Add "check" function to Control.Monad

[JonFairbairn]

Generalisation of "guard".

check (not . isSpace) 'c' == Just 'c'

check (not . isSpace) ' ' == Nothing

See example in attached darcs patch

[JonFairbairn]

darcs patch on base

[JonFairbairn]

Timescale: 14 days.

[igloo]

I think 14 days is unrealistic given the timing relative to ICFP et al, and suggest end of Sept as the deadline.

[JonFairbairn]

I suspect the consensus on this is not to add this function, but to add mfilter instead. I'll open a separate ticket for that.

[JonFairbairn]

I've opened ticket #3524 to add mfilter.

To paraphrase what I said on the libraries mailing list, the consensus is not to add check because it is a small function that readily be written in terms of other well known ones, though this writing will be much easier in the presence of mfilter.

I fear I should have made it clear in the proposal that the point of check is that it's more algebraically handy than guard, but as it's too late to replace guard we would have had to have both.