5. `parser_run` is a library function intended to be linked together
with the generated parser tables to complete the implementation of
a parser.
-6. `calc.cgm` is a test grammar for a simple calculator.
+6. Finally `calc` is a test grammar for a simple calculator. The
+ `parsergen` program built from the C code in this file can extract
+ that grammar directly from this file and process it.
###### File: parsergen.c
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
## parser includes
- ## parser
-###### File: calc.cgm
- ## demo grammar
+ ## parser functions
+ ## parser_run
###### File: parsergen.mk
CFLAGS += -Wall -g
all :: parsergen calc
- parsergen.c parsergen.mk calc.cgm libparser.c parser.h : parsergen.mdc
+ parsergen.c parsergen.mk libparser.c parser.h : parsergen.mdc
./md2c parsergen.mdc
## Reading the grammar
literally copied into the generated `.c` and `.h`. files. The last
contains the grammar. This is tokenised with "[scanner][]".
+If the `--tag` option is given, then any top level header that doesn't
+start with the tag is ignored, and the tag is striped from the rest. So
+`--tag Foo`
+means that the three needed sections must be `Foo: header`, `Foo: code`,
+and `Foo: grammar`. The tag `calc` is used to extract the same calculator
+from this file.
+
[mdcode]: mdcode.html
[scanner]: scanner.html
_ad hoc_ parsing as we don't have a parser generator yet.
The precedence and associativity can be set for each production, but
-can be inherited from symbols. The data type is potentially defined
-for each non-terminal and describes what C structure is used to store
-information about each symbol.
+can be inherited from symbols. The data type (either structure or a
+reference to a structure) is potentially defined for each non-terminal
+and describes what C structure is used to store information about each
+symbol.
###### declarations
enum assoc {Left, Right, Non};
struct symbol {
struct text struct_name;
+ int isref;
enum assoc assoc;
unsigned short precedence;
## symbol fields
struct production {
unsigned short precedence;
enum assoc assoc;
+ char line_like;
## production fields
};
struct grammar {
};
The strings reported by `mdcode` and `scanner` are `struct text` which have
-length rather than being null terminated. To help with printing an
+length rather than being null terminated. To help with printing and
comparing we define `text_is` and `prtxt`, which should possibly go in
`mdcode`. `scanner` does provide `text_dump` which is useful for strings
which might contain control characters.
+`strip_tag` is a bit like `strncmp`, but adds a test for a colon,
+because that is what we need to detect tags.
+
###### functions
static int text_is(struct text t, char *s)
{
printf("%.*s", t.len, t.txt);
}
+ static int strip_tag(struct text *t, char *tag)
+ {
+ int skip = strlen(tag) + 1;
+ if (skip >= t->len ||
+ strncmp(t->txt, tag, skip-1) != 0 ||
+ t->txt[skip-1] != ':')
+ return 0;
+ while (skip < t->len && t->txt[skip] == ' ')
+ skip++;
+ t->len -= skip;
+ t->txt += skip;
+ return 1;
+ }
+
### Symbols
Productions are comprised primarily of symbols - terminal and
[TK_mark] = "MARK",
[TK_string] = "STRING",
[TK_multi_string] = "MULTI_STRING",
- [TK_indent] = "INDENT",
- [TK_undent] = "UNDENT",
+ [TK_in] = "IN",
+ [TK_out] = "OUT",
[TK_newline] = "NEWLINE",
[TK_eof] = "$eof",
};
int num_syms;
###### functions
- static int text_cmp(struct text a, struct text b)
- {
- int len = a.len;
- if (a.len > b.len)
- len = b.len;
- int cmp = strncmp(a.txt, b.txt, len);
- if (cmp)
- return cmp;
- else
- return a.len - b.len;
- }
-
static struct symbol *sym_find(struct grammar *g, struct text s)
{
struct symbol **l = &g->syms;
precedence, otherwise it specifies a data type.
The data type name is simply stored and applied to the head of all
-subsequent productions. It must be the name of a structure, so `$expr`
-maps to `struct expr`.
+subsequent productions. It must be the name of a structure optionally
+preceded by an asterisk which means a reference or "pointer". So
+`$expression` maps to `struct expression` and `$*statement` maps to
+`struct statement *`.
+
+Any productions given before the first data type declaration will have
+no data type associated with them and can carry no information. In
+order to allow other non-terminals to have no type, the data type
+`$void` can be given. This does *not* mean that `struct void` will be
+used, but rather than no type will be associated with future
+non-terminals.
