haskell - Map lookup function with fold -

i create "map lookup" function using folds.

this "map" (just list of tuples):

phonebook:: [([char], [char])] phonebook = [("bob", "00-21-55")             ,("jack", "55-51-55")             ,("joe", "10-61-25")             ,("susy", "06-21-55")             ,("clara", "50-31-95")             ] 

and kind of function write:

lookup :: (eq k) => k -> [(k,v)] -> v lookup k = foldl1 (\acc (x,y) -> if k == y y else acc) 

however, not compile, yields "cannot construct infinite type" error.

could explain why wrong , how can make work?

please note aware of data.map , map functions exports, want solely learn how folds work.

first here working version - i'll come , explain in few minutes:

lookup :: (eq k) => k -> [(k,v)] -> maybe v lookup k = foldl (\ acc (k',v) -> if k == k' v else acc) nothing             

the problems have are:

• for foldl1 have type foldl1 :: (a -> -> a) -> [a] -> a need same type elements , result - here want value result fold on tuples of key/value-pairs.
• this why switched more general foldl (note not interested in if right choice here - see foldl' vs foldl vs foldr)
• this function can never total (if key not in dictionary/key-value-pair-list) opted reflect in result type, maybe v - of course makes easy find initial value: nothing

if don't maybe part can use error instead too:

lookup :: (eq k) => k -> [(k,v)] -> v lookup k = foldl (\ acc (k',v) -> if k == k' v else acc) (error "key not found") 

remarks:

this not efficient implementation through of list no matter if found key yet - feel free try come

hint: haskell lazy - maybe can find filtering out right key/value-pairs, taking safe-head of , maping value-part ;) )