On Tue, 18 Jul 2006, Jared Warren wrote:
>> % defining natural numbers
>> natural(zero).
>> natural(s(X)) :- natural(X).
>>
>> % translate to integers
>> toInt(zero, 0).
>> toInt(s(X), N) :- toInt(X, Y), N is Y + 1.
> Thank you. I can now more precisely state that what I'm trying to
> figure out is: what is 's', a predicate or a data structure? If it's a
> predicate, where are its instances? If not, what is the difference
> between the type language and Prolog such that the type language
> requires data structures?
it's data structure, to be exact, it's data constructor - just like,
for example, "Just" in Haskell. Haskell requires that all data
constructors should be explicitly defined before they can be used. you
can use "Just" to construct data values only if your program declares
"Just" as data constructor with "data" definition like this:
data Maybe a = Just a | Nothing
Prolog is more liberate language and there you can use any data
constructors without their explicit declarations, moreover, there is
no such declarations anyway
[deletia]
i once said you about good paper on type-classes level programming. if
you want, i can send you my unfinished article on this topic which shows
correspondences between logic programming, type classes and GADTs
So predicates and data constructors have similar syntax but different
semantics in Prolog?
That is right. The syntax and naming are different from Haskell though.
They are called respectively predicate and function symbols/functors, and
written like f(x1,...,xn) rather than F x1 ... xn. Terms are made of
possibly nested function symbols and variables, e.g.
f(f(f(a,X))) where f and a are used as function symbols
Atoms are made of a predicate symbol and terms,e.g.
p(f(f(f(a,X)))) where p is used as a predicate symbol
f and a are used as function symbols
Note that the same name can be used with multiple argument numbers and
both as a predicate and a function symbol.
Say, for the sake of argument, if I wanted to do
automatic translation, how would I tell which was which in a Prolog
program?
Based on the syntax. Basically the top-level symbols are predicate symbols
and the nested ones are function symbols, e.g. in a clause
p(f(X)) :-
q(h(i)),
j(k) = l(m).
p, q and '=' are predicate symbols and all the rest are function symbols.
It's probably good to read a bit about it in a proper reference.
Wikipedia's Prolog entry lists a number of tutorials.
Cheers,
Tom
--
Tom Schrijvers
Department of Computer Science
K.U. Leuven
Celestijnenlaan 200A
B-3001 Heverlee
Belgium
tel: +32 16 327544
e-mail: [EMAIL PROTECTED]
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