Redefining a Function


You want to temporarily or permanently redefine a function, but functions can’t be assigned to.


To redefine a function, assign a new code reference to the typeglob of the name of the function. Use a local if you want it to be temporary.

undef &grow;                    # silence -w complaints of redefinition
*grow = \&expand;           
grow();                         # calls expand()

    local *grow = \&shrink;     # only until this block exists
    grow();                     # calls shrink()


Unlike a variable but like a handle, a function cannot be directly assigned to. It’s just a name. You can manipulate it almost as though it were a variable, because you can directly manipulate the run-time symbol table using typeglobs like *foo to produce interesting aliasing effects.

Assigning a reference to a typeglob changes what is accessed the next time a symbol of that type is needed. This is what the Exporter does when you import a function or variable from one package into another. Since this is direct manipulation of the package symbol table, it only works on package variables (globals), not lexicals.

*one::var = \%two::Table;   # make %one::var alias for %two::Table
*one::big = \&two::small;   # make &one::big alias for &two::small

A typeglob is something you can use local on, but not my. Because of the local, this aliasing effect is then limited to the duration of the current block.

local *fred = \&barney;     # temporarily alias &fred to &barney

If the value assigned to a typeglob is not a reference but itself another typeglob, ...

Get Perl Cookbook now with the O’Reilly learning platform.

O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.