Question

I do the following in GHCI:

:m + Data.Map
let map = fromList [(1, 2)]
lookup 1 map

GHCI knows that map is a (Map Integer Integer). So why does it claim an ambiguity between Prelude.lookup and Data.Map.lookup when the type is clear and can I avoid?

<interactive>:1:0:
    Ambiguous occurrence `lookup'
    It could refer to either `Prelude.lookup', imported from Prelude
                          or `Data.Map.lookup', imported from Data.Map

> :t map
map :: Map Integer Integer
> :t Prelude.lookup
Prelude.lookup :: (Eq a) => a -> [(a, b)] -> Maybe b
> :t Data.Map.lookup
Data.Map.lookup :: (Ord k) => k -> Map k a -> Maybe a

Answer 1

The types are clearly different but Haskell doesn’t allow ad-hoc overloading of names, so you can only choose one lookup to be used without a prefix.

The typical solution is to import Data.Map qualified:

> import qualified Data.Map as Map

Then you can say

> lookup 1 [(1,2), (3,4)]
Just 2
> Map.lookup 1 Map.empty
Nothing

Usually, Haskell libraries either avoid re-using names from the Prelude, or else re-use a whole bunch of them. Data.Map is one of the second, and the authors expect you to import it qualified.

[Edit to include ephemient’s comment]

If you want to use Data.Map.lookup without the prefix, you have to hide Prelude.lookup since it’s implicitly imported otherwise:

import Prelude hiding (lookup) 
import Data.Map (lookup)

This is a bit weird but might be useful if you use Data.Map.lookup a whole bunch and your data structures are all maps, never alists.