Thu Jan 15 05:48:18 PST 2009 [email protected]
* Robustify lookupFamInstEnv, plus some refactoring
Ignore-this: 493fdbd370a714bf9677b16d2ba533d7
This patch deals with the following remark
Suppose we have
type family T a :: * -> *
type instance T Int = []
and now we encounter the type (T Int Bool). If we call
lookupFamInstEnv on (T Int Bool) we'll fail, because T has arity 1.
Indeed, I *think* it's a precondition of lookupFamInstEnv that the
supplied types exactly match the arity of the type function. But
that precondition is neither stated, nor is there an assertion to
check it.
With this patch, lookupFamInstEnv can take "extra" type arguments in
the over-saturated case, and does the Right Thing.
There was a nearly-identical function lookupFamInstEnvUnify, which
required the precisely analogous change, so I took the opportunity
to combine the two into one function, so that bugs can be fixed in one
place. This was a bit harder than I expected, but I think the result
is ok. The conflict-decision function moves from FamInst to FamInstEnv.
Net lines code decreases, although there are more comments.
M ./compiler/typecheck/FamInst.lhs -29 +3
M ./compiler/types/FamInstEnv.lhs -72 +96
View patch online:
http://darcs.haskell.org/ghc/_darcs/patches/20090115134818-1287e-a50094beaaa7211634457cf6ebd53f6d3da3b1ab.gz
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