Re: darcs patch: Add justIf to Data.Maybe

On 22/08/2009 16:22, Jon Fairbairn wrote:

Duncan Coutts writes:

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On Sat, 2009-08-22 at 10:18 +0200, Joachim Breitner wrote:

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Hi,

I propose the attached change

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Fri Aug 21 19:17:12 CEST 2009 Joachim Breitner * Add justIf to Data.Maybe

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As Jón was suggesting we might want to generalise to MonadPlus rather than putting it in Data.Maybe, it becomes rather similar to the existing function 'guard':

[...] some alpha conversion

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checkA :: MonadPlus m => Bool -> a -> m a checkA True x = return x checkA False _ = mzero

Using a Bool for the condition is a tad more general than a predicate like:

checkB :: MonadPlus m => (a -> Bool) -> a -> m a

I'd say the generality comparison goes the other way:

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In particular, the use case in Cabal does not fit the predicate pattern because the condition is not a condition on the value being returned but on something else in the environment.

But you can write any

checkA e as checkB $ const e

Right, but checkB p x = checkA (p x) x. Also, checkA p = (guard p >>) . return

whereas a function like

all_spaces = checkB (all . isSpace)

I think that for consistency with if/then/else and guard we should use checkA. It's not that hard to write all_spaces xs = checkA (all (isSpace xs)) xs if that's really what you want. I wouldn't call checkB more "general"; to me it's just a minor convenience issue, which is outweighed by consistency and simplicity.

is hard to write using checkA (suppose you are doing something like “map (checkB predicate)”) since checkA doesn't look at the second argument.

the "applying check to a list" case is not hard either: zipWithM checkA (map p xs) xs again, if that's what you want to do. Cheers, Simon