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
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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