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
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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