Re: [Haskell-cafe] naturally, length :: a -> Int
Dear Cafe, thanks for ideas and arguments.
[Ben Franksen] I would support a move Int->Word in all libraries where that makes sense.
I find it hard to think of a library where that does NOT make sense... The following is somewhat related but actually orthogonal: I mentioned that numbers often arise as cardinalities (so they are natural, by definition) and serve as pointers. The next step is then: pointer type safety. Vincent Hanquez' Foundation https://hackage.haskell.org/package/foundation-0.0.25 defines class (IsList c, Item c ~ Element c) => Collection c where length :: c -> CountOf (Element c) class ( ... Collection c) => Sequential c where take :: CountOf (Element c) -> c -> c with newtype CountOf ty (and there's also newtype Offset ty) I tend to agree. The phantom type argument for CountOf would catch errors like adding the number of students to the number of exercises (in my application). (Well and the next step after that would be to have the size of the collection in the (dependent) type as well.) Current implementation is newtype CountOf ty = CountOf Int and the author adds (and that was also the point I was making) "Int is a terrible backing type which is hard to get away from, considering that GHC/Haskell are mostly using this for [counting and] offset. Trying to bring some sanity by a lightweight wrapping." NB: I found out about this from Michael Snoyman's (very much entertaining, and too much true) https://www.snoyman.com/blog/2020/12/haskell-bad-parts-3/ Best regards, J.W.
You might want to revisit the following paper by Colin Runciman -: author = {Colin Runciman}, title = {What About the Natural Numbers}, journal = {Computer Languages}, year = {1989}, volume = {14}, pages = {181--191} It makes similar points and fills out details ... http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.56.3442 regards David On Tue, Mar 2, 2021 at 2:56 PM Johannes Waldmann < johannes.waldmann@htwk-leipzig.de> wrote:
Dear Cafe,
thanks for ideas and arguments.
[Ben Franksen] I would support a move Int->Word in all libraries where that makes sense.
I find it hard to think of a library where that does NOT make sense...
The following is somewhat related but actually orthogonal: I mentioned that numbers often arise as cardinalities (so they are natural, by definition) and serve as pointers. The next step is then: pointer type safety. Vincent Hanquez' Foundation https://hackage.haskell.org/package/foundation-0.0.25 defines
class (IsList c, Item c ~ Element c) => Collection c where length :: c -> CountOf (Element c) class ( ... Collection c) => Sequential c where take :: CountOf (Element c) -> c -> c
with newtype CountOf ty (and there's also newtype Offset ty)
I tend to agree. The phantom type argument for CountOf would catch errors like adding the number of students to the number of exercises (in my application).
(Well and the next step after that would be to have the size of the collection in the (dependent) type as well.)
Current implementation is newtype CountOf ty = CountOf Int and the author adds (and that was also the point I was making) "Int is a terrible backing type which is hard to get away from, considering that GHC/Haskell are mostly using this for [counting and] offset. Trying to bring some sanity by a lightweight wrapping."
NB: I found out about this from Michael Snoyman's (very much entertaining, and too much true) https://www.snoyman.com/blog/2020/12/haskell-bad-parts-3/
Best regards, J.W. _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.
-- David Duke Emeritus Professor of Computer Science School of Computing University of Leeds UK E:duke.j.david@gmail.com W:https://engineering.leeds.ac.uk/staff/334/Professor_David_Duke
participants (2)
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David Duke -
Johannes Waldmann