Wed Dec 13 08:29:15 PST 2006 [EMAIL PROTECTED]
* Add left-to-right impredicative instantiation
People keep complaining, with some justification, that
runST $ foo
doesn't work. So I've finally caved in. The difficulty with the above
is that we need to decide how to instantiate ($)'s type arguments based
on the first argument (runST), and then use that info to check the second
argumnent. There is a left-to-right flow of information.
It's not hard to implement this, and it's clearly useful. The main
change is in TcExpr.tcArgs, with some knock-on effects elsewhere.
I was finally provoked into this by Trac #981, which turned out, after some
head-scratching, to be another instance of the same problem.
(There was some bug-fixing too; a type like ((?x::Int) => ...) is a polytype
even though it has no leading for-alls, but the new TcUnify code was not
treating it right.)
Test for this is tc222
M ./compiler/typecheck/TcExpr.lhs -37 +60
M ./compiler/typecheck/TcMType.lhs -2 +1
M ./compiler/typecheck/TcUnify.lhs -38 +75
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