent. To continue working, this program needs to be updated to add the `in` intent to ``f``, as in ``proc f(in arg: owned C)``. Note that for totally generic arguments, the 1.18 and 1.19 compiler would instantiate the argument with the borrow type when passed ``owned`` or ``shared`` classes. For example: .. code-block:: chapel class C { var x:int; } proc f(arg) { } var myC = new owned C(1); f(myC); // does this call transfer ownership out of myC? writeln(myC); // prints `nil` if ownership transfer occurred This example functions the same in 1.18 and 1.20, but for different reasons. In 1.18, ``f`` is instantiated as accepting an argument of type ``borrowed C``. In the call ``f(myC)``, the compiler applies a coercion from ``owned C`` to ``borrowed C``, so ownership transfer does not occur. In 1.20, ``f`` is instantiated as accepting an argument of type ``owned C`` but this type uses the default intent (``const ref``). As a result, ownership transfer does not occur. .. _readme-evolution.new-C-is-owned: new C is owned ************** Supposing that `C` is a class type, `new C()` was equivalent to `new borrowed C()` before this release - meaning that it resulted in something of type `borrowed C`. However, it is now equivalent to `new owned C()` which produces something of type `owned C`. version 1.18, September 2018 ---------------------------- Version 1.18 includes many language changes that address problems with classes. In summary: * constructors are deprecated and replaced with initializers See :ref:`readme-evolution.initializers-replace-constructors` * memory management for class types has changed See :ref:`readme-evolution.class-memory-management` * `override` is now required on overriding methods See :ref:`readme-evolution.mark-overriding` .. _readme-evolution.initializers-replace-constructors: initializers replace constructors ********************************* Code that contained user-defined constructors will need to be updated to use an initializer. For example: .. code-block:: chapel record Point { var x, y: real; proc Point() { x = 0; y = 0; writeln("In Point()"); } proc Point(x: real, y: real) { this.x = x; this.y = y; writeln("In Point(x,y)"); } } var a:Point; var b = new Point(1.0, 2.0); will now compile with deprecation warnings. Here is the same program updated to use initializers: .. code-block:: chapel record Point { var x, y: real; proc init() { x = 0; y = 0; writeln("In Point.init()"); } proc init(x: real, y: real) { this.x = x; this.y = y; writeln("In Point.init(x,y)"); } } var a:Point; var b = new Point(1.0, 2.0); The change to initializers is much more than a change in the name of the method. See the language specification for further details. .. _readme-evolution.class-memory-management: class memory management *********************** Before 1.18, if ``C`` is a class type, a variable of type ``C`` needed to be deleted in order to prevent a memory leak. For example: .. code-block:: chapel class C { var x: int; } proc main() { var instance: C = new C(1); delete instance; } Version 1.18 introduced four memory management strategies that form part of a class type and are used with `new` expressions: ``owned C`` ``owned`` classes will be deleted automatically when the ``owned`` variable goes out of scope, but only one ``owned`` variable can refer to the instance at a time. Such instances can be created with ``new owned C()``. ``shared C`` ``shared`` classes will be deleted when all of the ``shared`` variables referring to the instance go out of scope. Such instances can be created with ``new shared C()``. ``borrowed C`` refers to a class instance that has a lifetime managed by another variable. Values of type ``borrowed C`` can be created with ``new borrowed C()``, by coercion from the other class ``C`` types, or by explicitly calling the ``.borrow()`` method on one of the other class ``C`` types. ``new borrowed C()`` creates a temporary instance that will automatically be deleted at the end of the current block. ``unmanaged C`` the instance must have `delete` called on it explicitly to reclaim its memory. Such instances can be created with ``new unmanaged C()``. Further note that the default is ``borrowed``, that is: ``C`` is now the same as ``borrowed C`` ``new C()`` is now the same as ``new borrowed C()`` Now, back to the example above. There are several ways to translate this program. First, the most semantically similar option is to replace uses of ``C`` with ``unmanaged C``: .. code-block:: chapel class C { var x: int; } proc main() { var instance: unmanaged C = new unmanaged C(1); delete instance; } Using ``unmanaged`` allows a Chapel programmer to opt in to manually managing the memory of the instances. A reasonable alternative would be to translate the program to use ``owned C``: .. code-block:: chapel class C { var x: int; } proc main() { var instance: owned C = new owned C(1); // instance will now be automatically deleted at the end of this block } If the program does not explicitly use ``owned C``, it can rely on ``new C()`` being equivalent to ``new borrowed C()``: .. code-block:: chapel class C { var x: int; } proc main() { var instance: C = new C(1); // instance will now be automatically deleted at the end of this block } See the *Class New* section in the *Classes* chapter of the language specification for more details. .. _readme-evolution.mark-overriding: overriding methods must be marked ********************************* Before 1.18, a class inheriting from another class can create an overriding method that is a candidate for virtual dispatch: .. code-block:: chapel class Person { var name: string; proc greet() { writeln("Hello ", name, "!"); } } class Student: Person { var grade: int; proc greet() { writeln("Hello ", name, ", welcome to grade ", grade); } } proc main() { var person: Person = new Student("Jeannie", 5); person.greet(); // uses the run-time type of person (Student) // and virtually dispatches to Student.greet() } Now such overriding methods must be marked with the `override` keyword: .. code-block:: chapel class Person { var name: string; proc greet() { writeln("Hello ", name, "!"); } } class Student: Person { var grade: int; override proc greet() { writeln("Hello ", name, ", welcome to grade ", grade); } } proc main() { var person: Person = new Student("Jeannie", 5); person.greet(); // uses the run-time type of person (Student) // and virtually dispatches to Student.greet() } version 1.15, April 2017 ------------------------ Version 1.15 includes several language changes to improve array semantics. In summary: * arrays are always destroyed when they go out of scope and in particular will not be preserved by use in `begin`. See :ref:`readme-evolution.array-lexical-scoping`. * the array alias operator `=>` has been deprecated in favor of creating references to an array or a slice of an array with `ref` or `const ref`. See :ref:`readme-evolution.alias-operator-deprecated`. * arrays now return by value by default instead of by `ref`. See :ref:`readme-evolution.arrays-return-by-value`. * arrays now pass by `ref` or `const ref` by default, depending on whether or not the formal argument is modified. See :ref:`readme-evolution.array-default-intent`. Additionally, the default intent for record method receivers has changed: * the method receiver for records is passed by `ref` or `const ref` by default, depending on whether or not the formal argument is modified. See :ref:`readme-evolution.record-this-default-intent`. .. _readme-evolution.array-lexical-scoping: array lexical scoping ********************* As described in the language changes for 1.12 in :ref:`readme-evolution.lexical-scoping`, using arrays beyond their scope is a user error. While such a program was in error starting with Chapel 1.12, such a pattern worked until Chapel 1.15. For example, this program will probably crash in Chapel 1.15: .. code-block:: chapel proc badBegin() { var A: [1..10000] int; begin { A += 1; } // Error: A destroyed here at function end, but the begin could still // be using it! } Similarly, using a slice after an array has been destroyed is an error: .. code-block:: chapel proc badBeginSlice() { var A: [1..10000] int; // slice1 sets up a slice using the => operator // note that the => operator is deprecated (see below) var slice1 => A[1..1000]; // slice2 sets up a slice by creating a reference to it ref slice2 = A[1..1000]; // either way, using the slice in a begin that can continue // after the function declaring the array exits is an error begin { slice1 += 1; slice2 += 1; } // Error: A destroyed here at function end, but the begin tries to // use it through the slices! } .. _readme-evolution.alias-operator-deprecated: array alias operator deprecated ******************************* The array alias operator, `=>`, has been deprecated in Chapel 1.15. Previously, the supported way to declare one array that aliases another (or a slice of another) was to use `=>`. Now, the supported way to do that is to use a `ref` or `const ref` variable: For example, before Chapel 1.15 you might have written: .. code-block:: chapel // pre-1.15 var A:[1..10] int; // set up a const alias of A const alias => A; // set up a mutable slice of A var slice => A[2..5]; // set up a re-indexing slice of A var reindex:[0..9] => A; In Chapel 1.15, use `ref` or `const ref` to create the same pattern: .. code-block:: chapel var A:[1..10] int; // set up a const alias of A const ref alias = A; // set up a mutable slice of A ref slice = A[2..5]; // set up a re-indexing slice of A ref reindex = A.reindex({0..9}); .. _readme-evolution.arrays-return-by-value: arrays return by value by default ********************************* Before Chapel 1.15, returning an array would return the array by reference. Now arrays return by value by default. That is, the act of returning an array can make a copy: .. code-block:: chapel var A: [1..4] int; proc returnsArray() { return A; } ref B = returnsArray(); B = 1; writeln(A); // outputs 1 1 1 1 historically // outputs 0 0 0 0 after Chapel 1.15 This behavior applies to array slices as well. The old behavior is available with the `ref` return intent. Note though that returning a `ref` to a local array is an error just like it is an error to return a local `int` variable by `ref`. .. code-block:: chapel proc returnsArrayReference() ref { return A; } .. _readme-evolution.array-default-intent: array default intent ******************** Before 1.15, the default intent for arrays was `ref`. The rationale for this feature was that it was a convenience for programmers who are used to modifying array formal arguments in their functions. Unfortunately, it interacted poorly with return intent overloading. Additionally, the implementation had several bugs in this area. The following example shows how it might be surprising that return intent