The RHS of the RULE for filterFB confuses p and q, which can be shown
as follows:
filterFB is defined as follows:
filterFB c p x r | p x = x `c` r
| otherwise = r
which is equivalent to
filterFB c p x r = if p x then c x r else r
(&&) is defined as follows
True && x = x
False && _ = False
which implies that
if a && b then c else d
is equivalent to
if a then (if b then c else d) else d
Now we can deduce
filterFB (filterFB c p) q a b
= if q a then filterFB c p a b else b
= if q a then (if p a then c a b else b) else b
= if q a && p a then c a b else b
= filterFB c (\x -> q x && p x) a b
which is *not* equivalent to
filterFB c (\x -> p x && q x) a b
Adventurous people can fix this this without recompilation by
hand-editing PrelList.hi: Search for "filterFB" near the end of the
file and flip p and q in the RHS. For more timid people: I'm checking
in a fixed version of PrelList.lhs. :-)
Cheers,
Sven
