Re: Potentially confusing syntax for injective type families

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 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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