The precedence line must contain a list of symbols - typically
terminal symbols, but not necessarily. It can only contain symbols
listed and may be inherited by any production which uses the symbol. A
production inherits from the last symbol which has a precedence.
+The symbols on the first precedence line have the lowest precedence.
+Subsequent lines introduce symbols with higher precedence.
+
###### grammar fields
struct text current_type;
+ int type_isref;
int prec_levels;
###### declarations
static const char *known[] = { "$$", "${", "}$" };
###### functions
- static char *dollar_line(struct token_state *ts, struct grammar *g)
+ static char *dollar_line(struct token_state *ts, struct grammar *g, int isref)
{
struct token t = token_next(ts);
char *err;
assoc = Non;
else {
g->current_type = t.txt;
+ g->type_isref = isref;
+ if (text_is(t.txt, "void"))
+ g->current_type.txt = NULL;
t = token_next(ts);
if (t.num != TK_newline) {
err = "Extra tokens after type name";
return NULL;
}
+ if (isref) {
+ err = "$* cannot be followed by a precedence";
+ goto abort;
+ }
+
// This is a precedence line, need some symbols.
found = 0;
g->prec_levels += 1;
s->precedence = g->prec_levels;
s->assoc = assoc;
found += 1;
+ t = token_next(ts);
}
if (found == 0)
err = "No symbols given on precedence line";
is given, the precedence is inherited from the last symbol in the
production which has a precedence specified.
-After the optional precedence may come the `${` mark. This indicates
+After the optional precedence may come the `${` mark. This indicates
the start of a code fragment. If present, this must be on the same
line as the start of the production.
be in one `code_node` of the literate code. The `}$` must be
at the end of a line.
-Text in the code fragment will undergo substitutions where `$N` for
-some numeric `N` will be replaced with a variable holding the parse
-information for the particular symbol in the production. `$0` is the
-head of the production, `$1` is the first symbol of the body, etc.
-The type of `$N` for a terminal symbol is `struct token`. For
-non-terminal, it is whatever has been declared for that symbol.
+Text in the code fragment will undergo substitutions where `$N` or
+`$<N`,for some numeric `N`, will be replaced with a variable holding
+the parse information for the particular symbol in the production.
+`$0` is the head of the production, `$1` is the first symbol of the
+body, etc. The type of `$N` for a terminal symbol is `struct token`.
+For a non-terminal, it is whatever has been declared for that symbol.
+The `<` may be included for symbols declared as storing a reference
+(not a structure) and means that the reference is being moved out, so
+it will not automatically be freed.
While building productions we will need to add to an array which needs to
grow dynamically.
return code;
}
-Now we have all the bit we need to parse a full production.
+Now we have all the bits we need to parse a full production.
###### includes
#include <memory.h>
struct symbol **body;
int body_size;
struct text code;
+ int code_line;
###### symbol fields
int first_production;
goto abort;
}
vs = sym_find(g, tk.txt);
- if (vs->type != Virtual) {
- err = "symbol after $$ must be virtual";
+ if (vs->num == TK_newline)
+ p.line_like = 1;
+ else if (vs->num == TK_out)
+ p.line_like = 2;
+ else if (vs->precedence == 0) {
+ err = "symbol after $$ must have precedence";
goto abort;
+ } else {
+ p.precedence = vs->precedence;
+ p.assoc = vs->assoc;
}
- p.precedence = vs->precedence;
- p.assoc = vs->assoc;
tk = token_next(state);
}
if (tk.num == TK_open) {
+ p.code_line = tk.line;
p.code = collect_code(state, tk);
if (p.code.txt == NULL) {
err = "code fragment not closed properly";
that we need to parse a grammar from a `code_node`.
The head of the first production will effectively be the `start` symbol of
-the grammar. However it wont _actually_ be so. Processing the grammar is
+the grammar. However it won't _actually_ be so. Processing the grammar is
greatly simplified if the real start symbol only has a single production,
-and expect `$eof` as the final terminal. So when we find the first explicit
-production we insert an extra production as production zero which looks like
+and expects `$eof` as the final terminal. So when we find the first
+explicit production we insert an extra production as production zero which
+looks like
###### Example: production 0
$start -> START $eof
-where `START` is the first non-terminal give.
+where `START` is the first non-terminal given.
