On Thu, 2011-03-03 at 11:39 +0530, Karthick Gururaj wrote:
What is the "()" type? Does it refer to a tuple? How can tuple be ordered, let alone be enum'd? I tried:
The () type is pronounced "unit". It is a type with only 1 value, also called () and pronounced "unit". Since it only has one possible value, it conveys no information at all, and is sometimes used in situations analogous to C's 'void' keyword. Okay, actually that was a little bit of a lie; () has two "values": () and bottom. Bottom is the "value" that corresponds to the program hanging in an infinite loop or dying with an error message. But if you have an actual, honest-to-goodness value that's not bottom, it has to be ().
But, surprise: Prelude> (1,1) > (1,2) False Prelude> (2,2) > (1,1) True Prelude> (1,2) > (2,1) False Prelude> (1,2) < (2,1) True
Okay, so this is no longer Enum, but just Ord. The ordering defined in the Ord instance for tuples is the normal lexicographic order: the comparison between the first elements dominates; but if the first elements coincide, then the second are compared instead. For larger tuple types, the same pattern continues. Think of it like organizing words in alphabetical order, where here you know the words all have the same number of letters.
Ok. Works. But on a non-commutative operation like division, we get: Prelude> let x = (/) 20.0 Prelude> x 10 2.0 Prelude> let y = (/20.0) Prelude> y 10 0.5
So a curried infix operator fixes the first argument and a "sectioned infix" operator fixes the second argument?
Sections can be either left sections or right sections, so you can pick which argument is provided. Prelude> let y = (/ 20.0) Prelude> y 10 0.5 Prelude> let y = (20.0 /) Prelude> y 10 2.0 Hope that helps, -- Chris Smith