Pragma not recognised when wrapped in #ifdef

[Simon Marlow]

Simon Peyton-Jones wrote:
> 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.

Spot on.

> If so, then this amounts to
>    a) a documentation bug: it should be clear what GHC does

Right, I checked the docs and it doesn't explicitly say this.

>    b) a feature request, to somehow allow cpp to affect in-file flags
>           I'm not sure what the spec would be

It needs a bit of thought - what should happen to pragmas that are there 
pre-CPP but not post-CPP, for example?

Cheers,
	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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> | Glasgow-haskell-users at haskell.org
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> 
> 
> 
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