Conversions

A conversion converts an expression of one type to another type, possibly producing a new value. In certain cases noted below, the source expression can be a type expression. We refer to these two types as the source and target types. Conversions can be either implicit (Implicit Conversions) or explicit (Explicit Conversions).

Implicit Conversions

An implicit conversion is a conversion that occurs implicitly—that is, without an explicit operation within the program. Implicit conversions fall into the following categories:

• implicit conversions for initialization and assignment (Implicit Conversions for Initialization and Assignment)

• implicit conversions for function calls (Implicit Conversions for Function Calls)

• implicit conversions for conditionals (Implicit Conversions for Conditionals)

If an implicit conversion for a function call is allowed from type T1 to type T2 then implicit conversion for initialization and assignment is allowed.

In addition, an implicit conversion from a type to the same type is allowed for any type. Such a conversion does not change the value of the expression.

Implicit conversion is not transitive. That is, if an implicit conversion is allowed from type T1 to T2 and from T2 to T3, that by itself does not allow an implicit conversion from T1 to T3.

Implicit conversion for function calls, initialization, and assignment are allowed between the following source and target types, as defined in the referenced subsections:

• boolean and numeric types types (Implicit Numeric and Bool Conversions),

• numeric types in the special case when the expression’s value is a compile-time constant (Implicit Compile-Time Constant Conversions),

• class types (Implicit Class Conversions), and

• when the source type is a subtype of the target type (including when the target type is generic) (Implicit Subtype Conversions)

Additionally, implicit conversions for initialization and assignment can be defined for record types, as specified in Implicit Conversions for Initialization and Assignment.

Implicit Numeric and Bool Conversions

Implicit conversions among numeric types are allowed when all values representable in the source type can also be represented in the target type, retaining their full precision. In addition, implicit conversions are permitted from int(s) and uint(s) values to real(t) and complex(2*t), for any widths s and t, even though these cases may result in a loss of precision.

Rationale.

We allow these additional conversions because they provide an important convenience for application programmers who want to mix integral and floating point values in mathematical expressions, and for computing using values using a specific bit-width. For these benefits, the loss of precision seemed like a reasonable tradeoff, particularly given that floating point types are approximate by nature.

Signed integral types int(s) can implicitly convert to uint(s) where s <= t.

Rationale.

We allow these conversions to avoid the situation that something similar to a binary operator produces surprising results when mixing int and uint types. In particular, without this rule, the plus function defined below would surprisingly produce values of a different width or a different kind:

proc plus(a: int(32), b: int(32)) : int(32) { ... }
proc plus(a: int(64), b: int(64)) : int(64) { ... }
proc plus(a: uint(32), b: uint(32)) : uint(32) { ... }
proc plus(a: uint(32), b: uint(32)) : uint(32) { ... }
proc plus(a: real(64), b: real(64)) : real(64) { ... }

var myInt32: int(32);
var myUint32: uint(32);
plus(myInt32, myUint32); // calls 'uint(32)' version, but
                         // without int->uint implicit conversion,
                         // would call the 'int(64)' version
var myInt64: int(64);
var myUint64: uint(64);
plus(myInt64, myUint64); // calls 'uint(64)' version, but
                         // without int->uint implicit conversion,
                         // would call the 'real(64)' version

While implicitly converting an int to a uint can lead to surprising behavior, this behavior is less problematic than the surprising behavior that comes from the above scenario.

A bool can be implicitly converted to any integral type by representing false as 0 and true as 1.

Rationale.

We disallow implicit conversion of a bool to a real, imaginary, or complex type because we expect that such conversions are most likely to be an unintended mistake by the programmer. Marking such cases as errors will draw the programmer’s attention to the issue, and if such a conversion is actually desired, a cast can be used (see Explicit Conversions).

