Hi,
How would you infer "a" from "F a"? Given "bar :: Bool", I can't see how
one could go from "Bool" to "F a = Bool" and determine "a" uniquely.
My question is not completely retorical, if there is an answer I would
like to know it :-)
Gruss,
Christian
* 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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