nope.
it is true that in the case of an infinite list it exibits the "desired"
behaviour but ...
return (replicate 1000000 'a') >>= \x->print $ spant (const True) x
ERROR - Garbage collection fails to reclaim sufficient space
i.e. as the function unfold, the thunk representing the second term builds
up on the heap. (not sure why it works for an infinite list, hugs must drop
the reference to the tail ?)
to obtain a function that will properly operate in constant space, for every
unfolding of the first term we need to enforce evaluation of the second
term.
From: "Claus Reinke" <[EMAIL PROTECTED]>
To: "john lask" <[EMAIL PROTECTED]>
CC: <[email protected]>
Subject: Re: [Haskell-cafe] RE: simple function: stack overflow in hugs
vsnonein ghc
Date: Mon, 24 Sep 2007 16:20:42 +0100
afraid not
the given example is too strict, the requirement is to generate the
matched portion lazilly, and return the tail (unconsumed portion).
ah yes, without optimisations, Prelude.span builds up stack,
while the continuation-based alternative i mentioned is too
strict for some uses.
In principle the function should be capable of being written to run in
constant space which the given example dose not.
return (repeat 'a') >>= \ x -> print $ span (const True) x
how about the old spec, then?
span p l = (takeWhile p l,dropWhile p l)
since takeWhile takes forever, here, it isn't even inefficient!-)
claus
with hugs you will get a stack error, in ghc it executes in constant
space, i.e. indefinitely. In essenece the above example does exactly the
same as my ealier code.
this thread might be relevant:
http://www.haskell.org/pipermail/hugs-bugs/2007-June/001815.html
http://www.haskell.org/pipermail/hugs-bugs/2007-June/001816.html
http://www.haskell.org/pipermail/hugs-bugs/2007-June/001817.html
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