[Haskell-cafe] Lazy evaluation and tail-recursion
limestrael at gmail.com
Wed Mar 16 20:02:54 CET 2011
> Yes, and a tail-recursive map couldn't run in constant space
Yes, I meant "if you are consuming it just once immediately".
> the above pattern [...] is better, have the recursive call as a non-strict
field of a constructor.
Which pattern? Mine or Tillman's? Or both?
2011/3/16 Daniel Fischer <daniel.is.fischer at googlemail.com>
> On Wednesday 16 March 2011 18:31:00, Yves Parès wrote:
> > Hello,
> > A question recently popped into my mind: does lazy evaluation reduce the
> > need to "proper" tail-recursion?
> > I mean, for instance :
> > fmap f  = 
> > fmap f (x:xs) = f x : fmap f xs
> > Here fmap is not tail-recursive, but thanks to the fact that operator
> > (:) is lazy, I think that it may still run in constant space/time, am I
> > right?
> Yes, and a tail-recursive map couldn't run in constant space, as far as I
> can see (time is O(length) for both of course, if the result is compeltely
> Tail recursion is good for strict stuff, otherwise the above pattern - I
> think it's called guarded recursion - is better, have the recursive call as
> a non-strict field of a constructor.
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