Pragma not recognised when wrapped in #ifdef

[Simon Peyton-Jones]

I'm guessing a bit here, but it looks as if you intend this:

* GHC should read Foo.hs, and see {-# LANGUAGE CPP #-}
* Then it should run cpp
* Then it should look *again* in the result of running cpp,
  to see the now-revealed {-# LANGUAGE DeriveDataTypeable #-}

I'm pretty sure we don't do that; that is, we get the command-line flags once for all from the pre-cpp'd source code.  Simon or Ian may be able to confirm.

If so, then this amounts to
   a) a documentation bug: it should be clear what GHC does
   b) a feature request, to somehow allow cpp to affect in-file flags
          I'm not sure what the spec would be

Simon

| -----Original Message-----
| From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-users-
| bounces at haskell.org] On Behalf Of Alistair Bayley
| Sent: Tuesday, February 10, 2009 12:03 PM
| To: GHC Users Mailing List
| Subject: Re: Pragma not recognised when wrapped in #ifdef
|
| > {-# LANGUAGE CPP #-}
| > #ifdef PRAGMA_DERIVE_TYPEABLE
| > {-# LANGUAGE DeriveDataTypeable #-}
| > #else
| > {-# OPTIONS -fglasgow-exts #-}
| > #endif
| > -- This file is Test/Fail.hs.
| > -- ghc --make  -optP-DPRAGMA_DERIVE_TYPEABLE -XCPP Test.Fail
| > module Test.Fail where
| > import Data.Generics
| > data Fail = Fail deriving Typeable
| >
| > If compile this wih the command
| >  ghc --make  -optP-DPRAGMA_DERIVE_TYPEABLE -XCPP Test.Fail
| > then I get this error from ghc-6.10.1:
| >
| > [1 of 1] Compiling Test.Fail        ( Test\Fail.hs, Test\Fail.o )
| >
| > Test\Fail.hs:11:26:
| >    Can't make a derived instance of `Typeable Fail'
| >      (You need -XDeriveDataTypeable to derive an instance for this class)
| >    In the data type declaration for `Fail'
|
|
| No response. I'd like to know if this is a bug (that's what it looks
| like to me), or just a mistake I've made.
|
| Thanks,
| Alistair
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