On: Who owns the fish? A Common Lisp solution to "Ein...

[Crito]

Hmm, seems like this could potentially be solved in a cool way using the amb operator. Might have to give that a shot later today.

[japr59 → Crito]

Definitely. For anybody interested, http://mitpress.mit.edu/sicp/full-text/sicp/book/node90.html offers a nice introduction to the amb operator.

One issue that I found when solving problems like this is that, to achieve speed, it can become necessary to use many nested lets. That could hurt readability. Nobody wants to see a line of code nested 20 tabs deep!

[tonetheman → japr59]

Oh node programmers love that... :)

[twfarland → Crito]

Haskell lists also work nicely for nondeterministic computation.

[andolanra → twfarland]

Indeed, we can use Haskell lists as the amb operator and implement require as

    require :: Bool -> [()]
    require True  = return ()
    require False = []

and then express amb-like computations in effectively the same way:

    sample :: [(Int, Int)]
    sample = do
      x <- [1,2,3,4,5]
      y <- [1,2,3,4,5]
      require (x == 2 * y)
      return (x, y)

after which the value of sample is [(2,1),(4,2)]. (Also, my require is effectively the same as the guard function found in Control.Monad, specialized for lists.)