Potentially confusing syntax for injective type families

[Jan Stolarek]

> 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.
Yes, I bumped into this issue quite early. After some discussion it was decided that this 
potential ambiguity is an acceptable trade-off.

Janek

Dnia niedziela, 14 lutego 2016, Matthew Pickering napisał:
> 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
> >
> > ---
> > Politechnika Łódzka
> > Lodz University of Technology
> >
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---
Politechnika Łódzka
Lodz University of Technology

Treść tej wiadomości zawiera informacje przeznaczone tylko dla adresata.
Jeżeli nie jesteście Państwo jej adresatem, bądź otrzymaliście ją przez pomyłkę
prosimy o powiadomienie o tym nadawcy oraz trwałe jej usunięcie.

This email contains information intended solely for the use of the individual to whom it is addressed.
If you are not the intended recipient or if you have received this message in error,
please notify the sender and delete it from your system.