bracketOnError, while, forever

[Malcolm Wallace]

"Simon Marlow" <simonmar at microsoft.com> writes:

> >> comparing :: (Ord a) => (b -> a) -> b -> b -> Ordering
> >> comparing p x y = compare (p x) (p y)
> 
>  sortBy (comparing fst)
> 
> is just too cute not to have.  Any objections?

Looks good to me.

> >> readM :: (Monad m, Read a) => String -> m a
> 
> Also subsumes System.IO.readIO.  Looks useful to me.

Yup.

> >> selections :: [a] -> [(a,[a])]
> >> selections []     = []
> >> selections (x:xs) = (x,xs) : [ (y,x:ys) | (y,ys) <- selections xs ]
> >> 
> >> permutations    :: [a] -> [[a]]
> >> permutations [] = [[]]
> >> permutations xs =
> >>     [ y : zs
> >>     | (y,ys) <- selections xs
> >>     , zs     <- permutations ys
> >>     ]

Here's another one.  I'm not sure what to call it, since 'permutation'
means something subtly different.

    -- Given a list of alphabets, return all possible strings with one
    -- symbol chosen from each alphabet respectively.
    permute         :: [[a]] -> [[a]]
    permute   []     = [[]]
    permute (xs:xss) = [ f:fs | f <- xs, fs <- permute xss ]

Regards, Malcolm