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? (*) More questions. I learned how to define (++), and then I wanted to see how (:) would be defined. The Haskell 98 Report mentions that -- The (:) operator is built-in syntax, and cannot legally be given -- a fixity declaration; but its fixity is given by: -- infixr 5 : What does ``built-in syntax'' mean? Paul Hudak, in ``The Haskell School of Expression'' mentions that he defines (:) legally, in Appendix A. After writing data [a] = [] | a : [a] -- more pseudo-code infixr 5 : and making a couple of observations about it, he writes: ``The way (:) is defined here is actually legal syntax. Infix constructors are permitted in *data* declarations, and are distinguished from infix operators (for pattern-matching purposes) by the fact that they must begin with a colon (a property trivially satisfied by ":").'' So I'm not sure what ``legal syntax'' and ``pseudo-code'' exactly mean. The program
module Main where
data [a] = [] | a : [a] infixr 5 :
main = putStrLn "hello world"
gives %runhugs.exe Colon.lhs runhugs: Error occurred ERROR "Colon.lhs":3 - Syntax error in data declaration (unexpected `[') (*) The 5. What does that 5 do in ``infixr 5 :''?