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

12 Feb 2015

      Hi Clinton.

I find this to be a fantastic idea. I really enjoyed how "type-level
functions" (TypeFamilies) being able to "un-apply" a type-function (code
like `type Element (f a) = a`) is a central piece of implementation here.

One thing I would add from myself is that names like Element and Container
were highly misleading for me, since they're rather "type from which map
function maps" and "end-result type". Not sure on new best names, just
shared my thoughts on these.

Thanks.

On Wed, Feb 11, 2015 at 9:00 PM, Clinton Mead  wrote:
...
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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Re: [Haskell-cafe] Rationale for two separate map functions in ClassyPrelude

Konstantine Rybnikov