From type/Routine

See Original text in context

multi sub trait_mod:<is>(Routine $r, :$pure!)

Marks a subroutine as pure, that is, it asserts that for the same input, it will always produce the same output without any additional side effects.

The is pure trait is a promise by the programmer to the compiler that it can constant-fold calls to such functions when the arguments are known at compile time.

sub syllables() is pure {
    say "Generating syllables";
    my @vowels = <a e i o u>;
    return  @vowels.append: <k m n sh d r t y> X~ @vowels;
}

You can mark function as pure even if they throw exceptions in edge cases or if they modify temporary objects; hence the is pure trait can cover cases that the compiler cannot deduce on its own. On the other hand, you might not want to constant-fold functions that produce a large return value (such as the string or list repetition operators, infix x and xx) even if they are pure, to avoid large precompilation files.

To see it an action with a particular compiler you can try this example:

BEGIN { say ‘Begin’ }
say ‘Start’;
say (^100).map: { syllables().pick(4).join("") };
 
 
# Example output: 
# Begin 
# Generating syllables 
# Start 
# (matiroi yeterani shoriyuru... 

Essentially this allows the compiler to perform some operations at compile time. The benefits of constant-folding may include better performance, especially in cases when the folded code is precompiled.

In addition, using a pure function or operator in sink context (that is, where the result is discarded) may lead to a warning. The code

sub double($x) is pure { 2 * $x };
double(21);
say "anything";
# WARNING: «Useless use of "double(21)" in expression "double(21)" in sink context (line 2)»

If you want to apply this trait to a multi, you need to apply it to the proto; it will not work otherwise, at least in versions 2018.08 and below.