How to define function only for old versions in GHC?

Question

I have a code that uses the fromRight function defined circa GHC 8.2. But I need to downgrade to GHC 8.0.2, which gives an error about Variable not in scope: for fromRight

I was wondering if it possible and how to add the missing definition

fromRight :: b -> Either a b -> b
fromRight _ (Right b) = b
fromRight b _         = b

so that it is only used when I use an GHC version than 8.2.1?

Answer 1

Usually, when you're wondering about a library function, you should use CPP.

{-# language CPP #-}

#if !MIN_VERSION_base (4,10,0)
fromRight :: ...
#endif

The MIN_VERSION_... macros used to be provided by Cabal; now they're provided by GHC. If you want to use them with sufficiently old versions of GHC, you'll need to use Cabal (using either cabal-install or stack).

---

Before you go to the trouble of doing this, note that there are several packages with names ending in -compat that do all the work for you. In this case, you can use the fromRight from Data.Either.Compat in the base-compat package. Then you don't have to care whether you're using a new enough base library.

Answer 2

You can always write

import Prelude hiding (fromRight)

which is valid even if fromRight does not exist in Prelude. Therefore, if you want to write a module which is compatible with both old and new versions of Prelude, you can simply choose to ignore the new fromRight function, and always use the one in your library.