The following code (full code available here[1], example taken from comments here[2]),

const' = \a _ -> a

test1 c = let a = const' (nthPrime 100000)
in (a c, a c)

test2 c = let a = \_ -> (nthPrime 100000)
in (a c, a c)

in a ghci session (with :set +s):

*Primes> test1 1
(9592,9592)
(0.89 secs, 106657468 bytes)
*Primes> test2 1
(9592,9592)
(1.80 secs, 213077068 bytes)

test1, although denotationally equivalent to test2 runs about twice as fast.

My questions are:

What is the optimization that test1 is taking advantage of called?
Is said optimization required to conform to the Haskell98 standard? If so, where is it stated?
Could someone explain or point to a precise explanation of this optimization? If I'm doing an operational reduction by hand on paper, how would I take account for this optimization?

Thanks,

David

--
David Sankel
Sankel Software
www.sankelsoftware.com