read_term(+Stream, ?Term, +Options)

Stream

Integer (stream number) or Atom (reserved or user-defined symbolic stream name)

Term

An term, usually a variable

Options

List of option terms

Calling Modes

• read_term(+, -, +)
• read_term(+, +, +)

Description

This is a generalisation of the predicates read/2 and readvar/3. Options is a (possibly empty) list of the following options:

variables(Vars)

returns a duplicate-free list of all the variables in the term that has been read (including anonymous variables).

variable_names(VarsNames)

returns a duplicate-free list of structures of the form Name=Var, where Var is a named (non-anonymous) variable which occurs in the term that has been read, and Name is an atom, representing the source name.

singletons(VarsNames)

returns a list of structures of the form Name=Var, where Var is a named (non-anonymous) variable which occurs only once in the term that has been read, and Name is an atom, representing the source name.

Exceptions

(4) instantiation fault

Stream is not instantiated.

(5) type error

Stream is not an atom or an integer.

(5) type error

Options is not a list of compound terms.

(6) out of range

Options list contains a unrecognised option.

(192) illegal stream mode

Stream is not an input stream.

(193) illegal stream specification

Stream is an illegal stream specification.

Examples

	?- read_term(T, []).
	 foo(X,_,bar(X,Y,_Z)).

	T = foo(X, _255, bar(X, Y, _Z))


	?- read_term(T, [variable_names(VN)]).
	 foo(X,_,bar(X,Y,_Z)).

	T = foo(X, _260, bar(X, Y, _Z))
	VN = ['X' = X, 'Y' = Y, '_Z' = _Z]


	?- read_term(T, [variables(V),variable_names(VN),singletons(S)]).
	 foo(X,_,bar(X,Y,_Z)).

	 T = foo(X, _278, bar(X, Y, _Z))
	 V = [X, _278, Y, _Z]
	 VN = ['X' = X, 'Y' = Y, '_Z' = _Z]
	 S = ['_Z' = _Z, 'Y' = Y]

See Also