f `unamb` g just needs f or g to be in weak head normal form I think. This should be much easier to test for I guess.
I always confuse weak head normal form with reduced head normal form, but the documentation of GHC does not help here: E.g.: -- | Reduces its argument to weak head normal form.rwhnf :: Strategy a rwhnf x = x `seq` () but the documentation of seq says *seq* :: a -> b -> bSource <file:///C:/app/ghc-6.10.2/doc/libraries/ghc-prim/src/GHC-Prim.html#seq>Evaluates its first argument to head normal form, and then returns its second argument as the result. Furthermore: *rnf* :: Strategy <file:///C:/app/ghc-6.10.2/doc/libraries/parallel/Control-Parallel-Strategies.html#t%3AStrategy> aSource <file:///C:/app/ghc-6.10.2/doc/libraries/parallel/src/Control-Parallel-Strategies.html#rnf>Reduces its argument to (head) normal form. So from the documentation rnf should be like seq, but it is not, rnf is a "deep seq". I find this very confusing. Is the documentation of seq wrong (should be weak head normal form)? Anyway, so I guess we would actually need a function: iswhnf :: a -> IO Bool But since the value of this iswhnf function depends on when it is called, I feel it has to be in the IO monad; actually multiple threads evaluating it have nothing to do with it? On Mon, Apr 20, 2009 at 10:05 PM, Jake McArthur <jake.mcart...@gmail.com>wrote: > Christopher Lane Hinson wrote: > >> What we'd like to avoid is duplicate verification that a thunk is hnf. Do >> we have evidence that this verification ever actually consumes a lot of >> resources? >> > > I think the OP is trying to avoid spawning unnecessary threads at the cost > of duplicate checks for HNF. > > - Jake >
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