Why is scanr strict in its third argument?
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
I think this discussion would be more appropriate to the libraries list.
On Nov 9, 2017 11:05 AM, "Geraint Jones"
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
participants (2)
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David Feuer -
Geraint Jones