[Haskell-cafe] type families and type signatures

[Thomas M. DuBuisson]

Id is an operation over types yielding a type, as such it doesn't make
much sense to me to have (Id a -> Id a) but rather something like (a ->
Id a).  One could make this compile by adding the obvious instance:

> type instance Id a = a

Curiously, is this a reduction from a real world use of families?  I
just can't think of how a (Fam a -> Fam a) function would be of use.

Cheers,
Thomas

Ganesh Sittampalam wrote:
> The following program doesn't compile in latest GHC HEAD, although it does 
> if I remove the signature on foo'. Is this expected?
> 
> Cheers,
> 
> Ganesh
> 
> {-# LANGUAGE TypeFamilies #-}
> module Test7 where
> 
> type family Id a
> 
> type instance Id Int = Int
> 
> foo :: Id a -> Id a
> foo = id
> 
> foo' :: Id a -> Id a
> foo' = foo
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