Procedures¶

A function is a code abstraction that can be invoked by a call expression. Throughout this specification the term “function” is used in this programming-languages sense, rather than in the mathematical sense. A function has zero or more formal arguments, or simply formals. Upon a function call each formal is associated with the corresponding actual argument, or simply actual. Actual arguments are provided as part of the call expression, or at the call site. Direct and indirect recursion is supported.

A function can be a procedure, which completes and returns to the call site exactly once, returning no result, a single result, or multiple results aggregated in a tuple. A function can also be an iterator, which can generate, or yield, multiple results (in sequence and/or in parallel). A function (either a procedure or an iterator) can be a method if it is bound to a type (often a class). An operator in this chapter is a procedure with a special name, which can be invoked using infix notation, i.e., via a unary or binary expression. This chapter defines procedures, but most of its contents apply to iterators and methods as well.

Functions are presented as follows:

procedures (this chapter)
operators Procedure Definitions, Operator Expressions
iterators Iterators
methods (when bound to a class) Class Methods
function calls Function Calls
various aspects of defining a procedure Procedure Definitions–Nested Functions
calling external functions from Chapel Calling External Functions
calling Chapel functions from external functions Calling Chapel Functions
determining the function to invoke for a given call site: function and operator overloading Function and Operator Overloading, function resolution Function Resolution

Function Calls¶

The syntax to call a non-method function is given by:

call-expression:
  lvalue-expression ( named-expression-list )
  lvalue-expression [ named-expression-list ]
  parenthesesless-function-identifier

named-expression-list:
  named-expression
  named-expression , named-expression-list

named-expression:
  expression
  identifier = expression

parenthesesless-function-identifier:
  identifier

A call-expression is resolved to a particular function according to the algorithm for function resolution described in Function Resolution.

Functions can be called using either parentheses or brackets.

Rationale.

This provides an opportunity to blur the distinction between an array access and a function call and thereby exploit a possible space/time tradeoff.

Functions that are defined without parentheses must be called without parentheses. Functions without parentheses are discussed in Functions without Parentheses.

A named-expression is an expression that may be optionally named. It provides an actual argument to the function being called. The optional identifier refers to a named formal argument described in Named Arguments.

Calls to methods are defined in Section Class Method Calls.

Procedure Definitions¶

Procedures are defined with the following syntax:

procedure-declaration-statement:
  privacy-specifier[OPT] procedure-kind[OPT] 'proc' identifier argument-list[OPT] return-intent[OPT] return-type[OPT] where-clause[OPT] function-body
  privacy-specifier[OPT] procedure-kind[OPT] 'operator' operator-name argument-list return-intent[OPT] return-type[OPT] where-clause[OPT] function-body

procedure-kind:
  'inline'
  'export'
  'extern'
  'override'

operator-name: one of
  'align' 'by'
  + - * / % ** : ! == != <= >= < > << >> & | ^ ~
  = += -= *= /= %= **= &= |= ^= <<= >>= <=> #

argument-list:
  ( formals[OPT] )

formals:
  formal
  formal , formals

formal:
  formal-intent[OPT] identifier formal-type[OPT] default-expression[OPT]
  formal-intent[OPT] identifier formal-type[OPT] variable-argument-expression
  formal-intent[OPT] tuple-grouped-identifier-list formal-type[OPT] default-expression[OPT]
  formal-intent[OPT] tuple-grouped-identifier-list formal-type[OPT] variable-argument-expression

formal-type:
  : type-expression

default-expression:
  = expression

variable-argument-expression:
  ... expression
  ...

formal-intent:
  'const'
  'const in'
  'const ref'
  'in'
  'out'
  'inout'
  'ref'
  'param'
  'type'

return-intent:
  'const ref'
  'ref'
  'param'
  'type'

return-type:
  : type-expression

where-clause:
  'where' expression

function-body:
  'do' statement
  block-statement

Functions do not require parentheses if they have no arguments. Such functions are described in Functions without Parentheses.

Formal arguments can be grouped together using a tuple notation as described in Splitting a Tuple into Multiple Formal Arguments in a Function Call.

Default expressions allow for the omission of actual arguments at the call site, resulting in the implicit passing of a default value. Default values are discussed in Default Values.

The intents const, const in, const ref, in, out, inout and ref are discussed in Argument Intents. The intents param and type make a function generic and are discussed in Generic Functions. If the formal argument’s type is omitted, generic, or prefixed with a question mark, the function is also generic and is discussed in Generic Functions.

