-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 David House wrote: > On 31/05/07, Isaac Dupree <[EMAIL PROTECTED]> wrote: >> foo undefined = undefined > > That's not true. When you evaluate foo undefined, it matches the first > (irrefutable) pattern immediately, without any deconstruction of the > undefined argument (which is the entire point of it being a lazy > pattern). So it becomes the right hand side, C <thunk>. Only when you > force that <thunk> would you have to force the undefined argument, so > foo undefined = C undefined: > > *Main> foo undefined > C *** Exception: Prelude.undefined >
Oh, I misinterpreted "must be non bottom" to be talking about becoming more strict, not less, compared to id, and then misanalysed the function :/ Now for the existential case... normally an existential "escaping" is useless, sometimes it seems to make sense. Looking at the GHC manual http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html#existential-quantification I think this is the relevant explanatory paragraph: In general, you can only pattern-match on an existentially-quantified constructor in a case expression or in the patterns of a function definition. The reason for this restriction is really an implementation one. Type-checking binding groups is already a nightmare without existentials complicating the picture. Also an existential pattern binding at the top level of a module doesn't make sense, because it's not clear how to prevent the existentially-quantified type "escaping". So for now, there's a simple-to-state restriction. We'll see how annoying it is. It seems like an existential as returned by (case y of E x -> x) is ("should" be) just "anything" -- you can put it in an existential container, or you can "seq" it, but not much else. To seq, (case y of E x -> (x `seq`)) should work, so the difficulty is in transferring an existential from one place to another with weird strictness effects... Is (data E' = forall a . E' !a) allowed? I seem to remember some discussion about its oddness... Type-class contexts on the existentials make it more complicated. Isaac -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iD8DBQFGYHDzHgcxvIWYTTURAn9oAJ45VLiD6W63ByUOrfpRILprPyKGdACbB+nu YIULFXH4KHDcbNmG/+cb5vI= =v2Co -----END PGP SIGNATURE----- _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
