Using a Prolog DCG to split a string

I'm trying to use a DCG to split a string into two parts separated by spaces. E.g. 'abc def' should give me back "abc" & "def". The program & DCG are below.

main:-
    prompt(_, ''),
    repeat,
    read_line_to_codes(current_input, Codes),
    (
        Codes = end_of_file
    ->
        true
    ;
        processData(Codes),
        fail
    ).

processData(Codes):-
    (
        phrase(data(Part1, Part2), Codes)
    ->
        format('~s, ~s\n', [ Part1, Part2 ])
    ;
        format('Didn''t recognize data.\n')
    ).

data([ P1 | Part1 ], [ P2 | Part2 ]) --> [ P1 | Part1 ], spaces(_), [ P2 | Part2 ].
spaces([ S | S1 ]) --> [ S ], { code_type(S, space) }, (spaces(S1); "").

This works correctly. But I found that having to type [ P1 | Part1 ] & [ P2 | Part2 ] was really verbose. So, I tried replacing all instances of [ P1 | Part1 ] w/ Part1 & likewise w/ [ P2 | Part2 ] in the definition of data, i.e. the following.

data(Part1, Part2) --> Part1, spaces(_), Part2.

That's much easier to type, but that gave me an Arguments are not sufficiently instantiated error. So it looks like an unbound variable isn't automatically interpreted as a list of codes in a DCG. Is there any other way to make this less verbose? My intent is to use DCG's where I would use regular expressions in other programming languages.

Your intuition is correct; the term-expansion procedure for DCGs (at least in SWI-Prolog, but should apply to others) with your modified version of data gives the following:

?- listing(data). 

data(A, D, B, F) :-
    phrase(A, B, C),
    spaces(_, C, E),
    phrase(D, E, F).

As you can see, the variable Part1 and Part2 parts of your DCG rule have been interpreted into calls to phrase/3 again, and not lists; you need to explicitly specify that they are lists for them to be treated as such.

I can suggest an alternative version which is more general. Consider the following bunch of DCG rules:

data([A|As]) --> 
    spaces(_), 
    chars([X|Xs]), 
    {atom_codes(A, [X|Xs])}, 
    spaces(_), 
    data(As).
data([]) --> [].

chars([X|Xs]) --> char(X), !, chars(Xs).
chars([]) --> [].

spaces([X|Xs]) --> space(X), !, spaces(Xs).
spaces([]) --> [].

space(X) --> [X], {code_type(X, space)}. 
char(X) --> [X], {\+ code_type(X, space)}.

Take a look at the first clause at the top; the data rule now attempts to match 0-to-many spaces (as many as possible, because of the cut), then one-to-many non-space characters to construct an atom (A) from the codes, then 0-to-many spaces again, then recurses to find more atoms in the string (As). What you end up with is a list of atoms which appeared in the input string without any spaces. You can incorporate this version into your code with the following:

processData(Codes) :-
    % convert the list of codes to a list of code lists of words
    (phrase(data(AtomList), Codes) ->
        % concatenate the atoms into a single one delimited by commas
        concat_atom(AtomList, ', ', Atoms),
        write_ln(Atoms)
    ;
        format('Didn''t recognize data.\n')
    ).

This version breaks a string apart with any number of spaces between words, even if they appear at the start and end of the string.