Functions can take a variable number of arguments. Such functions are discussed in Variable Number of Arguments.

The return-intent can be used to indicate how the value is returned from a function. return-intent is described further in Return Intents.

Open issue.

Parameter and type procedures are supported. Parameter and type iterators are currently not supported.

The return-type is optional and is discussed in Return Types. A type function may not specify a return type.

The where-clause is optional and is discussed in Where Clauses.

The function-body defines the function’s behavior and is defined in Function Bodies. Function bodies may contain return statements (see The Return Statement).

Function and operator overloading is supported in Chapel and is discussed in Function and Operator Overloading. Operator overloading is supported on the operators listed above (see operator-name).

The optional privacy-specifier keywords indicate the visibility of module level procedures to outside modules. By default, procedures are publicly visible. More details on visibility can be found in Visibility Of A Module’s Symbols.

The linkage specifier inline indicates that the function body must be inlined at every call site.

Rationale.

A Chapel compiler is permitted to inline any function if it determines there is likely to be a performance benefit to do so. Hence an error must be reported if the compiler is unable to inline a procedure with this specifier. One example of a preventable inlining error is to define a sequence of inlined calls that includes a cycle back to an inlined procedure.

See the chapter on interoperability (Interoperability) for details on exported and imported functions.

Functions without Parentheses¶

Functions do not require parentheses if they have empty argument lists. Functions declared without parentheses around empty argument lists must be called without parentheses.

Example (function-no-parens.chpl).

Given the definitions
proc foo { writeln("In foo"); }
proc bar() { writeln("In bar"); }
the procedure foo can be called by writing foo and the procedure bar can be called by writing bar(). It is an error to use parentheses when calling foo or omit them when calling bar.

Formal Arguments¶

A formal argument’s intent (Argument Intents) specifies how the actual argument is passed to the function. If no intent is specified, the default intent (The Default Intent) is applied, resulting in type-dependent behavior.

Named Arguments¶

A formal argument can be named at the call site to explicitly map an actual argument to a formal argument.

Example (named-args.chpl).

Running the code
proc foo(x: int, y: int) { writeln(x); writeln(y); }

foo(x=2, y=3);
foo(y=3, x=2);
will produce the output
2
3
2
3
named argument passing is used to map the actual arguments to the formal arguments. The two function calls are equivalent.

Named arguments are sometimes necessary to disambiguate calls or ignore arguments with default values. For a function that has many arguments, it is sometimes good practice to name the arguments at the call site for compiler-checked documentation.

Default Values¶

Default values can be specified for a formal argument by appending the assignment operator and a default expression to the declaration of the formal argument. If the actual argument is omitted from the function call, the default expression is evaluated when the function call is made and the evaluated result is passed to the formal argument as if it were passed from the call site. While the default expression is evaluated at the time of the function call, it is resolved in the scope of the definition of the called function, immediately before the called function is resolved. As a result, a default value expression can refer to previous formal arguments.

When a default value is provided for a formal argument without a type, the argument type will be inferred to match the type of the default value. This inference is similar to the type inference for variable declarations (see Local Type Inference). However, there is one difference: when the call provides a corresponding actual argument, and the actual argument is of a type that includes a runtime component (see Types with Runtime Components), the runtime component of the formal argument’s type will come from the actual argument, rather than from the default value expression.

Example (default-values.chpl).

The code
proc foo(x: int = 5, y: int = 7) { writeln(x); writeln(y); }

foo();
foo(7);
foo(y=5);
writes out
5
7
7
7
5
5
Default values are specified for the formal arguments x and y. The three calls to foo are equivalent to the following three calls where the actual arguments are explicit: foo(5, 7), foo(7, 7), and foo(5, 5). The example foo(y=5) shows how to use a named argument for y in order to use the default value for x in the case when x appears earlier than y in the formal argument list.

Example (default-array-runtime-type.chpl).

This example shows that the runtime type of the default expression does not impact the runtime type of the formal argument in the case that an actual argument was provided.
var D = {1..4};
proc createArrayOverD() {
  var A:[D] int;
  return A;
}

proc bar(arg = createArrayOverD()) {
  writeln(arg.domain);
}

bar(); // arg uses the default, so outputs {1..4}

var B:[0..2] int;
bar(B); // arg refers to B and so has the runtime type from B
        // so outputs {0..2}

Argument Intents¶

Argument intents specify how an actual argument is passed to a function where it is represented by the corresponding formal argument.

