[Haskell-cafe] Newbie question: Where is StackOverflow on the Wiki?

[Andrew Coppin]

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...