Warn if an always-match clause is not the last alternative

[hildjj]

Consider:

foo = "b"* / "foo" 

This is unlikely to produce the output that the author intended, since the "foo" will never be checked. I wonder if we can detect this pattern and at least warn on it?

[Mingun]

It is definitely possible. The idea that for each node you should calculate statistic -- how much characters node will consume when matched? It is better to calculate the minimum and maximum. Here we only need the minimum, and the maximum may be needed in other analyzes. If alternative have a branch with minimum=0, then other alternatives will not be tried.

Actually, I already wrote such pass (but not used it in any project)

[hildjj]

This probably also applies to lookaheads. !("b"*) always fails, and &("b"*) always succeeds.

[Mingun]

Maybe. Also, I realized, that the proposed solution will catch only part of situations. . always match everything, but it also always consume 1 character when matched. Actually we need static analysis which will create a representation of what inputs are possible at certain points. Then we would catch even such situations:

start1
  = [a-z]
  / [bcd] // never matched because the previous alternative matches all what this matches
start2
  = 'prefix'
  / 'prefix-literal' // never matched because the previous alternative matches all what this matches