overloading behaves very differently for arrays than for other types. As the example shows, this issue affects program behavior and not just const-checking error messages from the compiler. .. code-block:: chapel // First, let's try some of these things with an // associative array of ints: { var D:domain(int); var A:[D] int; // This adds index 1 to the domain, implicitly A[1] = 10; writeln(D.member(1)); // outputs `true` // This will halt, because index 2 is not in the domain //var tmp = A[2]; // This will also halt, for the same reason //writeln(A[3]); } // Now, let's try the same things with an array of arrays: { var D:domain(int); var AA:[D] [1..4] int; var value:[1..4] int = [10,20,30,40]; // This adds index 4 to the domain, implicitly AA[4] = value; writeln(D.member(4)); // outputs `true` // This will halt, because index 5 is not in the domain //var tmp = AA[5]; // It seems that this *should* halt, but it does not (pre 1.15) // Instead, it adds index 6 to the domain writeln(AA[6]); writeln(D.member(6)); // outputs `true` ! } See `GitHub issue #5217 `_ for more examples and discussion. In order to make such programs less surprising, version 1.15 changes the default intent for arrays to `ref` if the formal argument is modified in the function and `const ref` if not. As a result, the above example behaves similarly for an associative array of integers and an associative array of dense arrays. For example, in the following program, the default intent for the formal argument `x` is `ref`: .. code-block:: chapel proc setElementOne(x) { // x is modified, so x has ref intent x[1] = 1; } var A:[1..10] int; setElementOne(A); In contrast, in the following program, the default intent for the formal argument `y` is `const ref`: .. code-block:: chapel proc getElementOne(y) { // y is not modified, so y has const ref intent var tmp = y[1]; } const B:[1..10] int; getElementOne(B); .. _readme-evolution.record-this-default-intent: record `this` default intent **************************** Before 1.15, the default intent for the implicit `this` argument for record methods was implemented as `ref` but specified as `const ref`. In 1.15, this changed to `ref` if the formal `this` argument is modified in the body of the function and `const ref` if not. See `GitHub issue #5266 `_ for more details and discussion. .. code-block:: chapel record R { var field: int; proc setFieldToOne() { // this is modified, so this-intent is ref this.field = 1; } proc printField() { // this is not modified, so this-intent is const ref writeln(this.field); } } version 1.13, April 2016 ------------------------ ref return intent ***************** Previous versions of Chapel included an implicit `setter` param of type `bool` for `ref` return intent functions. In addition, the compiler created a getter and setter version of each ref return intent function. The getter version would return an rvalue, and the setter version would return an lvalue by ref. For example: .. code-block:: chapel var x = 1; proc refToX() ref { if setter then return x; // setter version else return 0; // getter version } refToX() = 3; // uses the setter version writeln(x); // prints 3 var tmp = refToX(); // uses the getter version writeln(tmp); // prints 0 This functionality has changed with version 1.13. It is still possible to write a getter and a setter, but these must be written as pair of related functions: .. code-block:: chapel var x = 1; // setter version proc refToX() ref { return x; } // getter version proc refToX() { return 0; } refToX() = 3; // uses the setter version writeln(x); // prints 3 var tmp = refToX(); // uses the getter version writeln(tmp); // prints 0 In some cases, when migrating code over to the new functionality, it is useful to put the old ref return intent function into a helper function with an explicit param `setter` argument, and then to call that function from the getter or setter. version 1.12, October 2015 -------------------------- .. _readme-evolution.lexical-scoping: lexical scoping *************** Prior to version 1.12 of Chapel, variables could be kept alive past their lexical scopes. For example: .. code-block:: chapel { var A: [1..n] real; var count$: sync int; var x: real; begin with (ref x) { ... A ...; ... count$ ...; ... x ...; } // ^^^ this task and its references to A, count$, and x could outlive // the scope in which those variables are declared. } // So, previously, Chapel kept these variables alive past their // logical scope. Disadvantages of this approach included: * It moves logical stack variables (like `x` and `count$` above) to the heap. * It complicates memory management by incurring reference counting overhead---or causing memory leaks in cases where reference counting hadn't been added. * It was not particularly valued or utilized by users. * It was arguably surprising ("x still exists even though it left scope?"). As of Chapel 1.12 (and moreso in subsequent releases), the implementation no longer provides this property. Instead, it is a user error to refer to a variable after it has left scope. For example: .. code-block:: chapel var flag$: sync bool; // flag$ starts empty { var x: real; begin with(ref x) { // create task referring to x flag$; // block task until flag$ is full ... x ... // user error: access to x occurs after it leaves scope } // end task } // x`s scope ends flag$ = true; // fill flag$ only after x's scope closes Code that refers