Argument intents are categorized as being either concrete or abstract. Concrete intents are those in which the semantics of the intent keyword are independent of the argument’s type. Abstract intents are those in which the keyword (or lack thereof) expresses a general intention that will ultimately be implemented via one of the concrete intents. The specific choice of concrete intent depends on the argument’s type and may be implementation-defined. Abstract intents are provided to support productivity and code reuse.

Concrete Intents¶

The concrete intents are in, out, inout, ref, const in, and const ref.

The In Intent¶

When in is specified as the intent, the formal argument represents a variable that is copy-initialized from the value of the actual argument, see Copy and Move Initialization.

For example, for integer arguments, the formal argument will store a copy of the actual argument.

An implicit conversion for a function call occurs from the actual argument to the type of the formal.

The formal can be modified within the function, but such changes are local to the function and not reflected back to the call site.

The Out Intent¶

The out intent on a formal argument supports return-like behavior. As such, the type of an out formal is not considered when determining candidate functions or choosing the best candidate (see Function Resolution).

When a function with the out intent returns, the actual argument is set to the formal argument using assignment or possibly initialized from the formal argument according to Split Initialization.

Within the function body, an out formal argument is initialized according Split Initialization. It will start with its default value if one is supplied and can use the default value for the declared type if no initialization point is found. The formal argument can be modified within the function.

Note that the way that type inference works with generic out formal arguments is very different from other formal arguments. In particular, the type of a generic out formal argument is inferred from the function body rather than from the call site.

Note

If the type of an out argument needs to be inferred based upon the call site, there are currently two approaches available:

use a separate type argument to pass the type

use the ref intent instead of the out intent

There is proposal that including a type query (e.g. ?t in an out argument will cause the type to be inferred based upon the call site. However this is not yet implemented, at the time of this writing.

The Inout Intent¶

When inout is specified as the intent, the actual argument is copy-initialized into the formal argument, the called function body is run, and then the actual argument is set to the formal argument with assignment. As a result the behavior of the inout intent is a combination of the in and out intents.

inout intent formals behave the same as in formals for the purposes of determining candidate functions and choosing the best candidate (see Function Resolution).

The actual argument must be a valid lvalue. The formal argument can be modified within the function.

The Ref Intent¶

When ref is specified as the intent, the actual argument is passed by reference. Any reads of, or modifications to, the formal argument are performed directly on the corresponding actual argument at the call site. The actual argument must be a valid lvalue. The type of the actual argument must be the same as the type of the formal.

The ref intent differs from the inout intent in that the inout intent requires copying from/to the actual argument on the way in/out of the function, while ref allows direct access to the actual argument through the formal argument without copies. Note that concurrent modifications to the ref actual argument by other tasks may be visible within the function, subject to the memory consistency model.

The Const In Intent¶

The const in intent is identical to the in intent, except that modifications to the formal argument are prohibited within the function.

The Const Ref Intent¶

The const ref intent is identical to the ref intent, except that modifications to the formal argument are prohibited within the dynamic scope of the function. Note that the same or concurrent tasks may modify the actual argument while the function is executing and that these modifications may be visible to reads of the formal argument within the function’s dynamic scope (subject to the memory consistency model).

Summary of Concrete Intents¶

The following table summarizes the differences between the concrete intents:

	`in`	`out`	`inout`	`ref`	`const in`	`const ref`
initializes formal from actual?	yes	no	yes	no	yes	no
sets actual from formal?	no	yes	yes	no	no	no
refers to actual argument?	no	no	no	yes	no	yes
formal can be read?	yes	yes	yes	yes	yes	yes
formal can be modified?	yes	yes	yes	yes	no	no
local changes affect the actual?	no	on return	on return	immediately	N/A	N/A

Abstract Intents¶

The abstract intents are const and the default intent (when no intent is specified).

The Const Intent¶

The const intent specifies that the function will not and cannot modify the formal argument within its dynamic scope. Whether const is interpreted as const in or const ref intent depends on the argument type. Generally, small values, such as scalar types, will be passed by const in; while larger values, such as domains and arrays, will be passed by const ref intent. The Abstract Intents Table below lists the meaning of the const intent for each type.

