Re: [Haskell-cafe] Constraints on data-types, mis-feature?
On Tuesday 10 July 2007, Jim Apple wrote: > On 7/9/07, Jonathan Cast <[EMAIL PROTECTED]> wrote: > > GADTs don't change anything (at least, not the last time I checked). > > GHC (in HEAD, at least) eliminates this wart for any datatype declared > with GADT syntax. > > http://www.haskell.org/ghc/dist/current/docs/users_guide/data-type-extensio >ns.html#gadt-style > > "Any data type that can be declared in standard Haskell-98 syntax can > also be declared using GADT-style syntax. The choice is largely > stylistic, but GADT-style declarations differ in one important > respect: they treat class constraints on the data constructors > differently. Specifically, if the constructor is given a type-class > context, that context is made available by pattern matching." Cool! Looks like it does work in HEAD (although it does /not/ in 6.6.1). Jonathan Cast http://sourceforge.net/projects/fid-core http://sourceforge.net/projects/fid-emacs ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Constraints on data-types, mis-feature?
On Tuesday 10 July 2007, Jim Apple wrote: > On 7/9/07, Jonathan Cast <[EMAIL PROTECTED]> wrote: > > GADTs don't change anything (at least, not the last time I checked). > > GHC (in HEAD, at least) eliminates this wart for any datatype declared > with GADT syntax. > > http://www.haskell.org/ghc/dist/current/docs/users_guide/data-type-extensio >ns.html#gadt-style > > "Any data type that can be declared in standard Haskell-98 syntax can > also be declared using GADT-style syntax. The choice is largely > stylistic, but GADT-style declarations differ in one important > respect: they treat class constraints on the data constructors > differently. Specifically, if the constructor is given a type-class > context, that context is made available by pattern matching." I'll definitely check this out; I've been quite annoyed by this restriction in the past. Jonathan Cast http://sourceforge.net/projects/fid-core http://sourceforge.net/projects/fid-emacs ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Constraints on data-types, mis-feature?
On 7/9/07, Jonathan Cast <[EMAIL PROTECTED]> wrote: GADTs don't change anything (at least, not the last time I checked). GHC (in HEAD, at least) eliminates this wart for any datatype declared with GADT syntax. http://www.haskell.org/ghc/dist/current/docs/users_guide/data-type-extensions.html#gadt-style "Any data type that can be declared in standard Haskell-98 syntax can also be declared using GADT-style syntax. The choice is largely stylistic, but GADT-style declarations differ in one important respect: they treat class constraints on the data constructors differently. Specifically, if the constructor is given a type-class context, that context is made available by pattern matching." Jim ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Constraints on data-types, mis-feature?
On Monday 09 July 2007, Daniil Elovkov wrote: > Hello > > In the archives of haskell-cafe I found a mention of constraints on > datatypes as a mis-feature of Haskell. In particular, that they're not > propagated well. Can someone elaborate on that? > > Also, are they still considered a mis-feature with the emergence of GADTs ? > > If I have > > data GADT a where >... >Alt :: (a -> b -> c) -> GADT a -> GADT b -> GADT c >... > > and I want to constrain all a, b, c. > > Would it be better to expose all of them as type vars, rather than > constrain in-place? It would lead to a rather verbose code. GADTs don't change anything (at least, not the last time I checked). If you say class C a where ... data GADT a where ... Alt :: (C a, C b, C c) => (a -> b -> c) -> GADT a -> GADT b -> GADT c ... when you pattern match on Alt, the compiler finds the instances for C a and C b, but the constraint C c is ignored. So constraints on data types work exactly the same way they always have, and the standard arguments against them all still work. (Although now I think the status of this `feature' can be down-graded to wart: after all, if you say newtype Id a = Id a data GADT a where ... Alt :: (C a, C b, C c) => (a -> b -> c) -> GADT (Id a) -> GADT (Id b) -> GADT (Id c) ... pattern-matching on Alt introduces all three constraints into the current context. . .) Jonathan Cast http://sourceforge.net/projects/fid-core http://sourceforge.net/projects/fid-emacs ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Constraints on data-types, mis-feature?
Hello In the archives of haskell-cafe I found a mention of constraints on datatypes as a mis-feature of Haskell. In particular, that they're not propagated well. Can someone elaborate on that? Also, are they still considered a mis-feature with the emergence of GADTs ? If I have data GADT a where ... Alt :: (a -> b -> c) -> GADT a -> GADT b -> GADT c ... and I want to constrain all a, b, c. Would it be better to expose all of them as type vars, rather than constrain in-place? It would lead to a rather verbose code. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
