[Haskell-cafe] Re: Datatypes - Haskell
Richard A. O'Keefe [EMAIL PROTECTED] writes:
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 [EMAIL PROTECTED]
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Re: [Haskell-cafe] Re: Datatypes - Haskell
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
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[Haskell-cafe] Re: Datatypes - Haskell
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
templateclass A, class B
struct Tuple
{
enum (One, Two) tag;
union
{
A either_one;
struct nOne
{
A either_two
B two;
};
};
}
Am I wrong. If not, how can I use it in the corresponding function in C++?
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, isn't it?
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.
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Re: [Haskell-cafe] Re: Datatypes - Haskell
On Feb 10, 2008 3:40 PM, Mattes Simeon [EMAIL PROTECTED] wrote:
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 class A, class B
class Tuple {
public:
static TupleA, B *One (A *a) { return new One (a); }
static TupleA, B *Two (A *a, B *b) { return new Two (a, b); }
virtual A *tuple1 () = 0;
virtual B *tuple2 () = 0;
};
template class A, class B
class One : TupleA, B
{
public:
One (A *a) { this-a = a; }
A *tuple1 () { return a; }
B *tuple2 () { return NULL; }
private:
A *a;
}
template class A, class B
class Two: TupleA, B
{
public:
Two (A *a, B *b) { this-a = a; this-b = b}
A *tuple1 () { return a; }
B *tuple2 () { return b; }
private:
A *a;
B *b;
}
--
vir
http://vir.comtv.ru/
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Re: [Haskell-cafe] Re: Datatypes - Haskell
G'day all.
On Feb 10, 2008 3:40 PM, Mattes Simeon [EMAIL PROTECTED] wrote:
Though in comparison with C or C++ I can't figure out so clear the syntax.
Quoting Victor Nazarov [EMAIL PROTECTED]:
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 boost/variant.hpp
templatetypename A, typename B
class Tuple : public boost::variantA, std::pairA,B
{
private:
typedef std::pairA,B pair_type;
typedef boost::variantA, pair_type base_type;
struct visitor_A : public boost::static_visitorconst A*
{
const A* operator()(const A a) { return a; }
const A* operator()(const pair_type p) { return p.first; }
};
public:
Tuple(const A a)
: base_type(a)
{
}
Tuple(const A a, const B b)
: base_type(pair_type(a,b))
{
}
const A* tuple1() const
{
return boost::apply_visitor(visitor_A(), *this);
}
const B* tuple2() const
{
const pair_type* p = boost::getpair_type(*this);
return p ? p-second : 0;
}
};
But in this specific case, this might be more appropriate:
templatetypename A,typename B
class Tuple
{
A m_a;
boost::optionalB m_b;
// etc
};
Cheers,
Andrew Bromage
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Re: [Haskell-cafe] Re: Datatypes - Haskell
Mattes Simeon [EMAIL PROTECTED] wrote: 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
