State variables

Per-closure persistent variables, initialised once and retained across calls.

A state variable lives inside a sub or block but keeps its value between calls. Its initialiser runs only the first time execution reaches it; after that the stored value persists.

Persistent counter #

sub counter {
    state $n = 0;
    return $n++;
}
say counter();
say counter();
say counter();

0
1
2

state $n = 0 initialises $n once; each call returns then increments the retained value.

Accumulating across calls #

Because the initialiser is skipped after the first call, state is a clean way to accumulate.

my &f = { state $x = 0; $x += $_ };
say f(10);
say f(5);

10
15

Notes #

• state differs from my: a my variable is re-created (and re-initialised) every call, whereas a state variable is created once per closure and remembered.
• Each closure gets its own state storage — two clones of the same block do not share it, just as with closures.
• The one-time initialiser makes state $cache = expensive() a tidy memoisation idiom.