A custom pre-processor might desugar terms into terms of a particular language extension, requiring a new language pragma. Currently, any language pragmas generated by a pre-processor will be ignored once the pre-processed file is picked up by GHC again.

As a contrived example, consider foo.hs:

{-# OPTIONS -F -pgmF ./preprocess.sh #-}

data Foo a where MkFoo :: Foo a

where preprocess.sh:

{{{ #!/bin/sh ( echo "# LANGUAGE GADTs #"; cat $2; ) > $3 }}}

Currently we get this behaviour:

bash-3.2$ ghc foo.hs

/tmp/ghc7191_0/ghc7191_0.hspp:4:0:
    Illegal generalised algebraic data declaration for `Foo'
      (Use -XGADTs to allow GADTs)
    In the data type declaration for `Foo'

bash-3.2$ ghc foo.hs -E
bash-3.2$ cat foo.hspp
{-# LANGUAGE GADTs #-}
{-# OPTIONS -F -pgmF ./preprocess.sh #-}

data Foo a where MkFoo :: Foo abash-3.2$ 

It would be nice if GHC noticed the new pragmas, particularly language pragmas, although perhaps not all pragmas should be picked up (maybe not new OPTION pragmas).