Daniel C. Bastos wrote:
There is something called infix constructors and something else called infix operators. I'm guessing that an infix operator is really a function, and an infix constructor I don't know what it is. How would you guys describe them?
An "infix operator" is literally a normal function with a funny name, for example (++). If a function name consists only of symbols, not letters, it works infix instead of prefix. On the other hand, you can take a normal function name and make it infix using backticks: filter odd [1,2,3] odd `filter` [1,2,3] An "infix constructor" is a constructor rather than a function. Normally, functions start with a lowercase letter, and constructors start with an uppercase letter. But when the names are symbols, everything becomes infix, and an infix constructor starts with a ":", and an infix function starts with any other symbol. data Tree x = Leaf x | Branch (Tree x) (Tree x) data Tree x = Leaf x | (Tree x) :## (Tree x)
I learned how to define (++), and then I wanted to see how (:) would be defined.
It's a constructor. Like Leaf and Branch in the example above. If the list type wasn't already defined, you could define it as data List x = Node x (List x) | End and then write lists as Node 1 (Node 2 (Node 3 End)) However, that's a lot of typing, so the actual definition looks more like data [x] = x : [x] | [] But that won't compile, because it doesn't obey the syntax rules of Haskell. You could, however, write data List x = x : (List x) | End and it would work. 1 : (2 : (3 : End))
What does ``built-in syntax'' mean?
The way lists are written (most especially the "[1,2,3]" syntax) is hard-wired into the compiler and cannot be changed. (Similarly for strings, actually.)
The program
data [a] = [] | a : [a] infixr 5 :
gives
%runhugs.exe Colon.lhs runhugs: Error occurred ERROR "Colon.lhs":3 - Syntax error in data declaration (unexpected `[')
It doesn't like you calling the type "[x]". If you call it "List x" like I showed above, it should work.
What does that 5 do in ``infixr 5 :''?
5 is the precedence. You know how "2 + 3 * 4" is treated as "2 + (3 * 4)"? That's because (*) has a higher precedence than (+).