[Haskell-cafe] Flattening tail recursion?
goldpython at gmail.com
Fri Dec 10 13:55:03 EST 2004
I did this:
countLines ls = foldl (\x y -> x + 1) 0 ls
On Fri, 10 Dec 2004 19:07:04 +0100 (MEZ), Henning Thielemann
<iakd0 at clusterf.urz.uni-halle.de> wrote:
> 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
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