Course of Raku / Advanced / Operators / Traits and pragmas 🆕

Pragmas

A pragma looks like a module loaded with use, but instead of bringing in code it changes a compilation rule for the rest of the current scope. Turn one on with use and (where it makes sense) off with no.

The most important pragma, strict, is on by default in Raku: it requires every variable to be declared before use, which is why a typo in a variable name is caught rather than silently creating a new variable.

Turning strict off with no strict lifts that requirement, so you can assign to a variable that was never declared with my:

no strict;

$x = 42;
say $x; # 42

With the default strict in force, that bare $x = 42 would be a compile-time error — Variable ‘$x’ is not declared. The no strict line relaxes the rule for the rest of the enclosing scope.

Other pragmas include fatal (which turns a quiet failure into a thrown exception — handy once you have met exception handling), isms (to allow idioms borrowed from other languages), and variables (to tune variable rules). A pragma’s effect is lexical — it lasts only to the end of the block it appears in — so you can tighten or loosen a rule for exactly the region of code where you want it.

One pragma takes an argument: lib adds a directory to the list Raku searches when loading modules — the in-program equivalent of the -I command-line option:

use lib 'lib';

You will reach for it once you start writing and loading your own modules.

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