Legal implicit conversions with numeric and boolean types may thus be summarized as follows:

	Destination Type
Source Type	uint(\(t\))	int(\(t\))	real(\(t\))	imag(\(t\))	complex(\(t\))
bool	all \(t\)	all \(t\)
uint(\(s\))	\(s \le t\)	\(s < t\)	all \(s,t\)		all \(s,t\)
int(\(s\))	\(s \le t\)	\(s \le t\)	all \(s,t\)		all \(s,t\)
real(\(s\))			\(s \le t\)		\(s \le t/2\)
imag(\(s\))				\(s \le t\)	\(s \le t/2\)
complex(\(s\))					\(s \le t\)

Implicit Compile-Time Constant Conversions

A param of numeric type can be implicitly converted to another numeric type in some cases if the param value can be represented exactly by the target type. In particular:

• param int(s) and uint(s) values that are exactly representable in the target type can implicit convert to int(t) and uint(t) regardless of the values of s and t.

• param real(s) that is exactly representable in the target type can implicitly convert to real(t) or to complex(t) regardless of the values of s and t.

• param imag(s) that is exactly representable in the target type can implicitly convert to imag(t) or to complex(t) regardless of the values of s and t.

• param complex(s) that is exactly representable in the target type can implicitly convert to complex(t).

As with the implicit numeric conversions, integral param values can implicitly convert:

• to uint of matching or greater size or to real

• or, to complex of any size.

Implicit Class Conversions

An expression of class type can be implicitly converted to:

• to a parent class type,

• to a nilable type, or

• to the borrow type.

Any combination of these three conversions is allowed.

The value nil can be implicitly converted to any nilable class type.

Conversion to a parent class type or to a nilable type is a subtype conversion and is discussed in the next section (Implicit Subtype Conversions).

Class types can be converted to the corresponding borrowed type. For example, owned C can be implicitly converted to borrowed C, and shared C? can be implicitly converted to borrowed C?. This coercion is equivalent to calling the .borrow() method. See Class Lifetime and Borrows. For example:

Example (implicit-conversion-to-borrow.chpl).

class C { }
var c:owned C = new owned C();

proc f(arg: borrowed C) { }
f(c); // equivalent to f(c.borrow())

Implicit Subtype Conversions

An implicit subtype conversion is allowed when the source type is a subtype of the target type.

Given any two types T1 and T2, the type T1 is considered to be a subtype of a type T2 if:

• T2 is a generic type (Generic Types) and T1 is an instantiation that type

• T1 is a class type that inherits from the class T2 (Inheritance)

• T1 is a non-nilable class type (e.g. borrowed C) and T2 is the nilable version of the same class type (e.g. borrowed C?) (Nilable Class Types)

• or a combination of the above.

The below examples use isSubtype to demonstrate when one type is a subtype of another.

Example (not-a-subtype.chpl)

The following code snippet demonstrates that int(8) is not a subtype of int. Note that, even though an int(8) value can be implicitly converted to int, int(8) is not a subtype of int.

isSubtype(int(8), int); // evaluates to false

Example (subtype-int8-integral.chpl)

However, int(8) is a subtype of the generic type integral according to the first rule above (Built-in Generic Types).

isSubtype(int(8), integral); // evaluates to true

Example (subtype-pass-int8-integral.chpl)

Since int(8) is a subtype of integral, the type int(8) can be passed to the type argument type t: integral (Legal Argument Mapping). As a result the following program will compile:

proc f(type t: integral) { }
f(int(8));

Example (subtype-parent-class.chpl)

This example demonstrates that ChildClass is a subtype of ParentClass.

class ParentClass { }
class ChildClass : ParentClass { }

writeln(isSubtype(ChildClass, ParentClass)); // outputs true
writeln(isSubtype(borrowed ChildClass, borrowed ParentClass)); // outputs true

proc f(type t: ParentClass) { }
f(ChildClass); // implicit subtype conversion

proc g(type t: borrowed ParentClass) { }
g(borrowed ChildClass); // implicit subtype conversion

// The implicit subtype conversion can also apply to non-type arguments:
proc h(in arg: owned ParentClass) { }
h(new owned ChildClass()); // implicit subtype conversion

Example (subtype-nilable.chpl).

This example shows that a non-nilable class type is a subtype of a nilable class type with the same management.

class C { }

writeln(isSubtype(C, C?)); // outputs true
writeln(isSubtype(owned C, owned C?)); // outputs true

Example (subtype-three.chpl).

