Hello Conal, The issue with your example is that it is ambiguous, so GHC can't figure out how to instantiate the use of `foo`. It might be easier to see why this is if you write it in this form:
foo :: (F a ~ b) => b foo = ...
Now, we can see that only `b` appears on the RHS of the `=>`, so there is
really no way for GHC to figure out what is the intended value for `a`.
Replacing `a` with a concrete type (such as `Bool`) eliminates the
problem, because now GHC does not need to come up with a value for `a`.
Another way to eliminate the ambiguity would be if `F` was injective---then
we'd know that `b` uniquely determines `a` so again there would be no
ambiguity.
If `F` is not injective, however, the only workaround would be to write the
type in such a way that the function arguments appear in the signature
directly (e.g., something like 'a -> F a' would be ok).
-Iavor
On Sun, Jan 13, 2013 at 11:10 AM, 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
_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users