darcs patch: Add justIf to Data.Maybe

[Jon Fairbairn]

On 2009-08-25 at 12:32BST Simon Marlow wrote:
> I think that for consistency with if/then/else and guard we
> should use checkA.

See my response to Sebastian.

> It's not that hard to write
> 
>      all_spaces xs = checkA (all (isSpace xs)) xs
> 
> if that's really what you want.

Not /hard/, but it involves using xs twice, which should tell us
something.

> I wouldn't call checkB more "general"; to me it's just a
> minor convenience issue, which is outweighed by consistency
> and simplicity.

See response to Sebastian.

> > 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

which involves duplication again, and on top of that zipWithM,
which while not /hard/ is hardly conceptually simple.

 Jón

-- 
Jón Fairbairn                              Jon.Fairbairn at cl.cam.ac.uk