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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