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: tom.schrijvers@cs.kuleuven.be