###### create production zero
struct production *p = calloc(1,sizeof(*p));
struct text start = {"$start",6};
struct text eof = {"$eof",4};
+ struct text code = {"$0 = $<1;", 9};
p->head = sym_find(g, start);
p->head->type = Nonterminal;
+ p->head->struct_name = g->current_type;
+ p->head->isref = g->type_isref;
+ if (g->current_type.txt)
+ p->code = code;
array_add(&p->body, &p->body_size, head);
array_add(&p->body, &p->body_size, sym_find(g, eof));
- g->start = p->head->num;
p->head->first_production = g->production_count;
array_add(&g->productions, &g->production_count, p);
-Now we are ready to read in the grammar.
-
-###### grammar fields
- int start; // the 'start' symbol.
+Now we are ready to read in the grammar. We ignore comments
+and strings so that the marks which introduce them can be
+used as terminals in the grammar. We don't ignore numbers
+even though we don't allow them as that causes the scanner
+to produce errors that the parser is better positioned to handle.
###### grammar_read
static struct grammar *grammar_read(struct code_node *code)
| (0 << TK_number)
| (1 << TK_string)
| (1 << TK_multi_string)
- | (1 << TK_indent)
- | (1 << TK_undent),
+ | (1 << TK_in)
+ | (1 << TK_out),
};
struct token_state *state = token_open(code, &conf);
- struct token tk = token_next(state);
+ struct token tk;
struct symbol *head = NULL;
struct grammar *g;
char *err = NULL;
else {
head->type = Nonterminal;
head->struct_name = g->current_type;
- if (g->start == 0) {
+ head->isref = g->type_isref;
+ if (g->production_count == 0) {
## create production zero
}
head->first_production = g->production_count;
else
err = "First production must have a head";
} else if (tk.num == TK_mark
- && text_is(tk.txt, "$")) {
- err = dollar_line(state, g);
+ && text_is(tk.txt, "$")) {
+ err = dollar_line(state, g, 0);
+ } else if (tk.num == TK_mark
+ && text_is(tk.txt, "$*")) {
+ err = dollar_line(state, g, 1);
} else {
err = "Unrecognised token at start of line.";
}
}
We will often want to form the union of two symsets. When we do, we
-will often want to know if anything changed (as they might mean there
+will often want to know if anything changed (as that might mean there
is more work to do). So `symset_union` reports whether anything was
added to the first set. We use a slow quadratic approach as these
-sets don't really get very big. If profiles shows this to be a problem is
+sets don't really get very big. If profiles shows this to be a problem it
can be optimised later.
static int symset_union(struct symset *a, struct symset *b)
return sl->ss;
}
-
### Setting `nullable`
We set `nullable` on the head symbol for any production for which all
}
}
+### Setting `line_like`
+
+In order to be able to ignore newline tokens when not relevant, but
+still include them in the parse when needed, we will need to know
+which states can start a "line-like" section of code. We ignore
+newlines when there is an indent since the most recent start of a
+line-like symbol.
+
+A "line_like" symbol is simply any symbol that can derive a NEWLINE.
+If a symbol cannot derive a NEWLINE, then it is only part of a line -
+so is word-like. If it can derive a NEWLINE, then we consider it to
+be like a line.
+
+Clearly the `TK_newline` token can derive a NEWLINE. Any symbol which
+is the head of a production that contains a line_like symbol is also a
+line-like symbol. We use a new field `line_like` to record this
+attribute of symbols, and compute it in a repetitive manner similar to
+`set_nullable`.
+
+###### symbol fields
+ int line_like;
+
+###### functions
+ static void set_line_like(struct grammar *g)
+ {
+ int check_again = 1;
+ g->symtab[TK_newline]->line_like = 1;
+ while (check_again) {
+ int p;
+ check_again = 0;
+ for (p = 0; p < g->production_count; p++) {
+ struct production *pr = g->productions[p];
+ int s;
+
+ if (pr->head->line_like)
+ continue;
+
+ for (s = 0 ; s < pr->body_size; s++) {
+ if (pr->body[s]->line_like) {
+ pr->head->line_like = 1;
+ check_again = 1;
+ break;
+ }
+ }
+ }
+ }
+ }
+
### Building the `first` sets
When calculating what can follow a particular non-terminal, we will need to
-know what the "first" terminal in an subsequent non-terminal might be. So
+know what the "first" terminal in any subsequent non-terminal might be. So
we calculate the `first` set for every non-terminal and store them in an
array. We don't bother recording the "first" set for terminals as they are
trivial.
`set_nullable`. This makes use of the fact that `symset_union`
reports if any change happens.
-The core of this which finds the "first" of part of a production body
+The core of this, which finds the "first" of part of a production body,
will be reused for computing the "follow" sets, so we split it out
into a separate function.
