Macros
Macros are a powerful
way
of manipulating source code at compile time. Macros must be declared
before they’re called. A call to a
macro
routine executes as soon as
it’s parsed. The parser substitutes the return value
from the macro into the parse tree in place of the macro call. If a
macro returns undef
, it makes no entry in the
parse tree. So, the macro disappear
takes a single
string argument and returns undef
. Any call to
disappear
is replaced at compile time with
nothing, just as if it were commented out.
macro disappear (Str $thinair) { return; } . . . disappear("Some text you'll never see");
If a macro returns a string, the string is parsed as Perl source
code, and the resulting parse tree replaces the macro call. So,
anywhere the macro twice
is called, it is replaced
at compile time by a for
modifier:
macro twice { return "for 1..2"; } . . . print "\n" twice; # same as: print "\n" for 1..2;
If a macro returns
a
block, that block is parsed as a closure, and the resulting parse
tree replaces the macro call. So, when the
reverse_numeric
macro is called, the parser
substitutes the block { $^b <=> $^a }
in
place of the call:
macro reverse_numeric { return { $^b <=> $^a }; } . . . sort reverse_numeric, @values;
If a macro returns a parse tree, the parser substitutes it directly for the macro call. The returned tree may be the original parse tree, a modified parse tree, or a manufactured parse tree.
By default, a call to a macro is parsed just like an ordinary subroutine call, ...
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