Re: darcs patch: Add justIf to Data.Maybe

Duncan Coutts writes:

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

...

Hi,

I propose the attached change

...

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

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

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:

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

BTW, I also don't want to take too strong a position on the name ;-)

me neither, but check (all . isOK) reads nicely ;-) -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk http://www.chaos.org.uk/~jf/Stuff-I-dont-want.html (updated 2009-01-31)