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.

Clinton Mead

Konstantine Rybnikov

adam vogt

Amos Robinson

Amos Robinson

adam vogt

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