[Haskell-cafe] Datatypes - Haskell
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) Thanks ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Datatypes - Haskell
On Feb 9, 2008, at 19:09 , Mattes Simeon wrote:
e.g. data Pair a b = Pair a b
struct Pair { a pair_a; b pair_b; };
data Either a b = Left a | Right b
union Either { enum { Left, Right } _tag; a either_left; b
either_right; };
(except that Haskell makes sure you use it properly, while C will let
you access foo.either_right when foo._tag == Left).
--
brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED]
system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED]
electrical and computer engineering, carnegie mellon universityKF8NH
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Re: [Haskell-cafe] Datatypes - Haskell
On Feb 10, 2008 12:09 AM, Mattes Simeon <[EMAIL PROTECTED]> wrote:
> 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
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