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get_var_info(?Var, ?InfoName, -Value)

Succeeds if Var is an uninstantiated variable, InfoName is a valid information name and the information value Value unifies with the value of the information.
Prolog Term, usually a variable.
Atom or variable.
Atom or variable.


This predicate is used to test the type of a free variable or to get its source name (when available).

The possible values for InfoName are type or name.

If InfoName unifies with name, the predicate will succeed if the source name of the free variable Var is available and unifies with Value.

If InfoName unifies with type, the predicate will succeed if Value unifies with the type of the variable Var.

The possible types are:

    meta      an attributed variable (metaterm)
    free      a free variable

Modes and Determinism

Fail Conditions

Fails if Var is not a variable


(5) type error
InfoName or Value is neither an atom nor a variable.
(6) out of range
InfoName is not an information name.


      ?- suspend:(Var1 > Var2), get_var_info(Var1, Info, Value).
      Var2 = Var2
      Var1 = Var1
      Info = name
      Value = 'Var1'
      Delayed goals:
       Var1 > Var2     More? (;)

      Var2 = Var2
      Var1 = Var1
      Info = type
      Value = meta

      Delayed goals:
       Var1 > Var2

      ?- get_var_info(Var1, type, Type).
      Var1 = Var1
      Type = free

      ?- set_flag(variable_names,off).

      ?- get_var_info(Var1, name, Name).
      no (more) solution.

      get_var_info(atom, name, Name).
      set_flag(variable_names, off),
          get_var_info(X, name, Name).

      get_var_info(X, not_an_info, Y).    (Error 6)
      get_var_info(X, type, 123).         (Error 5)

See Also

set_flag / 2, type_of / 2, var / 1