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 [1] http://hpaste.org/40033/operational_semantics [2] http://conal.net/blog/posts/lazier-functional-programming-part-2/ -- David Sankel Sankel Software www.sankelsoftware.com