to lexically scoped variables within tasks in this manner should use `sync` variables or blocks in order to guarantee the tasks's completion before the enclosing block exits. Note that the more commonly used `cobegin`, `coforall`, and `forall` statements already guarantee that the tasks they create will complete before the enclosing block exits. version 1.11, April 2015 ------------------------ forall intents ************** In previous versions of Chapel, the bodies of forall-loops have referred to all lexically visible variables by reference. In this release of Chapel, such variables are treated more consistently with the task intent semantics and syntax introduced in versions 1.8 and 1.10 respectively (described below). Specifically, prior to this release, a loop like the following would represent a data race: .. code-block:: chapel var sum = 0.0; forall a in A do sum += a; since multiple iterations of the loop could execute simultaneously, read the identical value from the shared variable ``sum``, update it, and write the result back in a way that could overwrite other simultaneous updates. Under the new forall intent semantics, such variables are treated as though they are passed by "blank intent" to the loop body (so ``const`` for variables of scalar type like ``sum``, preventing races in such cases). This mirrors the task intent semantics for variables referenced within begin, ``cobegin``, and ``coforall`` constructs. As in those cases, a user can specify semantics other than the default via a *with-clause*. For example, to restore the previous race-y semantics, one could write: .. code-block:: chapel var sum = 0.0; forall a in A with (ref sum) do sum += a; (Of course, the safe way to write such an idiom would be to use a reduction, or a synchronization type like ``sync`` or ``atomic``). type select statement ********************* Chapel has traditionally supported a ``type select`` statement that was like a ``select`` statement for types. However, this seemed inconsistent with the fact that other constructs like ``if...then`` operate on types directly. For that reason, this release removed support for ``type select x``. Instead, use the equivalent ``select x.type``. version 1.10, October 2014 -------------------------- task intents syntax ******************* Task intent clauses were added to Chapel in version 1.8 to support passing variables by reference into tasks. Since then, the need to pass variables by other intents and into other parallel constructs has arisen. But, the previous syntax was problematic to extend to other intents, while also generating syntactic ambiguities for other additions we wished to make to the language. For these reasons, a new task intent syntax was designed to cleanly support intents other than ``ref`` (particularly in looping contexts), to address the pending ambiguity, and to better reflect the similarity of task intents to formal argument lists. Where previously, task constructs could be followed by a ``ref`` clause, they can now be followed by a ``with`` clause that takes a list of intents and variables, specifying how to pass them into the task. Thus, where one would have previously written: .. code-block:: chapel begin ref(x) update(x); cobegin ref(x, y) { process(x); process(y); } coforall x in A ref(y) { process(x, y); } you would now write: .. code-block:: chapel begin with (ref x) update(x); cobegin with(ref x, ref y) { process(x); process(y); } coforall x in A with (ref y) { process(x, y); } As of the 1.10 release, only ``ref`` intents are supported, though we plan to expand this set of intents for the 1.11 release while also extending forall-loops to support task intents. 'var' function return intents changed to 'ref' ********************************************** A ``var`` function return intent has traditionally been used to indicate that a call to the function (referred to as a *var function*) could appear in either an r-value or l-value context. The ``var`` keyword was chosen since the function could be used in the same contexts as a variable could. Since that time, the ``ref`` keyword has been introduced into Chapel to support passing variables by reference to functions. Since returning an expression by reference supports similar capabilities as ``var`` functions require, while also being less unusual/more orthogonal, this release replaces ``var`` function return intents with ``ref`` intents. Thus, where one would previously write: .. code-block:: chapel proc getFoo() var { ... } now you would write: .. code-block:: chapel proc getFoo() ref { ... } The ``var`` as a return intent is deprecated and generates a warning for the current release, after which it will be removed. version 1.9, April 2014 -------------------------- operator precedence changes to benefit common cases *************************************************** Historically, Chapel's operator precedence choices have tended to follow the lead of C for operators that are common to both languages, figuring that following an established convention would be better than forging our own path. With this change, we modified the precedence of bitwise operators to better reflect what we think it intuitive to users and correct what is viewed in many circles to be a regrettable mistake in C. At the same time, we changed the binding of ``in`` and ``..