This example demonstrates a combination of all three rules. Note that ParentClass indicates a generic memory management strategy (Class Types).

class ParentClass { }
class ChildClass : ParentClass { }

writeln(isSubtype(ChildClass, ParentClass?)); // outputs true

proc f(type t: ParentClass?) { }
f(ChildClass); // uses implicit subtype conversion

proc g(in arg: ParentClass?) { }
g(new owned ChildClass()); // uses implicit subtype conversion

Implicit Conversions for Initialization and Assignment

An implicit conversion for initialization or assignment occurs at each of the following program locations:

• In an assignment, the expression on the right-hand side of the assignment is converted to the type of the expression on the left-hand side of the assignment.

• In a variable or field declaration that is not a ref variable, the initializing expression is converted to the type of the variable or field. The initializing expression is the right-hand side of the = in the declaration, if present, or in the field initialization statement in an initializer.

• The return or yield expression within a function without a ref or const ref return intent is converted to the return type of that function.

• For a call to a function with a formal argument with out or inout intent. The value of the formal argument is converted to the type of the corresponding actual argument when setting that actual with assignment or initialization (see The Out Intent).

Implicit conversions for initialization or assignment are allowed between numeric and boolean types (Implicit Numeric and Bool Conversions), numeric types in the special case when the expression’s value is a compile-time constant (Implicit Compile-Time Constant Conversions), class types (Implicit Class Conversions), and for generic target types (Implicit Subtype Conversions).

In addition, these implicit conversions can be defined for record types by implementing init= and possibly the = operator between two types as described in Advanced Copy Initialization and Function and Operator Overloading. init= will be called for initialization as described in Split Initialization and the = operator will be invoked for other uses of assignment.

In the event that an = overload is provided to support assignment between two types, the compiler will check that a corresponding init= also exists and emit an error if not. Additionally, if init= is provided to initialize one type from another, the corresponding : overload must also exist. See also Explicit Conversions for more information on the : operator. It is possible to provide : without init= or to provide init= without =.

Example (implementing-assignment.chpl)

Suppose that we have defined a record type to wrap an integer:

record myInteger {
  var intValue: int;
}

We might wish to support assignments setting a myInteger from int. In that event, we can provide the following functions:

operator =(ref lhs: myInteger, rhs: int) {
  lhs.intValue = rhs;
}
proc myInteger.init=(rhs: int) {
  this.intValue = rhs;
}
operator :(from: int, type toType: myInteger) {
  var tmp: myInteger = from; // invoke the init= above
  return tmp;
}

Since we defined operator =, it is necessary to also define init= and operator : between these types.

We can invoke these functions like this:

var a = 1:myInteger;  // cast -- invokes operator :

var b: myInteger = 2; // initialization -- invokes init=

var c: myInteger;
c = 3;                // split-initialization -- invokes init=

var d = new myInteger();
d = 4;                // assignment -- invokes operator =

Implicit Conversions for Function Calls

An implicit conversion for a function call - also called a coercion - occurs when the actual argument of a function call is converted to the type of the corresponding formal argument, if the formal’s intent is param, in, const in, or an abstract intent (Abstract Intents) with the semantics of in or const in.

Implicit conversions for function calls are allowed between numeric and boolean types (Implicit Numeric and Bool Conversions), numeric types in the special case when the expression’s value is a compile-time constant (Implicit Compile-Time Constant Conversions), class types (Implicit Class Conversions), and for generic target types (Implicit Subtype Conversions).

Additionally, an implicit conversion for a function call occurs when the actual type is a subtype of the formal type. This rule applies to in, const in, const ref, and type intent formals and includes generic formal types. See Implicit Subtype Conversions.

Implicit conversions are not applied for actual arguments passed to ref formal arguments.

Open issue.

For the const ref intent, subtype conversions can be allowed while keeping the const ref formal referring to the original actual argument’s value. However, this feature is still under discussion.

Open issue.

Should Chapel allow user-defined implicit conversions for function calls? If so, how would the user define them?

Implicit Conversions for Conditionals

An implicit conversion for a conditional occurs for the condition of:

• a conditional expression,

• a conditional statement,

• a while-do loop, or

• a do-while loop.

In such a condition, the following implicit conversions to bool are supported:

• An expression of integral type is taken to be false if it is 0 and is true otherwise.

• An expression of a class type is taken to be false if it is nil and is true otherwise.