The Default Intent¶

When no intent is specified for a formal argument, the default intent is applied. It is designed to take the most natural/least surprising action for the argument, based on its type. In practice, this is const for most types (as defined by The Const Intent) to avoid surprises for programmers coming from languages where everything is passed by in or ref intent by default. Exceptions are made for types where modification is considered part of their nature, such as types used for synchronization (like atomic) and arrays.

Default argument passing for tuples applies the default argument passing strategy to each tuple component as if it was passed as a separate argument. See Tuple Argument Intents.

The Abstract Intents Table that follows defines the default intent for each type.

Abstract Intents Table¶

The following table summarizes what these abstract intents mean for each type:

Type	`const` intent meaning	Default intent meaning	Notes
scalar types (`bool`, `int`, `uint`, `real`, `imag`, `complex`)	`const in`	`const in`
string-like types (`string`, `bytes`)	`const ref`	`const ref`
ranges	`const in`	`const in`
domains / domain maps	`const ref`	`const ref`
arrays	`const ref`	`ref` / `const ref`	see Default Intent for Arrays and Record ’this’
records	`const ref`	`const ref`	see Default Intent for Arrays and Record ’this’
auto-managed classes (`owned`, `shared`)	`const ref`	`const ref`	see Default Intent for ’owned’ and ’shared’
non-managed classes (`borrowed`, `unmanaged`)	`const in`	`const in`
tuples	per-element	per-element	see Tuple Argument Intents
unions	`const ref`	`const ref`
synchronization types (`atomic`, `sync`, `single`)	`const ref`	`ref`

Default Intent for Arrays and Record ’this’¶

The default intent for arrays and for a this argument of record type (see The Method Receiver and the this Argument) is ref or const ref. It is ref if the formal argument is modified inside the function, otherwise it is const ref. Note that neither of these cause an array or record to be copied by default. The choice between ref and const ref is similar to and interacts with return intent overloads (see Return Intent Overloads).

Default Intent for ’owned’ and ’shared’¶

The default intent for owned and shared arguments is const ref. Arguments can use the in or const in intents to transfer or share ownership if those arguments apply to owned or shared types.

Example (owned-any-intent.chpl).

proc defaultGeneric(arg) {
  writeln(arg.type:string);
}
class SomeClass { }
var own = new owned SomeClass();
defaultGeneric(own);
writeln(own != nil);

Variable Number of Arguments¶

Functions can be defined to take a variable number of arguments where those arguments can have any intent or can be types. A variable number of parameters is not supported. This allows the call site to pass a different number of actual arguments. There must be at least one actual argument.

If the variable argument expression contains an identifier prepended by a question mark, the number of actual arguments can vary, and the identifier will be bound to an integer parameter value indicating the number of arguments at a given call site. If the variable argument expression contains an expression without a question mark, that expression must evaluate to an integer parameter value requiring the call site to pass that number of arguments to the function.

Within the function, the formal argument that is marked with a variable argument expression is a tuple of the actual arguments. If the actual arguments all have the same type, the formal will be a homogeneous tuple, otherwise it will be a heterogeneous tuple.

Example (varargs.chpl).

The code
proc mywriteln(xs ...?k) {
  for x in xs do
    writeln(x);
}
defines a generic procedure called mywriteln that takes a variable number of arguments of any type and then writes them out on separate lines. The type of xs can also be constrained in the formal argument list to require that the actuals all have the same type. For example xs: string...?k would accept a variable number of string arguments.

Example (varargs-with-type.chpl).

Either or both the number of variable arguments and their types can be specified. For example, a basic procedure to sum the values of three integers can be written as
proc sum(x: int...3) do return x(0) + x(1) + x(2);
Specifying the type is useful if it is important that each argument have the same type. Specifying the number is useful in, for example, defining a method on a class that is instantiated over a rank parameter.

Example (varargs-returns-tuples.chpl).

The code
proc tuple(x ...) do return x;
defines a generic procedure that is equivalent to building a tuple. Therefore the expressions tuple(1, 2) and (1,2) are equivalent, as are the expressions tuple(1) and (1,).

Return Intents¶

The return-intent determines how the value is returned from a function and in what contexts that function is allowed to be used. The rules for returning tuples are specified in Tuple Return Behavior.

The Default Return Intent¶

When no return-intent is specified explicitly, the function returns a value that cannot be used as an lvalue. This value is obtained by copy-initialization from the returned expression, see Copy and Move Initialization.