`` to support some other Chapel idioms more naturally, like ``1..10 == 1..10``. To see the current operator precedence, refer to the `Quick Reference sheet `_. improved interpretation of {D} ****************************** Historically, for a domain D, Chapel has interpreted ``{D}`` as being equivalent to ``D``, inheriting a precedent of sorts set by the ZPL language, and dating from a time when we used square brackets for both domain literals and array types. With this change, we began interpreting ``{D}`` as a domain literal with a single index, ``D`` (i.e., an associative domain of domains). Associative domains of domains are not yet implemented in the language, so the new syntax is not yet useful, but at least the incongruity of ignoring the curly brackets has been removed. version 1.8, October 2013 -------------------------- task functions and intents; ref-clauses Chapel has three constructs for creating tasks: ``begin``, ``cobegin``, and ``coforall``. Historically, variable references within tasks followed standard lexical scoping rules. For example, the following code: .. code-block:: chapel var x = 0; begin writeln(x); x += 1; could print either the value 0 or 1, depending on whether the ``writeln()`` task was executed before or after the increment of ``x``. With this change, we view the creation of a task as an invocation of a *task function* — a compiler-created function that implements the task. Any references to variables outside of the task's scope (like ``x`` in the example above) are treated as implicit arguments to the task function, passed by blank intent. Thus, when ``x`` is an integer, as in the above code, the task will always print the value of 0, even if the increment of ``x`` is executed before the ``writeln()`` task, since the value of ``x`` will have been passed to the task function by blank intent (implying a ``const`` copy for integer arguments). In contrast, if x were a sync variable in the example above, the blank intent would cause it to be passed by reference to the task, permitting the task to see either of the values 0 or 1. To return to the previous behavior, a *ref-clause* can be added to the tasking construct to indicate that a variable should be passed to the task function by reference rather than blank intent. For example, the following code: .. code-block:: chapel var x = 0; begin ref(x) writeln(x); x += 1; would revert to the previous behavior, even if ``x`` were an integer. For more information on this feature, please refer to the *Task Intents* section of the *Task Parallelism* and *Synchronization* chapter of the language specification. version 1.6, October 2012 ------------------------- domain literals *************** Chapel's domain literals were historically specified using square brackets, based on ZPL's region syntax. Thus ``[1..m, 1..n]`` represented an *m × n* index set. In this change, we made domain literals use curly brackets in order to reflect their role as sets of indices, and also to make square brackets available for supporting array literals. Thus, ``{1..m, 1..n}`` is an *m × n* index set, ``[1.2, 3.4, 5.6]`` is a 3-element array of reals and ``[1..m, 1..n]`` is a 2-element array of ranges. Emacs users working on updating existing code can use the following recipe to update old-style domain literals to the new syntax: .. code-block:: text M-x query-replace-regexp: \([=|,] *\)\[\(.*?\)\]\([;|)]\) with: \1{\2}\3 zippered iteration ****************** Zippered iteration in Chapel was traditionally supported simply by iterating over a tuple of values. For example, forall ``(i,a)`` in ``(1..n, A)`` would iterate over the range ``1..n`` and the n-element array ``A`` in a zippered manner. In this change, we introduced the zip keyword to make these zippered iterations more explicit and to permit iteration over a tuple's values directly. Thus, the zippered iteration above would now be written: .. code-block:: chapel forall (i,a) in zip(1..n, A) ignoring tuple components/underscore ************************************ Overtime, the mechanism used to ignore a tuple component when destructuring a tuple has changed. Originally, an underscore was used to drop a value on the floor. For example, given a 3-tuple ``t``, the first and last components could be stored in ``x`` and ``z``, dropping the second component on the floor using: ``var (x, _, z) = t;``. In version 1.1 (Apr 2010), we changed this to use a blank space instead of an underscore, for simplicity and to permit underscore to be used as an identifier name. Thus, the example above would have been written as ``var (x, , z) = t;`` during this time period. However, in 2012, we changed back to using the underscore again in order to support the specification of 1-tuples using a dangling comma, similar to Python. Thus, dropping a tuple component is expressed as ``var (x, _, z) = t;`` again while ``(1.2, )`` is a 1-tuple of reals. version 1.4, October 2011 -------------------------- function declaration keywords ***************************** Prior to this change, the keyword ``def`` was used to define both procedures and iterators; the compiler inspected the body of the function for yield statements to determine whether it was a procedure or an iterator. In this change, we introduced the ``proc`` and ``iter`` keywords to distinguish between these two cases for the sake of clarity, to avoid mistakes, to support semantics that clearly distinguish between these cases, and to better support specifying interfaces.