[Haskell-cafe] Flattening tail recursion?

[Henning Thielemann]

On Fri, 10 Dec 2004, Robert Dockins wrote:

> >    countLines [] = 0
> >    countLines (_:ls) = 1 + countLines ls
> > 
> > I would have thought that this was tail recursive and would be
> > flattened into iteration by the compiler. Can anyone explain why? 
> 
> countlines = aux 0
>     where aux x [] = x
>           aux x (_:ls)
>                 | x `seq` False = undefined
>                 | otherwise     = aux (x+1) ls
> 
> The `seq` causes the x to be evaluated, so it should run in constant space.

Is it also possible to do that with 'foldl'?

Why is Prelude.length not defined this way (according to the Haskell98
report)?