Re: [Haskell-cafe] Some random newbie questions

On Sunday 09 January 2005 21:30, Marcin 'Qrczak' Kowalczyk wrote:

Jorge Adriano Aires writes:

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No, it would work with strict foldl too. In fact in the absence of optimization it would work better (uses less time and space). The optimization required is inlining and strictness analysis.

Is this also true if your just going to use the first few elements after reversing it?

Yes. A strict fold would evaluate cons cells of the result while they are constructed, not list elements. They are all defined (flip (:) x y is always defined), so a strict foldl is correct.

Yes, now I was refering to the efficiency issue only.

Making a cons cell should be not more expensive than making a thunk which will make a cons cell when evaluated.

Ok, I wasn't sure about this...

Well, unless the implementation doesn't inline flip and thus making these thunks is actually faster than running them. In this case a lazy foldl is more efficient than a strict foldl, as long as a sufficiently small part of the result is used; it's always less efficient if the whole result is examined.

Right. J.A.