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.
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).