For example all functions with Int -> Int are type equivalent, because structural equivalency is used. Most of the time the functions are not compatible because they have different pre/post conditions, even though they seem to have the same input output types. I rather make my functions an instance of a type-class to tell the compiler that they can be used interchangably. For example sqrt :: Float -> Float sqrt a = /* pre : a >= 0 */ negate :: Float -> Float /* no precondition */ are different functions. You cannot just plug one in place of another (you can in Haskell because of structural type equivalency). Also a function working over (Int,Int) will do so even if the numbers are totally irrelevant to that function. They maybe the number of (apples,oranges) or number of (books,authors). However, data D a b = MkD a b data E a b = MkE a b (D Int Int) and (E Int Int) are trated as different because of name equivalency testing. Basically there are 2 type constructors -> ( , ) with structural equivalency, which is odd. Someone just dumped some idiosyncracies of lamda calculus into the language. Thanks again.
Cagdas Ozgenc writes: | Greetings. | | I know 2 special type constructors(there might be other that I do | not know yet) -> and ( , ) where structural type equivalency is | enforced and we can also create new types with an algebric type | constructor notation where name equivalency is enforced. | | What is the rationale? I mean why 2 special type constructors, but | not 5, or 10 or N? | | Thanks for taking time.
-> and (,) are names of type contructors, each with kind *->*->*.
Offhand, I don't see any difference in the way we'd test equivalency involving -> or (,) or D, where:
data D a b = MkD a b
If that doesn't help, will you please give a more concrete example of what you mean?
Regards, Tom