Derek Elkins wrote:
The compiler is supposed to be able to reorder non-strict
evaluation to do optimisations, but that can't be done if effects
could happen.
There's nothing special about non-strict evaluation that makes the
antecedent true. Replacing "non-strict" with "strict" gives just as
much of a valid statement. It is purity that allows (some) reordering
of evaluation.
Here are two effectful statements that can safely be reordered.
print "foo"
x := 5
here are two more
y := 2
z := 3
(provided y and z don't alias)
Purity allows some reordering of evaluation, so does knowing that
two effectful computations won't interfere.
Ben.
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