Syntax extensions (was: RE: The Future of Haskell discussionat the Haskell Workshop)

[Simon Marlow]

>    {-# LANGUAGE <specification> #-}
> 
> where <specification> is one or more (if compatible) of keywords like
> 
>     Haskell98				Pure Haskell 98, no extensions.
>     SharedExtenisons (Haskell02???)	A set of agreed-upon extensions
> 					implemented by all "major"
> 					Haskell systems.
>     RecursiveDo
>     ArrowSyntax
>     TemplateHaskell
>     OverlappingInstances
>     UndecidableInstances
>     FFI					Foreign Function
Interface
>     MPTC				Multi-parameter Type Classes

Looks fine to me.  A few things to think about:

  - Some of the keywords specify an entire language (eg. Haskell98),
    whereas some are language modifiers (eg. FFI).  We might want
    to make a distinction.  Currently GHC supports only Haskell98 +
    modifiers.

  - Are extensions always additive?  Are there any extensions
    which are incompatible?

  - There are features you might want to *disable*.  eg.
    GHC lets you turn off the monomorphism restriction.

Perhaps something like this:

  {-# LANGUAGE Haskell98 +FFI -MonomorphismRestriction #-}


> (abbreviations used when three or more words).
> 
> The OPTION pragma would be used for compiler-specific 
> options, although,
> in the interest of supporting portable code without having to 
> resort to
> preprocessing using CPP, maybe it would make sense to provide
> 
>    OPTIONS-GHC
>    OPTIONS-Hugs
>    OPTIONS-NHC
>    ...
> 
> as well, the idea being that a compiler/interpreter then only 
> would look at options pertinent to itself.

Yes, I've been meaning to rename GHC's version of the pragma to
GHC_OPTIONS for some time.

Cheers,
	Simon