On 08 February 2005 12:33, Thomas Jäger wrote:
> First of all
>> -- Cale Gibbard
>> comparing :: (Ord a) => (b -> a) -> b -> b -> Ordering
>> comparing p x y = compare (p x) (p y)
> fits nicely with the ...By functions from Data.List.
sortBy (comparing fst)
is just too cute not to have. Any objections?
> Since there's no easy way to catch failure of read operations,
>> readM :: (Monad m, Read a) => String -> m a
>> readM s = case [x | (x,t) <- reads s, ("","") <- lex t] of
>> [x] -> return x
>> [] -> fail "Prelude.readM: no parse"
>> _ -> fail "Prelude.readM: ambiguous parse"
> this function seems to be quite natural.
Also subsumes System.IO.readIO. Looks useful to me.
> Finally,
>> -- Koen Claessen
>> 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
>> ]
> are quite useful (maybe they should be named select and permute since
> most Data.List names seem to be imperatives).
Both look reasonable. Not all the names in Data.List are imperative (eg. inits, tails, lines, words). I think the names are fine.
Cheers,
Simon
