Parsing and vetting the kinds of literal lists written in braces.

§1. Literal lists in Inform are written in braces: for example,

let L be {2, 12, 13};

Here "{2, 12, 13}" is parsed to an rvalue specification. Various syntactic things can go wrong with those commas and braces, which must be detected, and Inform must also work out the kind — here, it's "list of numbers".

Note that the empty list "{}" is valid as a constant, but that it contains no indication of its kind — this must be determined from context.

The following Preform grammar handles the syntax, at least:

<s-literal-list> ::=
    \{ \} |              ==> { -, Lists::at(Lists::empty_literal(Wordings::last_word(W)), W) }
    \{ <list-conts> \}   ==> { -, Lists::at(RP[1], W) }

<list-conts> ::=
    <list-entry> , <list-conts> | ==> { 0, Lists::add(RP[1], RP[2], W, R[1]) }
    <list-entry>                  ==> { 0, Lists::add(RP[1], Lists::empty_literal(W), W, R[1]) }

<list-entry> ::=
    <s-value> |          ==> { FALSE, RP[1] }
    ......               ==> { TRUE, Specifications::new_UNKNOWN(W) }

• This is Preform grammar, not regular C code.

§2. And the result of this grammar, if it matches, is an rvalue made thus:

parse_node *Lists::at?Usage of Lists::at:
§1(literal_list *L, wording W) {
    return Rvalues::from_wording_of_list(Lists::kind_of_ll(L, FALSE), W);
}

§3. Each different literal list (LL) found in the source text generates an instance of the following structure. Note that:

• (1) every LL structure represents a syntactically well-formed list, in which braces and commas balance; and
• (2) there can be at most one LL structure at any word position.

typedef struct literal_list {
    struct wording unbraced_text;  position in the source of quoted text, excluding braces
    struct parse_node *list_text;  used for problem reporting only
    int listed_within_code;  appears within a phrase, rather than (say) a table entry?

    struct kind *entry_kind;  i.e., of the entries, not the list
    int kinds_known_to_be_inconsistent;  problem(s) thrown when parsing these
    int explicitly_cast;  has reading a wider context determined the kind?
    struct llist_entry *first_llist_entry;  linked list of contents

    struct package_request *ll_package;  which will be the enclosure for...
    struct inter_name *ll_iname;
    CLASS_DEFINITION
} literal_list;

• The structure literal_list is private to this section.

§4. I believe "llath" is the Welsh word for "yard": not sure about "llist".

typedef struct llist_entry {
    struct parse_node *llist_entry_value;
    struct llist_entry *next_llist_entry;
    CLASS_DEFINITION
} llist_entry;

• The structure llist_entry is private to this section.

§5. These structures are built incrementally, adding one llist_entry at a time. They begin with a call to:

literal_list *Lists::empty_literal?Usage of Lists::empty_literal:
§1(wording W) {
    literal_list *ll = Lists::find_literal(Wordings::first_wn(W));
    if (ll == NULL) ll = CREATE(literal_list);
    ll->unbraced_text = W; ll->entry_kind = K_nil;
    ll->listed_within_code = FALSE;
    ll->kinds_known_to_be_inconsistent = FALSE;
    ll->ll_iname = NULL;
    ll->first_llist_entry = NULL;
    ll->explicitly_cast = FALSE;
    ll->list_text = NULL;
    ll->ll_package = Emit::current_enclosure();
    TheHeap::ensure_basic_heap_present();
    return ll;
}

§6. Parsing is quadratic in the number of constant lists in the source text, which is in principle a bad thing, but in practice the following causes no speed problems even on large-scale tests. If it becomes a problem, we can easily trade the time spent here for memory, by attaching a pointer to each word in the source text, or for complexity, by constructing some kind of binary search tree.

literal_list *Lists::find_literal?Usage of Lists::find_literal:
§5, §7, §9, §10(int incipit) {
    literal_list *ll;
    LOOP_OVER(ll, literal_list)
        if (Wordings::first_wn(ll->unbraced_text) == incipit)
            return ll;
    return NULL;
}

§7. Note that the entry kind is initially unknown, and it's not even decided for sure when we add the first entry. Here's how each entry is added, recursing right to left (i.e., reversing the direction of reading):

literal_list *Lists::add?Usage of Lists::add:
§1(parse_node *spec, literal_list *ll, wording W, int bad) {
    llist_entry *lle = CREATE(llist_entry);
    lle->next_llist_entry = ll->first_llist_entry;
    ll->first_llist_entry = lle;
    lle->llist_entry_value = spec;
    literal_list *ll2 = Lists::find_literal(Wordings::first_wn(W));
    if (ll2) ll = ll2;
    ll->unbraced_text = W;
    if (bad) ll->kinds_known_to_be_inconsistent = TRUE;
    return ll;
}

§8. With all the entries in place, we now have to reconcile their kinds, if that's possible. Problems are only issued on request, and with the current sentence cut down to just the list itself — since otherwise we might be printing out an entire huge table to report a problem in a single entry which happens to be a malformed list.

kind *Lists::kind_of_ll?Usage of Lists::kind_of_ll:
§2, §9(literal_list *ll, int issue_problems) {
    if (ll->explicitly_cast) return Kinds::unary_con(CON_list_of, ll->entry_kind);
    parse_node *cs = current_sentence;
    if (issue_problems) {
        if (ll->list_text == NULL)
            ll->list_text = Diagrams::new_UNPARSED_NOUN(ll->unbraced_text);
        current_sentence = ll->list_text;
    }
    kind *K = K_nil;
    llist_entry *lle;
    for (lle = ll->first_llist_entry; lle; lle = lle->next_llist_entry) {
        parse_node *spec = lle->llist_entry_value;
        if (!Node::is(spec, UNKNOWN_NT)) {
            if (AConditions::is_action_TEST_VALUE(spec))
                Dash::check_value_silently(spec, K_stored_action);
            else
                Dash::check_value_silently(spec, NULL);
        }
        spec = NonlocalVariables::substitute_constants(spec);
        kind *E = NULL;
        Work out the entry kind E8.1;
        if (Kinds::eq(K, K_nil)) K = E;
        else Revise K in the light of E8.2;
    }
    if (ll->kinds_known_to_be_inconsistent) K = K_nil;
    ll->entry_kind = K;
    current_sentence = cs;
    return Kinds::unary_con(CON_list_of, K);
}