The Ref Return Intent¶

When using a ref return intent, the function call is an lvalue (specifically, a call expression for a procedure and an iterator variable for an iterator).

The ref return intent is specified by following the argument list with the ref keyword. The function must return an lvalue that exists outside of the function’s scope.

Example (ref-return-intent.chpl).

The following code defines a procedure that can be interpreted as a simple two-element array where the elements are actually module level variables:
var x, y = 0;

proc A(i: int) ref {
  if i < 0 || i > 1 then
    halt("array access out of bounds");
  if i == 0 then
    return x;
  else
    return y;
}
Calls to this procedure can be assigned to in order to write to the “elements” of the array as in
A(0) = 1;
A(1) = 2;
It can be called as an expression to access the “elements” as in
writeln(A(0) + A(1));
This code outputs the number 3.

The Const Ref Return Intent¶

The const ref return intent is also available. It is a restricted form of the ref return intent. Calls to functions marked with the const ref return intent are not lvalue expressions.

Return Intent Overloads¶

In some situations, it is useful to choose the function called based upon how the returned value is used. In particular, suppose that there are two functions that have the same formal arguments and differ only in their return intent. One might expect such a situation to result in an error indicating that it is ambiguous which function is called. However, the Chapel language includes a special rule for determining which function to call when the candidate functions are otherwise ambiguous except for their return intent. This rule enables data structures such as sparse arrays.

See Choosing Return Intent Overloads Based on Calling Context for a detailed description of how return intent overloads are chosen based upon calling context.

Example (ref-return-intent-pair.chpl).

Return intent overload can be used to ensure, for example, that the second element in the pseudo-array is only assigned a value if the first argument is positive. The following is an example:

var x, y = 0;

proc doA(param setter, i: int) ref {
  if i < 0 || i > 1 then
    halt("array access out of bounds");

  if setter && i == 1 && x <= 0 then
    halt("cannot assign value to A(1) if A(0) <= 0");

  if i == 0 then
    return x;
  else
    return y;
}
proc A(i: int) ref {
  return doA(true, i);
}
proc A(i: int) {
  return doA(false, i);
}

A(0) = 0;
A(1) = 1;

The Param Return Intent¶

A parameter function, or a param function, is a function that returns a parameter expression. It is specified by following the function’s argument list by the keyword param. It is often, but not necessarily, generic.

It is a compile-time error if a parameter function does not return a parameter expression. The result of a parameter function is computed during compilation and substituted for the call expression.

Example (param-functions.chpl).

In the code
proc sumOfSquares(param a: int, param b: int) param do
  return a**2 + b**2;

var x: sumOfSquares(2, 3)*int;
sumOfSquares is a parameter procedure that takes two parameters as arguments. Calls to this procedure can be used in places where a parameter expression is required. In this example, the call is used in the declaration of a homogeneous tuple and so is required to be a parameter.

Parameter functions may not contain control flow that is not resolved at compile-time. This includes loops other than the parameter for loop Parameter For Loops and conditionals with a conditional expressions that is not a parameter.

The Type Return Intent¶

A type function is a function that returns a type, not a value. It is specified by following the function’s argument list by the keyword type, without the subsequent return type. It is often, but not necessarily, generic.

It is a compile-time error if a type function does not return a type. The result of a type function is computed during compilation.

As with parameter functions, type functions may not contain control flow that is not resolved at compile-time. This includes loops other than the parameter for loop Parameter For Loops and conditionals with a conditional expression that is not a parameter.

Example (type-functions.chpl).

In the code
proc myType(x) type {
  if numBits(x.type) <= 32 then return int(32);
  else return int(64);
}
myType is a type procedure that takes a single argument x and returns int(32) if the number of bits used to represent x is less than or equal to 32, otherwise it returns int(64). numBits is a param procedure defined in the standard Types module.

Function Bodies¶

The body of a procedure or iterator is made up of one or more statements that are executed when a call to the function is made. Function bodies can always be specified using a compound or _block_ statement (Blocks), set off by curly brackets. When a function’s body is just a single statement, the do keyword can be used as a shorthand for defining the body instead, similar to other forms of control flow.

The Return Statement¶

The return statement can only appear in a function body. It causes control to exit that function, returning it to the point at which that function was called.

