Re: [Haskell-cafe] How does GHC's implementation of par work?

I saw your paper on LtU last week. Here's an extra plug: I really enjoyed it. When can we expect to see Strict Core replacing System Fc? :) -- ryan On Tue, May 26, 2009 at 5:32 PM, Max Bolingbroke wrote:

2009/5/26 Ryan Ingram :

...

What causes par# to get executed?  Does GHC overload case for unboxed return values to force the evaluation of the scrutinee even if its return value isn't used?

As you noted, the type of "par" is a -> Int#. Furthermore, the only possible evaluation semantics for unboxed types in GHC's intermediate language is strict. This means that your case expression has to evaluate the scrutinee eagerly.

Indeed, consider the following program (which looks even lazier than your example):

let x :: Int# = error "boom!" in 10

This will diverge in either Haskell or Core, because both languages promise that if have an unlifted thing in your environment /it really has been evaluated/. SPJ and I recently wrote a paper [1] proposing an intermediate language for GHC that would allow Core to express thunks of unlifted types, but even so I don't think we would actually want to change the semantics of bindings of unlifted types in Haskell.

Cheers, Max

[1] http://www.cl.cam.ac.uk/~mb566/papers/tacc-hs09.pdf _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe