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% @(#)umsterm.tex	1.3 94/01/11
%
%
% umsterm.tex
%
% REL	DATE	AUTHOR		DESCRIPTION
% 2.10	080489	David Miller 	convert to Latex and update for rel 2.10
%
\chapter{Terminology}
\label{terminology}
\label{chapterm}
%HEVEA\cutdef[1]{section}

This chapter defines the terminology which is used throughout the manual and
in related documentation.

\begin{description}

% -------------------------------------------------------------------
\item[\Index{+X}]
This denotes an input argument. Such an argument must be instantiated before
a built-in is called.

% -------------------------------------------------------------------
\item[\Index{++X}]
This denotes a ground argument. Such an argument can be complex,
but must be fully instantiated, i.e., not contain any variables.

% -------------------------------------------------------------------
\item[\Index{--X}]
This denotes an output argument. Such an argument must be not
instantiated before a built-in is called.

% -------------------------------------------------------------------
\item[\Index{?X}]
This denotes an input or an output argument. Such an argument may be either
instantiated or not  when a built-in is called.

% -------------------------------------------------------------------
%\item[Array]
%\index{Array}
%An array specification is a compound term the form Name(Dim_1,...,Dim_n),
%where Name is the array name, the arity denotes the number of dimensions and
%the arguments are integers indicating the size in each dimension.
%An array element is selected with a similar term.

% -------------------------------------------------------------------
\item[Arity\index{arity}]
Arity is the number of arguments to a term.
Atoms are considered as functors with zero arity.
The notation \patternidx{Name/Arity}
is used to specify a functor by giving its name and arity.

% -------------------------------------------------------------------
\item[Atom\index{atom}]
An arbitrary name chosen by the user to represent objects from the
problem domain.
A Prolog atom corresponds to an identifier in other languages.  It can be
written as a conventional identifier (beginning with a lower-case letter), or
a character sequnce enclosed in single quotes.

% -------------------------------------------------------------------
\item[Atomic\index{atomic}]
An atom, string or a number. A term which does not contain other terms.

% -------------------------------------------------------------------
\item[Body\index{body of a clause}\index{clause!body}]
A clause body can either be of the form
\begin{quote}
\begin{verbatim}
Goal_1, Goal_2, ..., Goal_k
\end{verbatim}
\end{quote}
or simply
\begin{quote}
\begin{verbatim}
Goal
\end{verbatim}
\end{quote}
Each \about{Goal_i} must be a \about{callable term}.

% -------------------------------------------------------------------
\item[Built-in Procedures\index{built_in procedure}\index{procedure!built_in}]
These are predicates provided for the user by the
{\eclipse} system, they are either written in Prolog or in the implementation
language (usually C).

% -------------------------------------------------------------------
\item[Callable Term\index{callable term}\index{term!callable}]
A callable term is either a compound term or an atom.

% -------------------------------------------------------------------
\item[Clause\index{clause}]
See \about{program clause} or \about{goal clause}.

% -------------------------------------------------------------------
\item[Compound Term\index{compound term}\index{term!compound}]
Compound terms are of the form
\begin{quote}
\begin{verbatim}
f(t_1, t_2, ..., t_n)
\end{verbatim}
\end{quote}
where \about{f} is the \aboutidx{functor} of the compound term, \about{n} is its
arity and \about{t_i} are terms.
\about{Lists} and \about{pairs} are also compound terms.

% -------------------------------------------------------------------
\item[Constant\index{constant}\index{term!constant}]
An \about{atom}, a \about{number} or a \about{string}.

% -------------------------------------------------------------------
\item[Determinism\index{determinism}]
The determinism specification of a built-in or library predicate says
how many solutions the predicate can have, and whether it can fail.
The six determinism groups are defined as follows:
\begin{quote}
\begin{verbatim}
                |   Maximum number of solutions
    Can fail?   |   0               1               > 1
    ------------+------------------------------------------
    no          |   erroneous       det             multi
    yes         |   failure         semidet         nondet
\end{verbatim}
\end{quote}
This classification is borrowed from the Mercury programming language,
but in {\eclipse} only used for the purpose of documentation.
Note that the determinism of a predicate usually depends on its calling mode.

% -------------------------------------------------------------------
\item[\Index{DID}\index{dictionary identifier}]
Each atom created within {\eclipse} is assigned a unique
identifier called the \about{dictionary identifier} or \about{DID}.