A procedure can return a value by executing a return statement that includes an expression. If it does, that expression’s value becomes the value of the invoking call expression.

A return statement in a procedure of a void return type (Return Types) or in an iterator must not include an expression. A return statement in a procedure of a non-void return type must include an expression. For procedures with ref or const ref return intent, the expression must have storage associated with it that will outlive the procedure itself.

The statements following a return statement in the same block are ignored by the compiler because they cannot be executed.

The syntax of the return statement is given by

return-statement:
  'return' expression[OPT] ;

Example (return.chpl).

The following code defines a procedure that returns the sum of three integers:
proc sum(i1: int, i2: int, i3: int) do
  return i1 + i2 + i3;

Return Types¶

Every procedure has a return type. The return type is either specified explicitly via return-type in the procedure declaration, or is inferred implicitly.

Explicit Return Types¶

If a return type is specified and is not void, each return statement of the procedure must include an expression. For a non-ref return intent, an implicit conversion occurs from each return expression to the specified return type. For a ref return intent (The Ref Return Intent), the return type must match the type returned in all of the return statements exactly, when checked after generic instantiation and parameter folding (if applicable).

Implicit Return Types¶

If a return type is not specified, it is inferred from the return statements. It is illegal for a procedure to have a return statement with an expression and a return statement without an expression. For procedures without any return statements, or when none of the return statements include an expression, the return type is void.

Otherwise, the types of the expressions in all of the procedure’s return statements are considered. If a function has a ref return intent (The Ref Return Intent), they all must be the same exact type, which becomes the inferred return type. Otherwise, there must exist exactly one type such that an implicit conversion is allowed between every other type and that type, and that type becomes the inferred return type. If the above requirements are not satisfied, it is an error.

Where Clauses¶

The list of function candidates can be constrained by where clauses. A where clause is specified in the definition of a function (Procedure Definitions). The expression in the where clause must be a boolean parameter expression that evaluates to either true or false. If it evaluates to false, the function is rejected and thus is not a possible candidate for function resolution.

Example (whereClause.chpl).

Given two overloaded function definitions
proc foo(x) where x.type == int { writeln("int"); }
proc foo(x) where x.type == real { writeln("real"); }
the call foo(3) resolves to the first definition because the where clause on the second function evaluates to false.

Nested Functions¶

A function defined in another function is called a nested function. Nesting of functions may be done to arbitrary degrees, i.e., a function can be nested in a nested function.

Nested functions are only visible to function calls within the lexical scope in which they are defined.

Nested functions may refer to variables defined in the function(s) in which they are nested.

Function and Operator Overloading¶

Functions that have the same name but different argument lists are called overloaded functions. Function calls to overloaded functions are resolved according to the function resolution algorithm in Function Resolution.

To define an overloaded operator, use the operator keyword to define a function with the same name as the operator. The operators that may be overloaded are listed in the following table:

arity	operators
unary	`+` `-` `!` `~`
binary	`+` `-` `` `/` `%` `*` `:`
binary	`==` `<=` `>=` `<` `>`
binary	`<<` `>>` `&` `\|` `^` `#` `align` `by`
binary	`=` `+=` `-=` `=` `/=` `%=` `*=`
binary	`&=` `\|=` `^=` `<<=` `>>=` `<=>`

The arity and precedence of the operator must be maintained when it is overloaded. Operator resolution follows the same algorithm as function resolution.

Assignment overloads are not supported for class types.

Function Resolution¶

Function resolution is the algorithm that determines which target function to invoke for a given call expression. Function resolution is defined as follows.

Identify the set of visible functions for the function call. A visible function is any function that satisfies the criteria in Determining Visible Functions. If no visible function can be found, the compiler will issue an error stating that the call cannot be resolved.
From the set of visible functions for the function call, determine the set of candidate functions for the function call. A candidate function is any function that satisfies the criteria in Determining Candidate Functions. If no candidate function can be found and the call is within a generic function, its point of instantiation(s) are visited searching for candidates as defined in Function Visibility in Generic Functions. If still no candidate functions are found, the compiler will issue an error stating that the call cannot be resolved. If exactly one candidate function is found, this is determined to be the target function.
From the set of candidate functions, determine the set of most specific functions as described in Determining Most Specific Functions. In most cases, if the set of most specific functions contains more than one element, it will result in an ambiguity error. However, there can be several if they differ only in return intent.
From the set of most specific functions, the compiler determines a best function for each return intent as described in Determining Best Functions. If there is more than one best function for a given return intent, the compiler will issue an error stating that the call is ambiguous. Otherwise, it will choose the target function from these best functions based on the calling context as described in Choosing Return Intent Overloads Based on Calling Context.