Other standard types also allow implicit conversion for conditionals as indicated in their documentation.

Open issue.

Should Chapel allow user-defined implicit conversions for conditionals? If so, how would the user define them?

Explicit Conversions

Explicit conversions require a cast in the code. Casts are defined in Casts. Explicit conversions are supported between more types than implicit conversions, but not between all types.

The allowed explicit conversions are described in the following sections:

• conversions among primitive numeric and bool types (see Explicit Numeric Conversions)

• tuple to complex (see Explicit Tuple to Complex Conversion)

• enumerated types (see Explicit Enumeration Conversions)

• class conversions (see Explicit Class Conversions)

• range conversions (see Explicit Range Conversions)

• domain conversions (see Explicit Domain Conversions)

• string to bytes conversions (see Explicit String to Bytes Conversions)

• type to string conversions (see Explicit Type to String Conversions)

• user-defined explicit conversions (see User-Defined Casts).

The available explicit conversions are a superset of the available implicit conversions for initialization and assignment (Implicit Conversions for Initialization and Assignment), which, in turn, are a superset of the implicit conversions for function calls. As a result, the implicit conversions described in Implicit Conversions are also available as explicit conversions.

An explicit conversion from a type to the same type is allowed for any type. Such a conversion does not change the value of the expression.

Explicit Numeric Conversions

Explicit conversions are allowed from bool or any numeric type to bytes or string, and vice-versa. When converting to bytes or string the result will hold the string true or false for a bool, or a representation of the expression’s numerical value in other cases. When converting from a string or bytes, the reverse occurs, converting the represented value into a numerical or bool value. If the string/bytes does not represent a legal value of the given type, an IllegalArgumentError is thrown.

When a bool is converted to an int or uint, false converts to the value 0 and true to 1.

When a int, uint, or real is converted to a bool, the result is false if the number was equal to 0 and true otherwise.

When an int is converted to a larger int or uint, its value is sign-extended to fit the new representation. When a uint is converted to a larger int or uint, its value is zero-extended. When an int or uint is converted to an int or uint of the same size, its binary representation is unchanged. When an int or uint is converted to a smaller int or uint, its value is truncated to fit the new representation.

Note

Future:.

There are several kinds of integer conversion which can result in a loss of precision. Currently, the conversions are performed as specified, and no error is reported. In the future, we intend to improve type checking, so the user can be informed of potential precision loss at compile time, and actual precision loss at run time. Such cases include: When an int is converted to a uint and the original value is negative; When a uint is converted to an int and the sign bit of the result is true; When an int is converted to a smaller int or uint and any of the truncated bits differs from the original sign bit; When a uint is converted to a smaller int or uint and any of the truncated bits is true;

Rationale.

For integer conversions, the default behavior of a program should be to produce a run-time error if there is a loss of precision. Thus, cast expressions not only give rise to a value conversion at run time, but amount to an assertion that the required precision is preserved. Explicit conversion procedures would be available in the run-time library so that one can perform explicit conversions that result in a loss of precision but do not generate a run-time diagnostic.

When converting from a real type to a larger real type, the represented value is preserved. When converting from a real type to a smaller real type, the closest representation in the target type is chosen. 1

When converting to a real type from an integer type, integer types smaller than int are first converted to int. Then, the closest representation of the converted value in the target type is chosen. The exact behavior of this conversion is implementation-defined.

When converting from real(k) to complex(2k), the original value is copied into the real part of the result, and the imaginary part of the result is set to zero. When converting from a real(k) to a complex(j) such that j > 2k, the conversion is performed as if the original value is first converted to real(j/2) and then to j.

The rules for converting from imag to complex are the same as for converting from real, except that the imaginary part of the result is set using the input value, and the real part of the result is set to zero.

Explicit Tuple to Complex Conversion

A two-tuple of numerical values may be converted to a complex value. If the destination type is complex(128), each member of the two-tuple must be convertible to real(64). If the destination type is complex(64), each member of the two-tuple must be convertible to real(32). The first member of the tuple becomes the real part of the resulting complex value; the second member of the tuple becomes the imaginary part of the resulting complex value.

