On Feb 10, 2008 12:09 AM, Mattes Simeon
Hello to everybody
I am an new user of Haskel and generally in functional programming and I could say that I am very impressed from this Language. Though I can't understand the use of datatypes.
Let's take a firly simple situtation
e.g. data Pair a b = Pair a b
i.e. an new type with name Pair parameterized over the types a,b with one Constructor named Paid which take two values of type a,b
a more complex one would be data Either a b = Left a | Right b
i.e a new type named Either parameterized over the types a, b and two Constructors 1. Left which take one value of type a and 2. Right which takes one value of type b
I consider that the definitions above are well formulated. Nevertheless I can't understand them quite well.
I have read that datatypes are used to define new structures. So, is there any corresponding example in C, sinch I am quite familiar with structures in it? I hope the word C here is allowed here :o)
I guess C++ would be closer... template< class A, class B> struct Pair { A fst; B snd; } template< class A, class B> struct Either { enum {Left, Right} tag; union{ A left; B right; }; } In the second example the tag would be used to figure out which of the two alternatives the structure actually is. In Haskell you can just pattern match on the constructor. -- Sebastian Sylvan +44(0)7857-300802 UIN: 44640862