Notation¶

This section uses the following notation:

\(X\) is a function under consideration
An actual argument under consideration is \(A_i\) of type \(T(A_i)\)
The formal argument in function \(X\) that will receive \(A_i\) is \(X_i\) and it has type \(T(X_i)\). When \(X\) is a generic function, \(X_i\) refers to the possibly generic argument and \(T(X_i)\) refers to the instantiated type.
When needed in the exposition, \(Y\) is another function under consideration, where \(A_i\) is passed to the formal \(Y_i\) of type \(T(Y_i)\).

Determining Visible Functions¶

Given a function call, a function \(X\) is determined to be a visible function if its name is the same as the name of the function call and one of the following conditions is met:

\(X\) is defined in the same scope as the function call or in a lexical outer scope of the function call, or
\(X\) is public and is declared in a module that is used from the same scope as the function call or from its lexical outer scope, see also Using Modules, or
\(X\) is public and is declared in a module that is imported from the same scope as the function call or from its lexical outer scope, and the call qualifies the function name with the module name, see also Importing Modules.
\(X\) is a method and it is defined in the same module that defines the receiver type.

Determining Candidate Functions¶

Given a function call, a function is determined to be a candidate function if there is a valid mapping from the function call to the function where each actual argument is mapped to a formal argument with a legal argument mapping.

Valid Mapping¶

The following algorithm determines a valid mapping from a function call to a function if one exists:

Each actual argument that is passed by name is matched to the formal argument with that name. If there is no formal argument with that name, there is no valid mapping.
The remaining actual arguments are mapped in order to the remaining formal arguments in order. If there are more actual arguments then formal arguments, there is no valid mapping. If any formal argument that is not mapped to by an actual argument does not have a default value, there is no valid mapping.
The valid mapping is the mapping of actual arguments to formal arguments plus default values to formal arguments that are not mapped to by actual arguments.

Legal Argument Mapping¶

An actual argument \(A_i\) of type \(T(A_i)\) can be legally mapped to a formal argument \(X_i\) according to the following rules.

First, if \(A_i\) is a type but \(X_i\) does not use the type intent, then it is not a legal argument mapping.

Then, if \(X_i\) is a generic argument:

if \(X_i\) uses param intent, then \(A_i\) must also be a param

if \(X_i\) uses type intent, then \(A_i\) must also be a type

there must exist an instantiation \(T(X_i)\) of the generic declared type of \(X_i\), if any, that is compatible with the type \(T(A_i)\) according to the rules below.

Next, the type \(T(X_i)\) - which is either the declared type of the formal argument \(X_i\) if it is concrete or the instantiated type if \(X_i\) is generic - must be compatible with the type \(T(A_i)\) according to the concrete intent of \(X_i\):

if \(X_i\) uses ref intent, then \(T(A_i)\) must be the same type as \(T(X_i)\)

if \(X_i\) uses const ref intent, then \(T(A_i)\) and \(T(X_i)\) must be the same type or a subtype of \(T(X_i)\) (see Implicit Subtype Conversions)

if \(X_i\) uses in or inout intent, then \(T(A_i)\) must be the same type, a subtype of, or implicitly convertible to \(T(X_i)\).

if \(X_i\) uses the out intent, it is always a legal argument mapping regardless of the type of the actual and formal. In the event that setting \(T(A_i)\) from \(X_i\) is not possible then a compilation error will be emitted if this function is chosen as the best candidate.

if \(X_i\) uses the type intent, then \(T(A_i)\) must be the same type or a subtype of \(T(X_i)\) (see Implicit Subtype Conversions).

Finally, if the above compatibility cannot be established, the mapping is checked for promotion. If \(T(A_i)\) is scalar promotable to \(T(X_i)\) (see Promotion), then the above rules are checked with the element type, index type, or yielded type. For example, if \(T(A_i)\) is an array of int and \(T(X_i)\) is int, then promotion occurs and the above rules will be checked with \(T(A_i)\) == int.

Determining Most Specific Functions¶

Given a set of candidate functions, the following steps are applied to remove candidates from the set. After the process completes, the remaining candidates in the set are the most specific functions.