Explicit Enumeration Conversions

Explicit conversions are allowed from any enumerated type to bytes or string and vice-versa, including param conversions. For enumerated types that are either concrete or semi-concrete (see Enumerated Types), conversions are supported from the enum to any numeric or boolean type, including param conversions. Explicit conversions are also supported from integer values back to concrete or semi-concrete enumerated types.

When converting from an enum to a bytes or string, the value becomes the name of the enumerator.

When converting from a bytes or string to an enum, the result is the constant whose name matches the source value. If no matching value exists, an IllegalArgumentError is thrown.

For a semi-concrete enumerated type, if a numeric conversion is attempted for a constant with no underlying integer value, it will generate a compile-time error for a param conversion or throw an IllegalArgumentError otherwise.

When converting from an enum to an integer type, the value is first converted to the enum’s underlying integer type and then to the target type, following the rules above for converting between integers.

When converting from an enum to a real, imaginary, or complex type, the value is first converted to the enum’s underlying integer type and then to the target type.

When converting from an enum to a bool, the value is first converted to the enum’s underlying integer type. If the result is zero, the value of the bool is false; otherwise, it is true.

When converting from an integer value to an enum, the value is converted to the enum’s underlying integer type and then converted to the matching symbol. If no symbol has the given integer value, an IllegalArgumentError is thrown.

Explicit Class Conversions

An expression of static class type C can be explicitly converted to a class type D provided that C is derived from D or D is derived from C.

When at run time the source expression refers to an instance of D or it subclass, its value is not changed. Otherwise, the cast fails and the result depends on whether or not the destination type is nilable. If the cast fails and the destination type is not nilable, the cast expression will throw a ClassCastError. If the cast fails and the destination type is nilable, as with D?, then the result will be nil.

An expression of class type can also be converted to a different nilability with a cast. For conversions from a nilable class type to a non-nilable class type, the cast will throw a NilClassError if the value was actually nil.

In some cases a new variant of a class type needs to be computed that has different nilability or memory management strategy. Supposing that T represents a class type, then these casts may compute a new type:

• T:owned - new management is owned, nilability from T

• T:shared - new management shared, nilability from T

• T:borrowed - new management borrowed, nilability from T

• T:unmanaged - new management unmanaged, nilability from T

• T:class - non-nilable type with specific concrete or generic management from T

• T:class? - nilable type with specific concrete or generic management from T

• T:owned class - non-nilable type with owned management

• T:owned class? - nilable type with owned management

• T:shared class - non-nilable type with shared management

• T:shared class? - nilable type with shared management

• T:borrowed class - non-nilable type with borrowed management

• T:borrowed class? - nilable type with borrowed management

• T:unmanaged class - non-nilable type with unmanaged management

• T:unmanaged class? - nilable type with unmanaged management

The conversions in this subsection apply when the source is either an expression or a type expression.

Explicit Range Conversions

An expression of stridable range type can be explicitly converted to an unstridable range type, changing the stride to 1 in the process.

Explicit Domain Conversions

An expression of stridable domain type can be explicitly converted to an unstridable domain type, changing all strides to 1 in the process.

Explicit String to Bytes Conversions

An expression of string type can be explicitly converted to a bytes. However, the reverse is not possible as a bytes can contain arbitrary bytes. Instead, bytes.decode() method should be used to produce a string from a bytes.

Explicit Type to String Conversions

A type expression can be explicitly converted to a string. The resultant string is the name of the type.

Example (explicit-type-to-string.chpl).

For example:

var x: real(64) = 10.0;
writeln(x.type:string);

This program will print out the string "real(64)".

User-Defined Casts

An explicit conversion can be defined by implementing operator : (see also Function and Operator Overloading). An operator : should accept two arguments: the value to convert and the type to convert it to.

Example (implementing-cast.chpl)

Suppose that we have defined a record type to wrap an integer:

record myInteger {
  var intValue: int;
}

We might wish to support casts from myInteger to int. In that event, we can provide this cast operator:

operator :(from: myInteger, type toType: int) {
  return from.intValue;
}

and we can invoke it using the cast syntax like this:

var x = new myInteger(1);
var y = x:int;

1

When converting to a smaller real type, a loss of precision is expected. Therefore, there is no reason to produce a run-time diagnostic.