On Thu, Jun 4, 2015 at 10:30 PM, Edward Z. Yang
Looks to me like it's confused about whether a ~ is part of an equality constraint or is a laziness annotation. The former would be illegal at
Excerpts from Brandon Allbery's message of 2015-06-04 13:06:52 -0700: that
point, though, I'd think? Somewhere it believes a constraint might be possible there, via btype.
As ezyang says in the message I see just came in, you'll need extra production rules to distinguish that top level. Although I'd wonder why it believes an equality constraint is acceptable there in the first place; is that a lurking bug in the parser?
In general, the parser is more liberal than is actually required; we can give better error messages this way.
Second, here is where it's ambiguous:
T a ~b
where T is a TyCon. Does this parse as
T a (~b)
or
T a ~ b
Parser doesn't currently distinguish these cases.
I guess we should parse it as T a (~b), just as we have unary minus bind "tighter" with the following token.
Edward _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs