Stefan O'Rear wrote:
sum = sum' 0 sum' k [] = k sum' k (x:xs) = (sum' $! (k+x)) xs
enum x y | x >= y = 0 | otherwise = x : enum (x+1) y
sum (enum 1 10) => sum' 0 (enum 1 10) => sum' 0 (1 : enum (1+1) 10) => (sum' $! (0+1)) (enum (1+1) 10) => sum' 1 (enum (1+1) 10) =>
sum' 1 (2 : enum (2+1) 10) => (sum' $! (1+2)) (enum (2+1) 10) => sum' 3 (enum (2+1) 10) =>
sum' 3 (3 : enum (3+1) 10) => (sum' $! (3+3)) (enum (3+1) 10) => sum' 6 (enum (3+1) 10) =>
sum' 6 (4 : enum (4+1) 10) => (sum' $! (6+4)) (enum (4+1) 10) => sum' 10 (enum (4+1) 10) =>
...
sum' 36 (9 : enum (9+1) 10) => (sum' $! (36+9)) (enum (9+1) 10) => sum' 45 (enum (9+1) 10) => sum' 45 [] => 45
(I need to find some way to automate making these trails :) )
I did have a fairly small Tcl implementation for this... I don't have the code now, and I wrote it early in my Haskell career, so there's masses of stuff it didn't handle. (*cough* type classes) Actually, I've often longed for some tool (maybe even integrated into Lambdabot) to show the reduction sequence of an arbitrary expression. But none exists, AFAIK...