Allow RULES for seq, and exploit them
Roman found situations where he had
case (f n) of _ -> e
where he knew that f (which was strict in n) would terminate if n did.
Notice that the result of (f n) is discarded. So it makes sense to
transform to
case n of _ -> e
Rather than attempt some general analysis to support this, I've added
enough support that you can do this using a rewrite rule:
RULE "f/seq" forall n. seq (f n) e = seq n e
You write that rule. When GHC sees a case expression that discards
its result, it mentally transforms it to a call to 'seq' and looks for
a RULE. (This is done in Simplify.rebuildCase.) As usual, the
correctness of the rule is up to you.
This patch implements the extra stuff. I have not documented it explicitly
in the user manual yet... let's see how useful it is first.
The patch looks bigger than it is, because
a) Comments; see esp MkId Note [seqId magic]
b) Some refactoring. Notably, I moved the special desugaring for
seq from MkCore back into DsUtils where it properly belongs.
(It's really a desugaring thing, not a CoreSyn invariant.)
c) Annoyingly, in a RULE left-hand side we need to be careful that
the magical desugaring done in MkId Note [seqId magic] item (c)
is *not* done on the LHS of a rule. Or rather, we arrange to
un-do it, in DsBinds.decomposeRuleLhs.