[Haskell-cafe] Rationale for two separate map functions in ClassyPrelude

11 Feb 2015

      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.

[Haskell-cafe] Rationale for two separate map functions in ClassyPrelude

Clinton Mead