ClassyPrelude has two map functions, namely:

1. "map"

2. "omap"

"map" works on any Functor. However, things like "Text" are not functors as they aren't generic containers. As can be seen in the following code:

module Main where

  import Prelude ()

  import ClassyPrelude

  import qualified Data.Text as T

  import Data.Char as C

  main = do

    let l = [1,2,3] :: [Int]

    let t = (T.pack "Hello")

    let m = Just 5

    print $ map (*2) l

    print $ map (*2) m

    print $ omap C.toUpper t

    return ()

Notice one has to use "omap" to deal with the Text. The thing is, I found it trivially easy to get "map" to work for both calls. Here's the code:

{-# LANGUAGE MultiParamTypeClasses #-}

{-# LANGUAGE TypeFamilies #-}

module Main where

  import Prelude hiding (map)

  import qualified Data.Text as T

  import Data.Char as C

  import Control.Monad (Functor)

  class CanMap a b where

    type Element a :: *

    type Container a b :: *

    map :: (Element a -> b) -> a -> Container a b

  instance (Functor f) => CanMap (f a) b where

    type Element (f a) = a

    type Container (f a) b = f b

    map = fmap    

      

  instance CanMap T.Text Char where

    type Element T.Text = Char

    type Container T.Text Char = T.Text

    map = T.map

    

  main = do

    let l = [1,2,3] :: [Int]

    let m = Just 5

    let t = (T.pack "Hello")

    print $ map (*2) l

    print $ map (*2) m

    print $ map C.toUpper t

    return ()

All that's required is to add instances to CanMap for any monomorphic containers. ClassyPrelude already does this anyway with "omap" in the Data.MonoTraversable module. I suspect however there's a good reason I'm missing about why there should be two separate map functions to deal with these alternate situations, but I'm wondering what that is.

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