help needed for adding isWHNF primop to 5.00.2

Bernard James POPE
Tue, 31 Jul 2001 22:58:23 +1000 (EST)

Hi all,

Apologies for my large number of postings to this list of late.

> the normal definition of WHNF applies only
> to types containing _|_, i.e. pointed types.  Unpointed types are never
> represented by thunks, so I would be inclined to include them in WHNF.

Yes, I agree here. It is useful for me since I am using WHNF to determine
whether expressions are evaluated. Perhaps it is a slight abuse of the
terminology, but it gives me what I want :)

> You also have to deal with indirections, although you can assume (at the
> moment) that an indirection will eventually lead to an object in WHNF.

Good point. I did make this assumption. Well to be precise I made the
assumption that the thunk and indirection closure flags were mututally
exclusive. If things change I guess I will have to follow the indirections.

currently my definition is:

   #define isHNFzh(r,a) r=(! closure_THUNK((StgClosure *)a))

I believe under this definition that unpointed types and indirections 
are considered to be in WHNF. My initial tests seem to indicate that
this is working well. 

> You have to declare the primitive like this,
> BTW:
> 	isWHNF# :: a -> (# State# RealWorld, Int# #)
> because the compiler doesn't know about the IO type and you can't return
> a Bool directly.

I'm not too fussed about the unboxed Int, although as Marcin points out
it does look like some of the comparision primops can return a Bool. 
I couldn't figure out how they did it so I left it with the Int#.

> There's no problem with the simplifier as long as you declare the
> primitive to have the correct properties, i.e. that the polymorphic
> argument is lazy.


> It sounds like you also need a way to take apart arbitrary objects and
> look at their components.  

Yes indeed. I use type classes for this with deriving rules for user 
defined types. 
I'm in the midst of writing this up for IFL, in Stockholm, mid September. 
In particular, I am looking for "portable" ways of taking objects
apart, that's why type classes are attractive. Of course, isWHNF
cannot be written in Haskell, but if I can get it to work for Hugs,
GHC and NHC, then that is good enough for me :) 

What I hope is that some of this can appear as a Meta-programming
library for Haskell, which is more powerful than Read and Show. 
Something a bit similar to the semantic domain of Meta-ML.

> I know that Andy Gill was also interested in
> having similar facilities, perhaps he can help out.

I wonder if Andy is reading this? Possibly there are other people
around who are also interested in this. I think the HAT people
at NHC headquarters are also interested in these things. There 
once was talk of starting a debugging group for Haskell, but I
don't know what happened.

Thanks heaps for your help.