
Calling Sequence


x :: T


Parameters


x



any expression

T



a type specification





Description


•

The :: operator serves several purposes in Maple, all related to types.

•

When used in a procedure parameter declaration, the lefthand side of the operator specifies the parameter name, and the righthand side specifies the expected type of the argument.

•

The return type of a procedure can be declared by following the procedure's parameter declaration sequence by ::T . In this context, :: serves as an assertion that the procedure returns a value of the specified type, T. If assertions are enabled, returning a value not matching the type will raise an exception.

•

When used as the lefthand side of an assignment statement or as the first argument to the assign function, x::T asserts that the value being assigned to x is of type T.

•

A local variable can be declared with a type by using the :: operator. Within the procedure in which the variable is declared, any assignments made to it are treated as if the lefthand side of the assignment were written using the :: declaration.

•

If a local variable was declared with a type, and the lefthand side of an assignment to that local variable also specifies a type, then the value being assigned must satisfy both types.

•

The :: operator can also be used to assert a type on the control variable(s) of a for loop. For example, when iterating over the contents of a container such as a list, the :: operator can assert that each entry is of a specified type.

•

In the condition of an if or while statement, :: serves as a type testing operator. In this context, x::T is equivalent to type(x,T).

•

The :: operator also acts as a type test when used as an argument to any of the Boolean operators, and, or, not, xor, implies, or the evalb function.

•

In any other context, :: evaluates its arguments, but does not itself evaluate any further. Thus, it can be used as a data structure or part of a larger data structure. Although it is not required, it is customary to use :: as a data structure only when the righthand side represents a type or a typelike concept. For example, see RealRange.

•

An expression using the :: operator is of type `::`.



Thread Safety


•

The :: operator is threadsafe as of Maple 15.



Examples


A common use of :: is to declare the type of procedure parameters.
>

f := proc( x::integer ) x^2 end proc:

The :: operator can assert the return type of a procedure.
>

f := proc( x::integer ) :: odd; x^2 end proc:

>

$\mathrm{kernelopts}\left(\mathrm{assertlevel}=2\right)\:$

Using :: on the lefthand side of an assignment, declaring a local variable with ::, or adding :: to a loop variable, asserts that the value being assigned is of the specified type.
>

$x::\'\mathrm{integer}\'\u22543.4$

>

f := proc( x ) local y::integer;
y := x
end proc:

>

$\mathbf{for}\phantom{\rule[0.0ex]{0.3em}{0.0ex}}z::\'\mathrm{integer}\'\phantom{\rule[0.0ex]{0.3em}{0.0ex}}\mathbf{in}\phantom{\rule[0.0ex]{0.3em}{0.0ex}}\left[1.2\,3\,4\right]\phantom{\rule[0.0ex]{0.3em}{0.0ex}}\mathbf{do}\phantom{\rule[0.0ex]{0.0em}{0.0ex}}\phantom{\rule[0.0ex]{2.0em}{0.0ex}}z\phantom{\rule[0.0ex]{0.0em}{0.0ex}}\mathbf{end}\phantom{\rule[0.0ex]{0.3em}{0.0ex}}\mathbf{do}$

In the condition of an if (or while) statement, the :: operator is equivalent to using the type function.
>

f := proc( x )
if x::integer then
print("an integer")
else
print("not an integer")
end if
end proc:

${''not\; an\; integer''}$
 (4) 
The :: operator is inert if used out of context.
>

$x::\'\mathrm{integer}\'$

${2}{::}{\mathrm{Typesetting}}{:}{\mathrm{\_Hold}}{}\left(\left[{\'}{\mathrm{integer}}{\'}\right]\right)$
 (6) 
The :: operator is not inert if used within a Boolean expression.
>

$x::\'\mathrm{integer}\'\phantom{\rule[0.0ex]{0.3em}{0.0ex}}{\textstyle \mathbf{and}}\phantom{\rule[0.0ex]{0.3em}{0.0ex}}x<3$

Expressions involving the :: operator are of type `::`.
>

$\mathrm{type}\left(x::\mathrm{symbol}\,\mathrm{`::`}\right)$

>

$\mathrm{evalb}\left(\left(x::\mathrm{string}\right)::\mathrm{`::`}\right)$



Compatibility


•

The ability to apply the :: operator to the control variable(s) of a for loop is new in Maple 2021.

•

The :: operator was updated in Maple 2021.



