haskell - Tuple evaluation -

i'm writing function of following type:

match :: [(string,a)] -> maybe (string, a, a) 

i want function traverse list of tuples, , determine if there tuples in first element (a string) same. if so, want return tuple containing string, second element in each of matching tuples. if there no matching tuples, return "nothing". if there more 1 matching, return first 1 finds.

for example:

match [("x", 3), ("y", 4), ("z", "5"), ("x", 6)] = ("x", 3, 6) match [("x", 3), ("y", 4), ("z", "5")] = nothing 

i'm thinking:

match (x:xs) = if (fst x) = (fst xs) return (fst x, snd x, snd xs) --if no matches, return nothing 

thank help!

what if there 3 or 4 tuples "x"? can't have variable-length tuples. maybe want return list:

match :: [(string, a)] -> maybe (string, [a]) 

what if there several tuples match? want them all, or first? if want them should return list of matches.

match :: [(string, a)] -> [(string, [a])] 

if think of this, can see grouping "x" pairs together, , "y" pairs, , on, start. can using

sortby (comparing fst) xs 

comparing takes function , 2 values, applies function each, , compares results. sortby uses first argument comparison function, sortby (comparing fst) sorts list order first element in each tuple.

then can use groupby collect elements together.

edit:

groupby has type

groupby :: (a -> -> bool) -> [a] -> [[a]] 

so need write function equalfirst give parameter.

groupby equalfirst $ sortby (comparing fst) xs 

will give you

[[("x", 3), ("x", 6)], [("y", 4)], [("z", 5)]] 

which list of lists. each sub-list contains tuples same letter.

now can write function takes 1 of these sublists , converts result want. apply list of lists using map.