Potentially confusing syntax for injective type families

[Edward Kmett]

When this was first discussed a bunch of alternatives were tossed around,
mostly involving new keywords, or putting a conditional result keyword in
place. Here you can pick the name of the result type, so it doesn't pick
any naming conventions for you.

My understanding is that the current syntax was selected because it avoids
magic conditional keywords and fits into a gap in the current grammar.

All of the alternatives seemed worse.

-Edward

On Sun, Feb 14, 2016 at 5:56 AM, Matthew Pickering <
matthewtpickering at gmail.com> wrote:

> I guess my point is that the most natural parsing of
>
> class Hcl a b where
>      type Ht a b = r | r -> a b
>
> is  (type Ht a b = r) (| r -> a b) rather than (type Ht a b) (= r | r -> a
> b).
>
> A concrete example, in the case of functional dependencies, the
> vertical bar is used to signal what we expect *additional* optional
> information. In this case, adding the injectivity annotation
> completely changes the meaning of what came before.
>
> I don't have a solution and I hate bike-shedding. I just made this
> message to make sure the fact had been considered before release.
>
> Matt
>
>
>
> On Sun, Feb 14, 2016 at 9:28 AM, Jan Stolarek <jan.stolarek at p.lodz.pl>
> wrote:
> >> 2. Without the infectivity annotation, this declares an associate type
> >> synonym default. This isn't valid because Ht is not declared as an
> >> associated type before hand and r is not mentioned on the LHS.
> >>
> >> class Hcl a b where
> >>     type Ht a b = r
> > Indeed, this is invalid and GHC rejects this, so I think we're OK here.
> In case of associated
> > types if you want to declare injectivity you need to provide the "| r ->
> ...." part. Otherwise
> > you're declaring a default. This is documented in the User's Guide for
> 8.0.
> >
> > Janek
> >
> > ---
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