TypeFamilies vs. FunctionalDependencies & type-level recursion

[Simon Peyton-Jones]

Yes, good point about idiom 1; I've added it.

S

| -----Original Message-----
| From: haskell-prime-bounces at haskell.org [mailto:haskell-prime-
| bounces at haskell.org] On Behalf Of AntC
| Sent: 10 June 2012 06:23
| To: haskell-prime at haskell.org
| Subject: Re: TypeFamilies vs. FunctionalDependencies & type-level
| recursion
| 
| Simon Peyton-Jones <simonpj at ...> 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
| 
| 
| 
| 
| 
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