A much better idea than making sum strict, would simply be to add a
sum'.
Even better to abstract over strictness, to keep a lid on code duplication?
{-# LANGUAGE TypeOperators #-}
sum = foldlS ($) (+) 0
sum' = foldlS ($!) (+) 0
-- identity on constructors of t (from a), modulo strictness in a
type a :-?> t = (a -> t) -> (a -> t)
foldlS :: (b :-?> ([a] -> b)) -> (a -> b -> b) -> (b -> [a] -> b)
foldlS ($) op n [] = n
foldlS ($) op n (h:t) = (foldlS ($) op $ (op h n)) t
Strictness is encoded as a constructor transformer - ($) keeps the
constructor in question unchanged, ($!) makes it strict. Also works
with container types (Maps strict or not strict in their elements can
share the same strictness-abstracted code, for instance). Though
sometimes there is more than one strictness choice to make in the
same piece of code..
Claus
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