Simon Marlow marlowsd at
Wed Aug 3 12:44:10 CEST 2011

On 03/08/2011 11:09, Victor Nazarov wrote:
> On Wed, Aug 3, 2011 at 11:30 AM, Simon Peyton-Jones
> <simonpj at>  wrote:

>> So perhaps that's the problem. parseDynamicFlags could perfectly well simply return any un-recognised flags. Indeed, I thought it did just that -- it certainly returns a list of un-consumed arguments.  If it doesn't perhaps that's a bug.
> parseDynamicFlags returns un-consumed arguments if they are something
> like filenames, but it throws error if un-consumed argument starts
> with dash.

So then parseDynamicFlags should be split into two layers, the lower 
layer returning unused flags, and the upper layer generating errors. 
You could use the lower layer in your front end.

Please send us a patch...

>> OK, so this is harder.  Presumably you want to use an *unmodified* Haskell parser to parse the Haskell programs. Adding *syntactic* extensions is therefore somewhat invasive:
>>         - change the lexer
>>         - change the parser
>>         - change the HsSyn data structure
>>         - change every function that traverses HsSyn
>> However in this particular case maybe things are not so bad.  I believe that perhaps *all* you want is to add a new calling convention. See ForeignImport in HsDecls, and CCallConv in ForeignCall.  Simply adding a new data constructor to CCallConv, and lexing the token for it, would not be too bad.  We could possibly add that part to the mainline compiler. The compiler would largely ignore such decls, and they'd just pop out at the other end for your back end to consume.
> This would be cool. I will try to provide patch to add all calling
> conventions that backend implementors can use.
> But GHC should report errors about unsupported calling conventions
> sometime during compilation when should it?

Right, you'll need some backend-specific desugaring of the FFI 
declarations.  Maybe we need desugarer plugins? :-) An easier approach 
would be to have a slot in the DynFlags for a callback, like we do for 
printing error messages, so the GHC API client passes in a callback to 
do whatever backend-specific desugaring is required.  The callback 
mechanism could be used for lots of things - I can imagine it growing 
into a record of backend-specific functions that the earlier stages of 
the compiler might need to call.

It's hard to predict exactly what's needed.  Again, I suggest you try 
doing this and send us a patch.


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