title: GHCi 7.8.2 doesn't use language extensions from file
body: <p>Here's some simple code that requires <code>-XRebindableSyntax</code>.</p>
  
  <pre><code>{-# 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 -&gt; Foo -&gt; Foo
  f x y = x + y
  
  g = f 3 5
  </code></pre>
  
  <p>Here's my GHCi transcript:</p>
  
  <pre><code>&gt; ghci Foo.hs
    GHCi, version 7.8.2
    ...
  *Main&gt; g
    Foo 8
  *Main&gt; f 3 5
  
    &lt;interactive&gt;: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&gt; :set -XRebindableSyntax
   *Main&gt; f 3 5
     Foo 8
  </code></pre>
  
  <p>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 <code>-XRebindableSyntax</code>, which I will usually need?)</p>