Joachim Breitner schrieb:
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 see. It may be that GHC actually eliminates common subexpression of primitive types. Its Core output will tell you. Nonetheless, maybe it is more sensible for a compiler not to answer the question whether a common subexpression should be shared or not, but it may try to evaluate the depending expressions in a way such that the shared data structure can be garbage collected as computation goes on.