I think this discussion would be more appropriate to the libraries list. On Nov 9, 2017 11:05 AM, "Geraint Jones" <geraint.jo...@wolfson.ox.ac.uk> wrote:
> There are two things you might think of when you think of scanr; > or rather, there are two things I think of: a specification > > scanr f e = map (foldr f e) . tails > > and an implementation > > scanr f e = foldr g [e] where g x yss@(ys:_) = f x ys : yss > > Of course these two differ, because the one I called an implemntation > is strict wheras the specification isn't. > > I wouldn't mind, but scanl goes to the trouble of not being strict, > as does Data.List.tails, which means inter alia that scanr (:) [] is > not an implementation of Data.List.tails as you might have expected. > (Well, as I did.) > > > In the context of > > > arb = error "not used in demo" > > bot = error "bottom" > > you get > > *Main> (null . map (foldr arb arb) . Data.List.tails) bot > False > *Main> (null . scanr arb arb) bot > *** Exception: bottom > *Main> (null . scanl arb arb) bot > False > _______________________________________________ > Glasgow-haskell-users mailing list > Glasgow-haskell-users@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users >
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