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only
Combined from primary sources listed below.

In Functions§

See primary documentation in context for only

The only keyword preceding sub or method indicates that it will be the only function with that name that inhabits a given namespace.

only sub you () {"Can make all the world seem right"};

This will make other declarations in the same namespace, such as

sub you ( $can ) { "Make the darkness bright" }

fail with an exception of type X::Redeclaration. only is the default value for all subs; in the case above, not declaring the first subroutine as only will yield exactly the same error; however, nothing prevents future developers from declaring a proto and preceding the names with multi. Using only before a routine is a defensive programming feature that declares the intention of not having routines with the same name declared in the same namespace in the future.

(exit code 1)
===SORRY!=== Error while compiling /tmp/only-redeclaration.p6
Redeclaration of routine 'you' (did you mean to declare a multi-sub?)
at /tmp/only-redeclaration.p6:3
------> <BOL>⏏<EOL>

Anonymous subs cannot be declared only. only sub {} will throw an error of type, surprisingly, X::Anon::Multi.