Question

I have a question about Haskell. I want to know how I can create a list of data from two lists, one with data, an other with some key values. I’ll explain it with an example:

Given two lists: [('a', "red"), ('b', "blue"), ('c', "green")] and [('a','b'), ('b', 'c'), ('c','a')]. Now I want to change the values of the second list with their colors given in the first list. So the function should return [("red","blue"), ("blue","green"), ("blue","red")].

I was thinking about list comprehension, but I’m very new to Haskell and I have no idea how I should do that. Or is there an easier way to do this?

This is probably a dumb question, but if someone can give me an example, I might get used to the think process of Haskell a bit more.

Answer 1

-- second element from first (head) dropping tuples with bad key
color :: Eq a => a -> [(a, b)] -> b
color c = snd.head.dropWhile ((/=c).fst)

recolor a b = map (\(x, y) -> (color x a, color y a)) b

running

Prelude> recolor [('a', "red"), ('b', "blue"), ('c', "green")] [('a','b'), ('b', 'c'), ('c','a')]
[("red","blue"),("blue","green"),("green","red")]

If you consider an element of the second list cannot be found in the first list. You can write

color :: Eq b => b -> [(b, a)] -> Maybe a
color c = fmap snd.listToMaybe.dropWhile ((/=c).fst)

then

Prelude> recolor [('a',"red"),('b',"blue"),('c',"green")] [('a','h'),('u','c'),('c','a')]
[(Just "red",Nothing),(Nothing,Just "green"),(Just "green",Just "red")]

(you need import Data.Maybe (listToMaybe))