[Haskell-cafe] Future imports in GHC/Base

João Cristóvão jmacristovao at gmail.com
Mon Jan 13 11:47:58 UTC 2014

Hello all,

I was wondering if there is any particular reason for Haskell/GHC not
having a kind of "import from Future", similar to Python import __future__,
or, in a different take, stuff like modernizr for javascript.

Was this done already?

My quite simple use case: I could really use the Foldable instance for
Either, and also ended up defining my own 'isLeft' and 'isRight'. Felt
guilty of re-inventing the wheel after seeing it defined elsewhere, it was
so obvious, of course.

Then I see it defined in GHC Head - but unfortunatelly not all packages
support it yet. And I can only guess that installing base 4.7 on top of GHC
7.6.3 would certainly result in Cabal Hell.

I ended up copying the parts that I needed. Any one else doing this?

My proposal is simple, and quite restricted:
1) Include only stuff with no dependencies on new compiler features
2) Focus mainly on additional typeclasses instances
3) Or new functions, that otherwise should not interfere with existing code.
4) Use CPP to translate the code into NOP when the new compiler finally
enters the Haskell Platform.

Thus, a very simple "FutureGHC" package (I'm open to alternative names)
would, for isLeft/isRight, be like the code proposed below - taken from GHC
Head libraries, of course.

What do you think?


-- (...)

module Data.Either.GHC707 (
  module Data.Either
#if __GLASGOW_HASKELL__ < 707
  , isLeft
  , isRight
 ) where

#if __GLASGOW_HASKELL__ < 707
import Data.Either

-- | Return `True` if the given value is a `Left`-value, `False` otherwise.
-- /Since:
isLeft :: Either a b -> Bool
isLeft (Left  _) = True
isLeft (Right _) = False

-- | Return `True` if the given value is a `Right`-value, `False` otherwise.
-- /Since:
isRight :: Either a b -> Bool
isRight (Left  _) = False
isRight (Right _) = True
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