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
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] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH
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
Thanks for your help. It was very useful.
Though in comparison with C or C++ I can't figure out so clear the syntax.
Maybe it has to do with the syntactic Sugar of each Language. I 'll give you a
similar example I saw in a book for Haskel
The following program just returns the value of the position of a datatype Tuple
which can hold one or two elements.
data Tuple a b = One a | Two a b
tuple1 (One a)= Just a
tuple1 (Two a b) = Just a
tuple2 (One a) = Nothing
tuple2 (Two a b) = Just b
The corresponding Version in C++, which seems to be more appropriate, would be
template
On Feb 10, 2008 3:40 PM, Mattes Simeon
Thanks for your help. It was very useful.
Though in comparison with C or C++ I can't figure out so clear the syntax. Maybe it has to do with the syntactic Sugar of each Language. I 'll give you a similar example I saw in a book for Haskel
The following program just returns the value of the position of a datatype Tuple which can hold one or two elements.
data Tuple a b = One a | Two a b tuple1 (One a)= Just a tuple1 (Two a b) = Just a
tuple2 (One a) = Nothing tuple2 (Two a b) = Just b
The corresponding Version in C++, which seems to be more appropriate, would be
I think this is the most native way to do it in C++:
template
G'day all.
On Feb 10, 2008 3:40 PM, Mattes Simeon
Though in comparison with C or C++ I can't figure out so clear the syntax.
Quoting Victor Nazarov
I think this is the most native way to do it in C++:
Herb Sutter and Andrei Alexandrescu will find you and beat you up if
you write this. This is considered more appropriate. (Warning: untested
code follows.)
#include
Mattes Simeon
Though in comparison with C or C++ I can't figure out so clear the syntax...I seems realy strange, and I'm confused.
Surely a solution to this would be to use the standard types of Haskel for tuples and check out each time if I have just a number or a tuple. But this is how somebody thinks in imperative languages. Functional programming is something more...
The data declarations in Haskell are well-suited to pattern matching -- you have a compact way to express each alternative, so you can match on that alternative. The C++ way offers a *uniform* interface to every alternative, so you can do if-tests that will type check. The "do an if test at runtime" way is very general, but basically unoptimizable; whereas pattern matching is very specific, and can be optimized (see page 5 of Luca Cardelli's [Compiling ML]).
Sorry for beeing so naive, but although unions, enum, structure are just some tools in C, surely something more in C++, in Haskell they are seem to be a standard.
It's certainly true that Haskell elevates certain common data structures to the level of 'native citizens', providing short cut syntax and so forth. Personally, I think C's approach is an example of neglect in this domain, not sparseness; however, at the time C was introduced, I would not have said the same thing at all. -- _jsn [Compiling ML]: http://lucacardelli.name/Papers/CompilingML.pdf
On the subject of data types, I've recently seen Haskell code using data Foo ... = Foo { ... } where I would have used newtype instead of data. When is it a good idea to avoid newtype?
ok:
On the subject of data types, I've recently seen Haskell code using data Foo ... = Foo { ... } where I would have used newtype instead of data. When is it a good idea to avoid newtype?
It depends what's in the ... If its just something with the same representation as an existing type, using a newtype makes sense. If it builds a more complicated single-contructor type, such as many record types, then data is required. -- Don
"Richard A. O'Keefe"
On the subject of data types, I've recently seen Haskell code using data Foo ... = Foo { ... } where I would have used newtype instead of data. When is it a good idea to avoid newtype?
When the code was written before newtype was introduced into the language? Also when { ... } is not a single type. -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk
participants (9)
-
ajb＠spamcop.net -
Brandon S. Allbery KF8NH -
Don Stewart -
Jason Dusek -
Jon Fairbairn -
Mattes Simeon -
Richard A. O'Keefe -
Sebastian Sylvan -
Victor Nazarov