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Predicates for analyzing/constructing terms

Vocabulary

  • analyzing/constructing a term or
    • disassembling/assembling a term or maybe
    • decomposing/composing a term
  • In Clojure there is also "Destructuring", which is pattern-matching a term and disassembling it into parts like one-way unification. In Prolog this would by copy_term, followed by unification.
    • This is also how calls work in Erlang
    • The Picat language both has both unification and pattern matching in the head of clauses.

Note these definitions from the Prolog ISO standard's glossary on pages 2-10 (as listed here for example):

  • arity: The number of arguments of a compound term. Syntactically, a non-negative integer associated with a functor or predicate.
  • functor: An identifier together with an arity.
  • predicate: An identifier together with an arity.
  • predicate indicator: A compound term A/N, where A is an atom and N is a non-negative integer, denoting one particular procedure
  • procedure: A control construct, a built-in predicate, or a user-defined procedure. A procedure is either static or dynamic. A procedure is either private or public.

So, functor and predicate are used the same way: to designate a particular program construct which is implemented by a procedure ... whereas predicate indicator is a concrete structure used by a program, with a certain form: Name/Arity. Although I feel that functor is more a syntactic concept, whereas a predicate comes with a specification, though not necessarily the code of a procedure.

functor is now a concept in Category Theory (since '45 actually) which has nothing to do with the Prolog functor. It's also used in linguistics; maybe it got into Prolog from there?

Predicates

These are predicates found in the section Analysing and Constructing Terms of the SWI-Prolog manual.

And this is one is for distinguishing compound terms from atomic terms. Both are complements of each other: Anything that is not a fresh variable is either compound or atomic (at least in the current implementation). Atoms are a sublcass of the atomic terms and are tested with atom/1.

For dicts, it is best to use

Which is more elegant than compound_name_arguments/3

Overview

Analyzing and Constructing Terms

Other formats

A note from O'Keefe's "The Craft of Prolog"

In O'Keefe's "The Craft of Prolog" (MIT Press, 1990), we read on p. 145:

The first is the meta-logical operations functor/3 and arg/3, particularly the latter. A word of warning here: it is almost always a bad idea to use =../2. Do not use =../2 to locate a particular argument of some term.

For example, instead of

Term =.. [_F,_,ArgTwo|_]

you should write

arg(2, Term, ArgTwo)

You will find it easier to get the explicit number "2" right than to write the correct number of "don't care" variables in the call to =../2.

Other people reading your program will find the call to arg/3 a much clearer expression of your intent. Your program will also be more efficient. Even if you need to locate several arguments of a term, it is clearer and more efficient to write

arg(1, Term, First),
arg(3, Term, Third),
arg(4, Term, Fourth)

than to write

Term =.. [_,First,_,Third,Fourth|_]

Do not use ../2 when you know the functor. (That is, when you know both the function symbol and the arity.)

For example, one Prolog programmer actually wrote the following code:

add_date(OldItem, Date, NewItem) :- 
   OldItem =.. [item, Type, Ship, Serial],
   NewItem =.. [item,Type,Ship,Serial,Date].

This could have been expressed more clearly and more efficiently as

add_date(OldItem, Date, NewItem) :-
   OldItem = item(Type,Ship,Serial),
   NewItem = item(Type,Ship,Serial,Date).

or, even better, as

add_date(item(Type,Ship,Serial),Date,item(Type,Ship,Serial,Date)).