If any candidate is more visible (or shadows) another candidate, discard all candidates that are less visible than (or shadowed by) another candidate.
If at least one candidate requires promotion and at least one candidate does not use promotion, discard all candidates that use promotion.
Discard any function that has a less specific argument mapping than any other function. See More Specific Argument Mappings below for details on the more specific argument mapping relation.
Discard any candidates that have more formals that require implicit conversion than other candidates. For this step, implicit conversions between real(w), imag(w), and complex(2*w) are not considered.
Discard any candidates that have more formals that require a negative param value is converted to an unsigned integral type of any width (i.e. a uint(?w)).
Discard any candidates that have more formals that require param narrowing conversions than another candidate. A param narrowing conversion is when a param value is implicitly converted to a type that would not normally be allowed. For example, an int value cannot normally implicitly convert to an int(8) value. However, the param value 1, which is an int, can implicitly convert to int(8) because the value is known to fit. See also Implicit Compile-Time Constant Conversions.

Note that where clauses are also considered but that happens in a later step. See Determining Best Functions.

More Specific Argument Mappings¶

Given candidate functions \(X\) and \(Y\) with formal arguments \(X_1\) \(X_2\) … and \(Y_1\) \(Y_2\) … that correspond to actual arguments \(A_1\) \(A_2\) …, which candidate function is more specific is determined in two steps. First, the non-param actual arguments and their corresponding formal arguments are considered. Then, any param actual arguments and their corresponding formal arguments are considered.

Within each of those steps, the candidate function \(X\) has a more specific argument mapping if:

for each argument \(i\) considered, the argument mapping from \(A_i\) to \(Y_i\) is not better than the argument mapping for the argument \(A_i\) to \(X_i\), and
for at least one argument \(j\) considered, the argument mapping from \(A_j\) to \(X_j\) is better than the argument mapping from \(A_j\) to \(Y_j\).

Better Argument Mapping¶

Given an actual argument \(A_i\) and the corresponding formal arguments \(X_i\) and \(Y_i\) (in the functions \(X\) and \(Y\) being considered), the following rules are applied in order to determine whether \(X_i\) or \(Y_i\) is a better argument mapping:

If one of the formals requires promotion and the other does not, the formal not requiring promotion is better
If both of the formals have the same type after instantiation and one of the formals is less generic than the other formal, the less-generic formal is better
If one of the formals is param and the other is not, the param formal is better
If one of the formals requires a param narrowing conversion and the other is not, the one not requiring such narrowing is better
If the actual and both formals are numeric types and one formals is a preferred numeric conversion target, that formal is better
If one of the formals matches the actual type exactly and the other does not, the matching formal is better
If the actual’s scalar type for promotion matches one of the formals but not the other, the matching formal is better
If an implicit conversion is possible from the type of one formal to the other, but not vice versa, then the formal that can be converted from is better. (I.e. if the type of \(X_i\) can implicitly convert to the type of \(Y_i\), then \(X_i\) is better). Similarly, if the type of one formal can be instantiated to produce the type of another formal, the type of the more-instantiated formal is better.

Preferred Numeric Conversion Target¶

To compute if a formal is a preferred numeric conversion target, apply the following rules in order:

Classify the actual and formals by numeric kind. If one formal has the same kind as the actual but the other does not, the formal with the same kind is better. Each of the following bullets represents a different numeric kind for this rule:
- bool
- int(?w) or uint(?w), that is, a signed or unsigned integral type of any width
- real(?w)
- imag(?w)
- complex(?w)
- all other types
Classify the actual and formals by numeric width. If one formal has the same numeric width as the actual but the other does not, the formal with the same width is better. Each of the following bullets represents a different numeric width for this rule:
- All numeric types that match the default width. This includes bool, int, uint, real, imag, and complex as well as their more specific names int(64), uint(64), real(64), imag(64), complex(128)
- All numeric types with 32-bit width: int(32), uint(32), real(32), imag(32), complex(64). complex(64) is in this category because the real element width is 32 bits.
- All numeric types with 16-bit width: int(16), uint(16)
- All numeric types with 8-bit width: int(8), uint(8)

Determining Best Functions¶

Given the set of most specific functions for a given return intent, only the following function(s) are selected as best functions:

all functions, if none of them contain a where clause;
only those functions that have a where clause, otherwise.