Commit 92428681 authored by simonpj's avatar simonpj
Browse files

[project @ 2001-07-25 13:29:07 by simonpj]

	Fix a bug in the defn of "both"

This bug made the bootstrapped GHC enter an absent argument.

The change is from

	both Lazy (Seq k Now ds) = Seq Keep Now ds
	both Lazy (Seq k l ds) = Seq Keep l ds

Simple, eh?  (Comments with the code.)
parent f7341243
......@@ -696,8 +696,21 @@ both Err d = d
both Lazy Bot = Lazy
both Lazy Abs = Lazy
both Lazy Err = Lazy
both Lazy (Seq k Now ds) = Seq Keep Now ds
both Lazy (Seq k l ds) = Seq Keep l ds
both Lazy d = d
-- Notice that the Seq case ensures that we have the
-- boxed value. The equation originally said
-- both (Seq k Now ds) = Seq Keep Now ds
-- but it's important that the Keep is switched on even
-- for a deferred demand. Otherwise a (Seq Drop Now [])
-- might both'd with the result, and then we won't pass
-- the boxed value. Here's an example:
-- (x-1) `seq` (x+1, x)
-- From the (x+1, x) we get (U*(V) `both` L), which must give S*(V)
-- From (x-1) we get U(V). Combining, we must get S(V).
-- If we got U*(V) from the pair, we'd end up with U(V), and that
-- can be a disaster if a component of the data structure is absent.
-- [Disaster = enter an absent argument.]
both Eval (Seq k l ds) = Seq Keep Now ds
both Eval (Call d) = Call d
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