% -------------------------------------------------------------------
\item[Difference List\index{difference list}\index{list!difference}]
A difference list is a special kind of a list.
Instead of being ended by \about{nil}, a difference list
has an uninstantiated tail so that new elements
can be appended to it in constant time.
A difference list is written as \about{List - Tail}
where \about{List} is the beginning of the list and \about{Tail}
is its uninstantiated tail.
Programs that use difference lists are usually more efficient
and always much less readable than programs without them.

% -------------------------------------------------------------------
\item[Dynamic Procedure\index{dynamic procedure}\index{procedure!dynamic}]
These are procedures which can be modified clause-wise, by adding or removing
one clause at a time. Note that this class of procedure is equivalent to
interpreted procedures in other Prolog systems. See also \about{static
procedures}.

%% -------------------------------------------------------------------
%\item[ElemSpec]
%\index{array}
%An \about{ElemSpec} specifies a global variable (an atom) or an array
%element (a ground compound term with as much arguments (integers) as
%the number of dimensions of the array).

% -------------------------------------------------------------------
\item[External Procedures\index{external procedure}\index{procedure!external}]
These are procedures which are defined in a language
other than Prolog, and explicitly connected to Prolog predicates by the user.

% -------------------------------------------------------------------
\item[Fact\index{fact}]
A fact or \aboutidx{unit clause}\index{clause!unit} is a term of the form:
\begin{quote}
\begin{verbatim}
Head.
\end{verbatim}
\end{quote}
where \notation{Head} is a \about{head}.

A fact may be considered to be a rule whose body is always \about{true}.

% -------------------------------------------------------------------
\item[Functor\index{functor}]
A functor is characterised by its name (which is an atom), and its arity
(which is its number of arguments).

% -------------------------------------------------------------------
\item[Goal Clause\index{clause!goal}\index{goal}]
See \about{query}.

% -------------------------------------------------------------------
\item[Ground\index{ground term}\index{term!ground}]
A term is ground when it does not contain any uninstantiated variables.

% -------------------------------------------------------------------
\item[Head\index{head of a clause}\index{clause!head}]
A clause head is a structure or an atom.

% -------------------------------------------------------------------
\item[Instantiated\index{instantiated variable}\index{variable!instantiated}]
A variable is instantiated when it has been bound to an atomic or a
compound term as opposed to being \about{uninstantiated}%
\index{uninstantiated variable}\index{variable!uninstantiated}
or \about{free}\index{free variable}\index{variable!free}.
See also \about{ground}.


% -------------------------------------------------------------------
\item[List\index{list}]
A list is a special type of term within Prolog. It is a
recursive data structure consisting of \about{pairs} (whose tails are lists).
A \about{list} is either the atom \notationidx{[]} called \notationidx{nil}
as in LISP,
or a pair whose tail is a list.
The notation :
\begin{quote}
\begin{verbatim}
[a , b , c]
\end{verbatim}
\end{quote}
is shorthand for:
\begin{quote}
\begin{verbatim}
[a | [b | [c | []]]]
\end{verbatim}
\end{quote}

% -------------------------------------------------------------------
\item[Mode\index{mode}]
A predicate mode is a particular instantiation pattern of its arguments
at call time.  Such a pattern is usually written as a predicate template, e.g.,
\begin{quote}
\begin{verbatim}
    p(+,-)
\end{verbatim}
\end{quote}
where the symbols \notation{+}, \notation{++}, \notation{-} and \notation{?}
represent
instantiated, ground, uninstantiated and unknown arguments respectively.

% -------------------------------------------------------------------
\item[Name/Arity]
The notation \patternidx{Name/Arity} is used to specify a functor by giving its
name and arity.

% -------------------------------------------------------------------
\item[Number\index{number}]
A number literal denotes a number, more or less like in all programming
languages.

% -------------------------------------------------------------------
\item[Pair\index{pair}]
A pair is a compound term with the functor \predspec{./2} (dot)
which is written as :
\begin{quote}
\begin{verbatim}
[H|T]
\end{verbatim}
\end{quote}
\about{H} is the \defnotionni{head}\index{head of a pair}\index{pair!head}
of the pair and \about{T} its
\defnotionni{tail}\index{tail of a pair}\index{pair!tail}.


% -------------------------------------------------------------------
%\item[Permanent Procedures]
%\index{procedure!permanent}
%These are procedures which cannot be changed in any way, they are statically
%linked to other procedure's calls.
%
% -------------------------------------------------------------------
\item[Predicate\index{predicate}]
A predicate is another term for a \about{procedure}.