§8.1. Work out the entry kind E8.1 =

    if (Node::is(spec, UNKNOWN_NT)) {
        if (issue_problems) Issue a bad list entry problem8.1.1;
        ll->kinds_known_to_be_inconsistent = TRUE;
        E = K;
    } else if ((Node::is(spec, CONSTANT_NT) == FALSE) &&
        (Lvalues::is_constant_NONLOCAL_VARIABLE(spec) == FALSE)) {
        if (issue_problems) Issue a nonconstant list entry problem8.1.2;
        ll->kinds_known_to_be_inconsistent = TRUE;
        E = K;
    } else {
        E = Specifications::to_kind(spec);
        if (E == NULL) {
            if (issue_problems) Issue a bad list entry problem8.1.1;
            ll->kinds_known_to_be_inconsistent = TRUE;
        }
    }

• This code is used in §8.

§8.2. The following broadens K to include E, if necessary, but never narrows K. Thus a list containing a person, a woman and a door will see K become successively "person", "person" (E being narrower), then "thing" (E being incomparable, and "thing" being the max of "person" and "door").

Revise K in the light of E8.2 =

    kind *previous_K = K;
    K = Latticework::join(E, K);
    if (Kinds::get_construct(K) != CON_phrase)
        if (Kinds::Behaviour::definite(K) == FALSE) {
            if (issue_problems) Issue a list entry kind mismatch problem8.2.1;
            ll->kinds_known_to_be_inconsistent = TRUE;
            break;
        }

• This code is used in §8.

§8.1.1. Issue a bad list entry problem8.1.1 =

    Problems::quote_source(1, current_sentence);
    Problems::quote_wording(2, Node::get_text(spec));
    StandardProblems::handmade_problem(Task::syntax_tree(),
        _p_(PM_BadConstantListEntry));
    Problems::issue_problem_segment(
        "The constant list %1 contains an entry '%2' which isn't any "
        "form of constant I'm able to read.");
    Problems::issue_problem_segment(
        "%PNote that lists have to be written with spaces after commas, "
        "so I like '{2, 4}' but not '{2,4}', for instance.");
    Problems::issue_problem_end();

• This code is used in §8.1 (twice).

§8.1.2. Issue a nonconstant list entry problem8.1.2 =

    Problems::quote_source(1, current_sentence);
    Problems::quote_wording(2, Node::get_text(spec));
    Problems::quote_spec(3, spec);
    StandardProblems::handmade_problem(Task::syntax_tree(),
        _p_(PM_NonconstantConstantListEntry));
    Problems::issue_problem_segment(
        "The constant list %1 contains an entry '%2' which does make sense, "
        "but isn't a constant (it's %3). Only constants can appear as entries in "
        "constant lists, i.e., in lists written in braces '{' and '}'.");
    Problems::issue_problem_end();

• This code is used in §8.1.

§8.2.1. Issue a list entry kind mismatch problem8.2.1 =

    Problems::quote_source(1, current_sentence);
    Problems::quote_wording(2, Node::get_text(spec));
    Problems::quote_kind(3, E);
    Problems::quote_kind(4, previous_K);
    StandardProblems::handmade_problem(Task::syntax_tree(),
        _p_(PM_IncompatibleConstantListEntry));
    Problems::issue_problem_segment(
        "The constant list %1 contains an entry '%2' whose kind is '%3', but "
        "that's not compatible with the kind I had established from looking at "
        "earlier entries ('%4').");
    Problems::issue_problem_end();

• This code is used in §8.2.

§9. The following allow other parts of Inform to find the kind of a constant list at a given word position; either to discover the answer, or to force problem messages out into the open —

kind *Lists::kind_of_list_at?Usage of Lists::kind_of_list_at:
Rvalues - §24.4(wording W) {
    int incipit = Wordings::first_wn(W);
    literal_list *ll = Lists::find_literal(incipit+1);
    if (ll) return Lists::kind_of_ll(ll, FALSE);
    return NULL;
}

void Lists::check_one?Usage of Lists::check_one:
Dash - §13.1.1.2(wording W) {
    int incipit = Wordings::first_wn(W);
    literal_list *ll = Lists::find_literal(incipit+1);
    if (ll) Lists::kind_of_ll(ll, TRUE);
}

§10. The interpretation of an empty list at a given position in the source is sometimes set by the assertion machinery, so we also need:

void Lists::set_kind_of_list_at(wording W, kind *K) {
    if (Kinds::get_construct(K) != CON_list_of) internal_error("not a list kind");
    int incipit = Wordings::first_wn(W);
    literal_list *ll = Lists::find_literal(incipit+1);
    if (ll) ll->entry_kind = Kinds::unary_construction_material(K);
    else internal_error("no list appears at this point");
    ll->explicitly_cast = TRUE;
}

int Lists::length_of_ll(wording W) {
    int incipit = Wordings::first_wn(W);
    literal_list *ll = Lists::find_literal(incipit+1);
    if (ll == NULL) internal_error("no list appears at this point");
    int count = 0;
    llist_entry *lle;
    for (lle = ll->first_llist_entry; lle; lle = lle->next_llist_entry)
        count++;
    return count;
}