both Err Abs = Err
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 d = d
+both Lazy Bot = Lazy
+both Lazy Abs = Lazy
+both Lazy Err = Lazy
+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
projects / ghc-hetmet.git / blobdiff