unfold/fold
A program transformation where a recursive call to a function is unfolded to an instance of the function's body and then later an instance of the function's body is replaced by a call. E.g.
sumdouble l = sum (double l)
double l = case l of [] -> [] x:xs -> 2*x + double xs
==> (unfold double)
sumdouble l = sum (case l of [] -> [] x:xs -> 2*x : double xs)
==> (distribute over case)
sumdouble l = case l of [] -> sum [] x:xs -> sum (2*x : double xs)
==> (unfold sum)
sumdouble l = case l of [] -> 0 x:xs -> 2*x + sum (double xs)
==> (fold sumdouble)
sumdouble l = case l of [] -> 0 x:xs -> 2*x + sumdouble xs
(1994-11-03)