Reset Regex state after calling 'test', resolves #7

[sharkdp]

Previously, the second assert in the following code would fail:

let pattern = regex "a" (parseFlags "g")
assert $ test pattern "a"
assert $ test pattern "a"

The reason is that the RegExp object in JavaScript maintains a global state (for details, see the discussion in #7).

This commit resets the lastIndex property on the RegExp object, such that both calls to test return the same result.

[davidchambers]

Should we restore the previous value of r.lastIndex instead?

[sharkdp]

In the Purescript Regex API, there is no other function that affects lastIndex (if I'm correct), so I thought resetting to 0 would be fine. But your solution is the safer alternative (there might be foreign code which affects the lastIndex property). Thanks.

[paf31]

It's a shame it works this way, but I can see why it might be useful in code where performance is a must. Perhaps we should find a way to represent things like test using Eff in the case where the global flag is set.

[davidchambers]

LGTM

[sharkdp]

It's a shame it works this way, but I can see why it might be useful in code where performance is a must. Perhaps we should find a way to represent things like test using Eff in the case where the global flag is set.

I agree that - with these changes - the global flag in a Regex is useless when using test. The question is: when would I be interested in using (an effectful version of) test? I actually cannot think of any reasonable use case apart from counting the number of matches [*]. However, to get the number of matches, I would rather use match directly, which gives me all results. But I might be missing something here.

I don't really have a strong opinion on this, but using Eff just to model this (arguably) weird behavior of the JavaScript RegExp API seems strange to me. An alternative might be to provide a version of test like this:

testFromIndex :: Regex -> Int -> String -> { match :: Boolean, newIndex :: Int }

which could be used in a fold/loop.

[*] Note that test only gives me a Boolean whether or not the Regex matched. Otherwise it might be interesting to use an effectful version of test in a loop and exit early.

[garyb]

Could you rebase this one please?

[garyb]

Thanks!