Hi Conal,
I agree that your initial example is a little puzzling, and I'm glad that the
new ambiguity checker prevents both definitions, not just one.
However, your initial question seems broader than just this example. I have run
into this problem (wanting injective type functions) several times myself, and
have been successful at finding workarounds. But, I can't think of any unifying
principle or solid advice. If you can post more information about your problem,
perhaps I or others can contribute.
For what it's worth, the desire for injective type functions has been entered
as ticket #6018 in the GHC Trac, but I see you're already on the cc: list. I
believe Simon PJ has given serious thought to implementing this feature and may
have even put in some very basic code toward this end.
Richard
On Jan 13, 2013, at 2:10 PM, Conal Elliott <co...@conal.net> wrote:
> I sometimes run into trouble with lack of injectivity for type families. I'm
> trying to understand what's at the heart of these difficulties and whether I
> can avoid them. Also, whether some of the obstacles could be overcome with
> simple improvements to GHC.
>
> Here's a simple example:
>
> > {-# LANGUAGE TypeFamilies #-}
> >
> > type family F a
> >
> > foo :: F a
> > foo = undefined
> >
> > bar :: F a
> > bar = foo
>
> The error message:
>
> Couldn't match type `F a' with `F a1'
> NB: `F' is a type function, and may not be injective
> In the expression: foo
> In an equation for `bar': bar = foo
>
> A terser (but perhaps subtler) example producing the same error:
>
> > baz :: F a
> > baz = baz
>
> Replacing `a` with a monotype (e.g., `Bool`) eliminates the error.
>
> Does the difficulty here have to do with trying to *infer* the type and then
> compare with the given one? Or is there an issue even with type *checking* in
> such cases?
>
> Other insights welcome, as well as suggested work-arounds.
>
> I know about (injective) data families but don't want to lose the convenience
> of type synonym families.
>
> Thanks, -- Conal
>
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