Permutation

A series can be matched as a permutation with perm, which takes its members in any order — each one exactly once. The members go in [].

A = perm["A" "B" "C"];

That matches ABC, CAB, BCA and the three others, and fails if any member is missing or repeated.

Anything a series can hold, a perm can hold — the members are ordinary units, so groups, counters and actions are all fair game:

# three named actions, in any order
head := perm[title author date];

# a group as a member, so one of the three is itself a choice
flags := perm[("r" | "w") "x" "s"];

# a counted member: the digits may come before or after the letters
code  := perm[<A:Z>+ <0:9>+];

Each member is taken exactly once. perm["A" "B"] matches AB and BA, and fails on A, on ABB and on AA.

Where a series would be written with the members in a fixed order, a perm is the same grammar with that order relaxed — which is why it is usually the right tool for attributes, record fields and command-line style flags.

Important

A perm does not skip between its members. If they can be separated, put the inbetween in yourself:

attrs = perm[(name .) (size .) (color .)];