term_string/2 results in a syntax error if the string starts with '?'

[Till Hofmann]

I'm trying to convert a string that starts with a '?' into a term:

?- term_string(T, "?x").
ERROR: Syntax error: Operator expected
ERROR:
ERROR: ** here **
ERROR: ?x .

I would expect the result to be the same as with atom_string/2:

?- atom_string(A, "?x").
A = '?x'.

I guess the error occurs because term_string actually parses the string, as the documentation mentions:

If String is instantiated, it is parsed and the result is unified with Term

 

However, I still wouldn't expect a syntax error as result. Is this a bug or is my expectation wrong?

[Raivo Laanemets]

It's a syntax error directly on console too:

?- A = ?x.

ERROR: Syntax error: Operator expected

ERROR: A = 

ERROR: ** here **

ERROR: ?x .

Atom must be quoted if it contains any special characters.

[Till Hofmann]

On Thursday, January 5, 2017 at 11:40:36 AM UTC+1, Raivo Laanemets wrote:

It's a syntax error directly on console too:

?- A = ?x.

ERROR: Syntax error: Operator expected

ERROR: A = 

ERROR: ** here **

ERROR: ?x .

Atom must be quoted if it contains any special characters.

Yes, and I would expect term_string/2 to quote the atom appropriately, just like atom_string/2 does.

[Jan Wielemaker]

term_string/2 is (de)serialization of a Prolog term to/from text (a
string). That means the text must have valid Prolog syntax. If you
want an atom, use atom_string/2. If you want an atom as fallback,
use catch/3 around term_string, as in:

catch(term_string(Term, String),
error(syntax_error(_),_),
atom_string(Term, String)).

I can't see many cases where this is a smart idea ...

Cheers --- Jan

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