% -------------------------------------------------------------------
\item[\Index{PredSpec}]
This is similar to \pattern{Name/Arity}.
Some built-ins allow the arity to be omitted and to specify the name only:
this stands for all (visible) predicates with that name and any arity.

% -------------------------------------------------------------------
\item[Program Clause\index{clause}\index{program clause}\index{clause!program}]
A program clause (or simply \about{clause}) is either the term
\begin{quote}
\begin{verbatim}
Head :- Body.
\end{verbatim}
\end{quote}
\index{body}
(i.e., a compound term with the functor \predspec{:-/2}), or only a fact.

% -------------------------------------------------------------------
\item[Query\index{query}]
A query  has the same form as a \about{body} and is also called a \about{goal}.
Such clauses occur mainly as input to the top level Prolog loop
and in files being compiled, then they have the form
\begin{quote}
\begin{verbatim}
:- Goal_1, ..., Goal_k.
\end{verbatim}
\end{quote}
or
\begin{quote}
\begin{verbatim}
?- Goal_1, ..., Goal_k.
\end{verbatim}
\end{quote}
The first of these two forms is often called a \aboutidx{directive}.

% -------------------------------------------------------------------
\item[Regular Prolog Procedure%
\index{regular procedure}\index{procedure!regular}]

A regular (Prolog) procedure is a sequence of user clauses whose heads
have the same functor, which then identifies the user procedure.


% -------------------------------------------------------------------
\item[Simple Procedures\index{simple procedure}\index{procedure!simple}]
Apart from regular procedures {\eclipse} recognises simple procedures
which are written not in Prolog but in the implementation language (i.e., C),
and which are deterministic.
There is a functor associated with each
simple procedure, so that
any procedure recognisable by {\eclipse} is identified by a functor,
\index{functor!of a procedure}\index{procedure!functor}
or by a compound term (or atom) with this functor.

% -------------------------------------------------------------------
\item[\Index{SpecList}]
The SpecList notation means a sequence of \about{PredSpec} terms of the form:
\begin{quote}
\begin{verbatim}
name_1/arity_1, name_2/arity_2, ..., name_k/arity_k.
\end{verbatim}
\end{quote}
The SpecList notation is used in many built-ins, for example,
to specify a list of procedures in the
\bipref{export/1}{../bips/kernel/modules/export-1.html} predicate.

% -------------------------------------------------------------------
\item[Static Procedures\index{static procedure}\index{procedure!static}]
These are procedures which can only be changed as a whole unit, i.e., removed or
replaced.

% -------------------------------------------------------------------
\item[Stream\index{stream}]
This is an I/O channel identifier and can be a physical stream number, one of
the predefined stream identifiers (\notation{input}, \notation{output},
\notation{error}, \notation{warning_output}, \notation{log_output},
\notation{null})
or a user defined stream name (defined using
\bipref{set_stream/2}{../bips/kernel/iostream/set_stream-2.html} or
 \bipref{open/3}{../bips/kernel/iostream/open-3.html}).

% -------------------------------------------------------------------
\item[String\index{string}]
A string is similar to those found in all other programming languages.  A string
is enclosed in double quotes.

% -------------------------------------------------------------------
\item[Structure\index{structure}]
Compound terms which are not pairs are also called \about{structures}.

% -------------------------------------------------------------------
\item[Term\index{term}]
A term is the basic data type in Prolog.
It is either a \about{variable}, a \about{constant} or a \about{compound term}.

% -------------------------------------------------------------------
\item[Variable\index{variable}\index{term!variable}]
A variable is more similar to a mathematical variable than to a variable in some
imperative language.  It can be free, or instantiated to a term, but once
instantiated it becomes indistinguishable from the term to which it was
instantiated: in particular, it cannot become free again (except upon
backtracking through the point of instantiation).
The name of a variable is written in the form of an identifier that begins with
an upper-case letter or with an underscore.  A single underscore represents an
\aboutidx{anonymous variable}\index{variable!anonymous} that has only one
occurrence (i.e., another occurrence of this name represents another variable).

% -------------------------------------------------------------------
\end{description}

The notation \pattern{Pred/N1, N2} is often used in this documentation
as a shorthand for \pattern{Pred/N1, Pred/N2}.

%HEVEA\cutend