GHCi 7.8.2 doesn't use language extensions from file

Question

Here's some simple code that requires -XRebindableSyntax.

{-# LANGUAGE RebindableSyntax, NoImplicitPrelude #-}

import NumericPrelude
import qualified Algebra.Additive (C)
import qualified Algebra.Ring (C)

newtype Foo = Foo Int deriving (Show)

instance Algebra.Additive.C Foo where
    (Foo x) + (Foo y) = Foo (x+y)

instance Algebra.Ring.C Foo where
    fromInteger = Foo . fromInteger

f :: Foo -> Foo -> Foo
f x y = x + y

g = f 3 5

Here's my GHCi transcript:

> ghci Foo.hs
  GHCi, version 7.8.2
  ...
*Main> g
  Foo 8
*Main> f 3 5

  <interactive>:3:3:
    No instance for (GHC.Num.Num Foo) arising from the literal ‘3’
    In the first argument of ‘f’, namely ‘3’
    In the expression: f 3 5
    In an equation for ‘it’: it = f 3 5

 *Main> :set -XRebindableSyntax
 *Main> f 3 5
   Foo 8

I'm 95% sure that when I loaded a file with an extension prior to GHCi 7.8, I wouldn't have to reset that extension in GHCi. Is this documented somewhere, or is it a bug? Is there an easy workaround (for example, telling GHCi to always use -XRebindableSyntax, which I will usually need?)

Answer 1

Is there an easy workaround (for example, telling GHCi to always use -XRebindableSyntax, which I will usually need?)

You can put a .ghci file in the same directory as your source files. Now if you start ghci from that directory, the .ghci file will be load and its content executed like the commands you type at the ghci prompt. In this case, you would put :set -XRebindableSyntax in there.

I think this is better than putting :set -XRebindableSyntax into the .ghci in your home directory because you might have other Haskell projects in other directories that need different extensions.

For more ideas what to put into project-specific ghci files, see Neil Mitchell's blog post.