Haskell ambiguous type variable compiler error when foldMap on empty List

Question

I'm trying to write a function that flattens nested Lists.

The code:

data NestedList a = Elem a | List [NestedList a]

flatten :: NestedList a -> [a]
flatten (Elem e) = [e]
flatten (List l) = foldMap flatten l

main = do
    let c0 = List []
    print $ flatten c0

When I try and provide an empty List from main I get a compiler error:

Ambiguous type variable ‘a0’ arising from a use of ‘print’
  prevents the constraint ‘(Show a0)’ from being solved.
  Probable fix: use a type annotation to specify what ‘a0’ should be.

I am wondering what causes this error and how to fix it.

Any help is much appreciated!

It's exactly as the error message says - GHC can't figure out what the a is in NestedList a for the value List []. To fix, again as GHC says, just give a type annotation, eg let c0 = (List [] :: NestedList Int) — Robin Zigmond
– Robin Zigmond, Commented Sep 12, 2020 at 23:11
Does this answer your question? Testing empty list [] with Eq type — Robin Zigmond
– Robin Zigmond, Commented Sep 12, 2020 at 23:12
I found this article, and it really helped me learn about this error and others: obround.blogspot.com/2020/09/… — xilpex
– xilpex, Commented Sep 13, 2020 at 22:47

Cactus · Accepted Answer · 2020-09-18 08:04:59Z

0

When you call print :: (Show a) => a -> IO (), the constraint on Show a needs to be solved somehow. In your case, the type of c0 is inferred to be NestedList a, which is of course not enough information to solve the Show [a] constraint from print $ flatten c0.

You solve this problem by adding a type signature to the binding of c0 to make it monomorphic.

answered Sep 18, 2020 at 8:04

Cactus

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