[Haskell-cafe] Advice on type families and non-injectivity?

[Jake McArthur]

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 <conal at conal.net> 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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