isAlpha
is in Data.Char
, not Data.List
import Data.List
import Data.Char
myFunc :: [String] -> String
myFunc ls = filter isAlpha . nub . concat $ ls
In the future, I'd suggest not fully qualifying names, just do
import Data.List as L
import Data.Char as C
-- Now use `C.isAlpha` and `L.nub`
And when searching for functions, I'd suggest hoogle