Creighton Hogg wrote:
> This may be silly of me, but I feel like this is an important point:
> so you're saying that tail recursion, without strictness, doesn't run
> in constant space?

It is an important point, and a classic space bug (see foldl in the
Prelude).

It it not the fault of tail recursion per se, in fact tail recursion is
often important in Haskell too.

> So for example in the case of,
> facTail 1 n' = n'
> facTail n n' = facTail (n-1) (n*n')

The problem with this example is that it will build up an expression of
the form:

   (n1 * n2 * n3 .....)

in the second argument. It's size will be proportional to the number of
recursive calls made (n).
> You'll just be building a bunch of unevaluated thunks until you hit
> the termination condition?
>

To fix it you will want the function to evaluate its second argument
eagerly:

facTail n n' = facTail (n-1) $! (n*n')