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readvar(+Stream, -Term, -VarList)

Succeeds if the next Prolog term from the input stream Stream is successfully read and unified with Term, and any variables in Term are collected in the list VarList, together with their names.

Stream: Stream handle or alias (atom)
Term: Prolog term.
VarList: A Variable.

This built-in predicate is obsolete!

Description

This predicate has been superseeded by read_term/3 with the variable_names(VarsNames) option.

Used to read the next term from the input stream Stream, unify it with Term and store any variables in Term to the list VarList. This is a list of pairs in the format [VarName|Var].

VarName is the literal input variable name expressed as an atom; Var is the variable. The first element of the pair Varname is the atom corresponding to the variable name, and the second element Var is the corresponding variable.

If there is more than one Prolog term in the file, the term must be in Prolog term format i.e. terminated by a period and a blank space character, neither of which are retained by Prolog.

Modes and Determinism

readvar(+, -, -) is semidet

Modules

This predicate is sensitive to its module context (tool predicate, see @/2).

Fail Conditions

Fails if a syntax error was detected and no term could be read

Exceptions

(4) instantiation fault: Stream is not instantiated.
(5) type error: Stream is not an atom or a stream handle, Varlist is not a variable.
(190) end of file reached: End of file was encountered before reading any character.
(192) illegal stream mode: Stream is not an input stream.
(193) illegal stream specification: Stream is an illegal stream specification.
(198) reading past the file end: Trying to read even after the error 190 was raised.

Examples

Success:
      ?- readvar(input,Term,VarList).
       atom.
      Term = atom
      VarList = []
      yes.

      ?- readvar(input,T,L).
       X.
      T = _50
      L = [['X'|_50]]
      yes.

      ?- system('cat file1').
      f(X,Y).
      g(1,X).
      yes.
      ?- open(file1,update,r), readvar(r,T1,V1),
         readvar(r, T2,V2).
      T1 = f(_120, _122)
      V1 = [['X'|_120], ['Y'|_122]]
      T2 = g(1, _146)              % the clauses are separate,
      V2 = [['X'|_146]]            % so the X's are different.
      yes.

Fail:
      ?- readvar(input, X + 2,V).
       X + 1.
      no.

Error:
      readvar(S,a(b,c),V).          (Error 4).
      readvar("string",a(b,c),V,).  (Error 5).
      readvar(output,X + 2,V).      (Error 192).
      readvar(atom,X + 2,V).        (Error 193).