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Simon Peyton Jones authored
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
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