function

#function

##Syntax A functionDeclaration is:

function id [ ( [paramDeclaration {, paramDeclaration } ] ) ] : typeSpec statementsAndDeclarations end id

##Description A function declaration creates (but does not run) a new function. The name of the function (id) is given in two places, just after function and just after end.

##Example

    function doubleIt ( x : real ) : real
        result 2.0 * x
    end doubleIt
    
    put doubleIt ( 5.3 )        % This outputs 10.6

##Details The set of parameters declared with the function are called formal parameters. For example, in the doubleIt function, x is a formal parameter. A function is called (invoked) by a function call which consists of the function's name followed by the parenthesized list of actual parameters (if any). For example, doubleIt (5.3) is a call having 5.3 as an actual parameter. If there are no parameters and no parentheses, the call does not have parentheses. The keyword function can be abbreviated to fcn. See also functionCall and procedureDeclaration.

Each actual non-var parameter must be assignable to the type of its corresponding formal parameter. See also assignability.

A function must finish by executing a result statement, which produces the function's value. In the above example, the result statement computes and returns the value 2.0 * x.

In principle, a function (1) should not change any variables outside of itself (global variables) or (2) should not have var parameters. In other words, it should have no side effects. The original implementation prevented (1) and (2) and thereby prevented function side effects. Current implementations of Turing do not enforce this restriction.

The upper bounds of arrays and strings that are parameters may be declared to be an asterisk (*), meaning the bound is that of the actual parameter. See paramDeclaration for details about parameters.

Procedures and functions cannot be declared inside other procedures and functions.

The syntax of a functionDeclaration presented above has been simplified by leaving out the optional result identifier, import list, pre condition, init clause, post condition and exception handler. The full syntax is

The resultId is the name of the result of the function and can be used only in the post condition.

A function must be declared before being called; to allow for mutually recursive procedures and functions, there are forward declarations with later declaration of the procedure or function body. See forward and body declarations for explanations.

You declare parameterless functions using an empty parameter list. When this is done, a call to the function must include an empty parameter list.

    function [ pervasive ] id
        [ ( [ paramDeclaration {,paramDeclaration } ] ) ] 
            [ resultId ] : typeSpec
        [ pre trueFalseExpn ]
        [ init id := expn {, id := expn } ]
        [ post trueFalseExpn ]
        [ exceptionHandler ]
        statementsAndDeclarations
    end id

##See also import.html list, pre.html condition.html, init.html clause, post.html condition.html and exceptionHandler for explanations of these additional features.

See also pervasive.html.