CODEWITHSUNDEEP
haskellfunctor
Babra Cunningham
Babra Cunningham

Reputation: 2967

Fmap over a list of custom types?

The below code won't compile:

data Outcome a = Fail | Pass a deriving (Show, Eq, Ord, Functor)

myList = [Pass 33, Pass 12, Fail, Pass 45]

main = do
   print $ fmap (+1) myList

I can't see why this doesn't work, as I've successfully applied the same approach to my binary tree of type 'data Tree a = Empty | Node a (Tree a) (Tree a)'?

Upvotes: 1

Views: 169

Answers (2)

redneb
redneb

Reputation: 23870

The problem is that you are trying to add 1 to each element of the list myList which has type Outcome Integer. Try this instead:

fmap (fmap (+1)) myList

or to make it more clear:

map (fmap (+1)) myList

So the outer fmap applies to the list and the inner one applies to the Outcome functor. So, to each element of the list, we apply the function fmap (+1), which in turn applies the function (+1) to the value inside the Outcome.

Upvotes: 11

Lee
Lee

Reputation: 144136

Lists are functors and you have an [Outcome] so you need to supply a function Outcome -> b. It looks like you want to apply (+1) to each Outcome within the list so you need to use:

fmap (\o -> fmap (+1) o) myList

or

fmap (fmap (+1)) myList

Upvotes: 6

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