On 5 Jul 2007, at 18:35, Chung-chieh Shan wrote:
oleg@pobox.com wrote in article <20070705061533.8965EAD43@Adric.metnet.fnmoc.navy.mil> in gmane.comp.lang.haskell.cafe:
Conor McBride has posed an interesting problem:
implement constructors P v for embedding pure values v O for holes f :$ a for application, left-associative and an interpreting function emmental such that emmental (P (+) :$ (P (*) :$ O :$ P 10) :$ O) 4 2 = 42
Hrm! I don't see the original message where the problem was posed, but it is indeed interesting. Here is my solution, but I don't need "P", "O", and ":$" to be constructors, so I rename them to "p", "o", and "$:", respectively:
emmental m = m id p x k = k x o k = k (m $: n) k = m (\f -> n (\x -> k (f x)))
infixl 0 $: test = emmental (p (+) $: (p (*) $: o $: p 10) $: o) 4 2 -- 42
Very nice! Thanks Conor