[Haskell-cafe] Equality constraints in type families

[Hugo Pacheco]

Sorry, I meant

type FList a x = Either One (a,x)
type instance F [a] = FList a

On Thu, Mar 27, 2008 at 4:45 PM, Hugo Pacheco <hpacheco at gmail.com> wrote:

>
>
> > The current implementation is wrong, as it permits
> >
> >   type S a b = a
> >   type family F a :: * -> *
> >   type instance F a = S a
> >
> > Why do we need to forbid this type instance?  Because it breaks the
> > confluence of equality constraint normalisation.  Here are two
> > diverging normalisations:
> >
> >   (1)
> >
> >     F Int Bool  ~  F Int Char
> >
> >   ==> DECOMP
> >
> >     F Int ~ F Int, Bool ~ Char
> >
> >   ==> FAIL
> >
> >
> >   (2)
> >
> >     F Int Bool  ~  F Int Char
> >
> >   ==> TOP
> >
> >     S Int Bool  ~  S Int Char
> >
> >   ==> (expand type synonym)
> >
> >     Int  ~  Int
> >
> >   ==> TRIVIAL
> >
> > This does mean that a program such as
>
> type FList a = Either One ((,) a)
> type instance F [a] = FList a
>
> will be disallowed in further versions?
> Doesn't this problem occur only for type synonyms that ignore one or more
> of the parameters? If so, this could be checked...
>
> hugo
>
>
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