Yes, good point about idiom 1; I've added it. S
| -----Original Message----- | From: haskell-prime-boun...@haskell.org [mailto:haskell-prime- | boun...@haskell.org] On Behalf Of AntC | Sent: 10 June 2012 06:23 | To: haskell-prime@haskell.org | Subject: Re: TypeFamilies vs. FunctionalDependencies & type-level | recursion | | Simon Peyton-Jones <simonpj@...> writes: | | > | > No I didn't intend to put more in the header, perhaps less. | > I've added more clarification. | > | > Simon | | Thanks Simon, I agree with keeping it terse; I agree with your "yuk" | rating for `of'. At risk of bikeshedding over surafce syntax (for a | feture that's still only a gleam in the eye) ... | | I think we're going to see two idioms for overlapping instances: | | Idiom 1: total instance (this would apply to all the HList examples). We | only need one instance group for the whole; then it's the type family | decl that seems superfluous. Perhaps we could allow: | | type family Equal a b :: Bool where | Equal a a = True | Equal a b = False | | type family HasMember a (b :: '[]) :: Bool where | HasMember a '[] = False -- (not | overlapping) | HasMember a ( a ': bs ) = True | HasMember a ( b ': bs ) = HasMember a bs | | Idiom 2: an instance group discriminated by the outermost type | constructor, or by one of the arguments (this might apply for Monad | Transformers). Then although the instance header is superfluous, it | might be useful documentation: | | module SomeLibrary where | type family F a b :: ... | | module MyModule where | data MyType = ... | type instance F MyType b where -- total function for a ~ | MyType | F MyType Int = ... | F MyType (Int, b) = ... | F MyType b = ... | | AntC | | | | | | _______________________________________________ | Haskell-prime mailing list | Haskell-prime@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-prime _______________________________________________ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
