Thanks, Jake! This suggestion helped a lot. -- Conal On Sun, Jan 13, 2013 at 1:59 PM, Jake McArthur wrote:

I have a trick that loses a little convenience, but may still be more convenient than data families.

{-# LANGUAGE TypeFamilies #-}

import Data.Tagged

type family F a

foo :: Tagged a (F a) foo = Tagged undefined

bar :: Tagged a (F a) bar = foo

This allows you to use the same newtype wrapper consistently, regardless of what the type instance actually is; one of the inconveniences of data families is the need to use different constructors for different types.

On Sun, Jan 13, 2013 at 2:10 PM, Conal Elliott 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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