List of Tuples as a function Parameter in Haskell -

List of Tuples as a function Parameter in Haskell -

how define function parameter list of tuples? illustration of input

[("hey", false), ("you", true)]

my function takes list of tuples as: [(string, bool)] here looks like:

afunc :: [(string, bool)] -> bool afunc ???? =

so fill in ???? able access tuple? help great. thanks.

edit:

afunc :: [(string, bool)] -> bool afunc atuple = mapm_ lookup atuple?

so how access tuple in function? doesn't work.

it looks you're trying implement own version of lookup. can write simple version using list comprehension:

lookup' :: string -> [(string,bool)] -> bool lookup' k lkp = head $ [v | (k',v) <- lkp, k'==k]

or using filter:

lookup'' :: string -> [(string,bool)] -> bool lookup'' k lkp = snd $ head $ filter ((==k) . fst) lkp

notice these versions unsafe - is, they'll fail ugly , uninformative error if list doesn't contain item:

ghci> lookup' "foo" [("bar",true)] *** exception: prelude.head: empty list

you can solve issue writing own custom error message:

lookuperr :: string -> [(string,bool)] -> bool lookuperr k lkp = case [v | (k',v) <- lkp, k'==k] of (v:_) -> v [] -> error "key not found!"

a improve approach homecoming maybe bool instead:

lookupmaybe :: string -> [(string,bool)] -> maybe bool lookupmaybe k lkp = case [v | (k',v) <- lkp, k'==k] of (v:_) -> v [] -> nil

the library version takes approach, , has more generic signature:

lookup :: (eq a) => -> [(a,b)] -> maybe b

you can read implementation here.

list function haskell parameters tuples