On 5 Jul 2007, at 18:35, Chung-chieh Shan wrote:
[EMAIL PROTECTED] wrote in article
<[EMAIL PROTECTED]> 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
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