Hello Henning,
Am Samstag, den 04.12.2010, 12:41 +0100 schrieb Henning Thielemann: > Joachim Breitner schrieb: > > Now consider the program: > > > >> avg' :: [Int] -> (Int, Int) > >> avg' n = (sum [0..n] `div` length [0..n], length [0..n]) > > It think this is not typecorrect, since 'n' denotes a list and is used > as upper bound in [0..n]. Should it be > > > avg' ns = (sum ns `div` length ns, length ns) > > ? Thanks for noting. The type annotation is wrong. In your case (which I had first), ns is shared and thus would not allow for constant-space calculation of sum and length and would fail to demonstrate my point. > > I’d say that CSE to > >> avg n = (sum [0..n] `div` l, l) > >> where l = length [0..n] > > is safe, because the shared expression (an Int) is smaller than the > > thunks and the compiler can tell. > > If this is meant to be > > > avg n = (sum ns `div` l, l) > > where l = length ns > > according to my substitution above, then 'ns' still has to be shared and > stored completely. Right, sorry for the confusion by the wrong type. Greetings, Joachim -- Joachim Breitner e-Mail: m...@joachim-breitner.de Homepage: http://www.joachim-breitner.de ICQ#: 74513189 Jabber-ID: nome...@joachim-breitner.de
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