Re: [Haskell-cafe] Course-of-value recursion by defining a sequence as a self-referential infinite list

I came up with a one-liner for computing coefficients of the generating function for integer partitions: part :: Int → [Integer ] part n = take n $ product [cycle (1 : replicate n 0) | n ← [0 . . (n − 2)]] Karczmarczuk’s solution via the Haskell prelude: part = 1 : b 1 where b n = (1 : b (n + 1)) + (replicate n 0 ++ b n) cycle and replicate are really underrated!

On Jan 20, 2025, at 8:54 AM, Douglas McIlroy wrote:

...

catalanNumbers :: Num a => [a] catalanNumbers = let xs = 1 : PowerSeries.mul xs xs in xs

This example of a generating function come to life as a program deserves to be better known. Bill Burge presented it 50 years ago in "Recursive Programming Techniques", Addison-Wesley, 1975. I revisited it in "Power series, power serious", JFP 9 (1999) 323-335, where, with overloadied arithmetic, it became ts = 1 : ts^2 The technique is laid bare in ten one-liners at https://www.cs.dartmouth.edu/~doug/powser.html.

Doug _______________________________________________ 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.