Comments on: Adding numbers http://conal.net/blog/posts/adding-numbers Inspirations & experiments, mainly about denotative/functional programming in Haskell Sat, 26 Sep 2020 21:06:12 +0000 hourly 1 http://wordpress.org/?v=4.1.17 By: conal http://conal.net/blog/posts/adding-numbers#comment-782 Sat, 06 Nov 2010 18:53:46 +0000 http://conal.net/blog/?p=240#comment-782 Daniel: thanks for the simple explanation.

As noted in the source code, if I use zipWith3' and init is, I get the first three digits out but not the last one.

]]> By: Daniel Fischer http://conal.net/blog/posts/adding-numbers#comment-781 Sat, 06 Nov 2010 13:45:49 +0000 http://conal.net/blog/?p=240#comment-781 “Note the mutual recursion in the two local definitions. I’m relying on the last element of is being dropped by zipWith3. I could instead pass in init is, but when I do so, no digits get out. I think the reason has to do with a subtlety in the definition of init.”

Not so subtle. zipWith3 pattern-matches on all three lists. (init is) is empty iff os is empty, so (assuming the digit lists to be nonempty), to decide whether it produces any output, it needs to see whether it produces any output: <>. The problem is that init can’t deliver an item until it knows that there’s a next one.

]]> By: augustss http://conal.net/blog/posts/adding-numbers#comment-780 Wed, 27 Oct 2010 08:37:14 +0000 http://conal.net/blog/?p=240#comment-780 Hmmm, I seem to remember that Russel O’Connor mentioned some problem with real numbers and data abstraction, but I can’t remember what. Ask him. Real numbers are beastly. :)

]]> By: conal http://conal.net/blog/posts/adding-numbers#comment-779 Tue, 26 Oct 2010 23:05:07 +0000 http://conal.net/blog/?p=240#comment-779 A Twitter exchange:

luqui: @conal, your addition is discontinuous. addsPR add10 (repeat 9) (repeat 0) False gives _|_. There are two contradictory right answers.

luqui: @conal, impossible to have continuous addition with stream of digits. First digit partitions R into 2 disjoint subsets, but R is connected.

conal: @luqui yep. fixable with redundant number system representation + data abstraction?

luqui: @conal, so I have heard. I tried for 3 or 4 hours today and couldn’t quite get it.

]]> By: conal http://conal.net/blog/posts/adding-numbers#comment-778 Tue, 26 Oct 2010 06:53:32 +0000 http://conal.net/blog/?p=240#comment-778 ja: Shhh. You’re getting ahead of the story.

]]> By: ja http://conal.net/blog/posts/adding-numbers#comment-777 Tue, 26 Oct 2010 00:11:15 +0000 http://conal.net/blog/?p=240#comment-777 You might also want to check out redundant number systems.

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