1. LR(0) or SLR(1), where no look-ahead is considered.
2. LALR(1) where we build look-ahead sets with each item and merge
the LA sets when we find two paths to the same "kernel" set of items.
-3. LR(1) where different look-ahead for any item in the code means
+3. LR(1) where different look-ahead for any item in the set means
a different state must be created.
###### forward declarations
itemset, so we need to ignore the offset=zero items which are added during
completion.
-To facilitate this, we modify the "DOT" number so that "0" sorts to the end of
-the list in the symset, and then only compare items before the first "0".
+To facilitate this, we modify the "DOT" number so that "0" sorts to
+the end of the list in the symset, and then only compare items before
+the first "0".
###### declarations
static inline unsigned short item_num(int production, int index)
a.data[i] - b.data[i];
}
+It will be helpful to know if an itemset has been "completed" or not,
+particularly for LALR where itemsets get merged, at which point they
+need to be consider for completion again. So a `completed` flag is needed.
+
+For correct handling of `TK_newline` when parsing, we will need to
+know which states (itemsets) can occur at the start of a line, so we
+will record a `starts_line` flag too whenever DOT is at the start of a
+`line_like` symbol.
+
+Finally, for handling `TK_out` we need to know whether productions in the
+current state started *before* the most recent indent. A state
+doesn't usually keep details of individual productions, so we need to
+add one extra detail. `min_prefix` is the smallest non-zero number of
+symbols *before* DOT in any production in an itemset. This will allow
+us to determine if the the most recent indent is sufficiently recent
+to cancel it against a `TK_out`. If it was seen longer ago than the
+`min_prefix`, and if the current state cannot be reduced, then the
+indented section must have ended in the middle of a syntactic unit, so
+an error must be signaled.
+
And now we can build the list of itemsets. The lookup routine returns
both a success flag and a pointer to where in the list an insert
should happen, so we don't need to search a second time.
short state;
struct symset items;
struct symset go_to;
+ enum assoc assoc;
+ unsigned short precedence;
char completed;
+ char starts_line;
+ int min_prefix;
};
###### grammar fields
}
Adding an itemset may require merging the LA sets if LALR analysis is
-happening. If any new LA set add symbol that weren't in the old LA set, we
+happening. If any new LA set adds any symbols that weren't in the old LA set, we
clear the `completed` flag so that the dependants of this itemset will be
recalculated and their LA sets updated.
them to a data structure, of freeing them.
static int add_itemset(struct grammar *g, struct symset ss,
+ enum assoc assoc, unsigned short precedence,
enum grammar_type type)
{
struct itemset **where, *is;
is->state = g->states;
g->states += 1;
is->items = ss;
+ is->assoc = assoc;
+ is->precedence = precedence;
is->next = *where;
is->go_to = INIT_DATASET;
*where = is;
#### The build
-To build all the itemsets, we first insert the initial itemset made from the
-start symbol, complete each itemset, and then generate new itemsets from old
-until no new ones can be made.
+To build all the itemsets, we first insert the initial itemset made
+from production zero, complete each itemset, and then generate new
+itemsets from old until no new ones can be made.
Completing an itemset means finding all the items where "DOT" is followed by
a nonterminal and adding "DOT=0" items for every production from that
We also collect a set of all symbols which follow "DOT" (in `done`) as this
is used in the next stage.
-
-NOTE: precedence handling should happen here - I haven't written this yet
-though.
+If any of these symbols are flagged as `line_like`, then this
+state must be a `starts_line` state so now is a good time to record that.
+
+When itemsets are created we assign a precedence to the itemset from
+the complete item, if there is one. We ignore the possibility of
+there being two and don't (currently) handle precedence in such
+grammars. When completing a grammar we ignore any item where DOT is
+followed by a terminal with a precedence lower than that for the
+itemset. Unless the terminal has right associativity, we also ignore
+items where the terminal has the same precedence. The result is that
+unwanted items are still in the itemset, but the terminal doesn't get
+into the go to set, so the item is ineffective.
###### complete itemset
for (i = 0; i < is->items.cnt; i++) {
struct symbol *s;
struct symset LA = INIT_SYMSET;
unsigned short sn = 0;
+ struct symset LAnl = INIT_SYMSET;
+ unsigned short snnl = 0;
+ if (is->min_prefix == 0 ||
+ (bs > 0 && bs < is->min_prefix))
+ is->min_prefix = bs;
if (bs == pr->body_size)
continue;
s = pr->body[bs];
- if (symset_find(&done, s->num) < 0)
+ if (s->precedence && is->precedence &&
+ is->precedence > s->precedence)
+ /* This terminal has a low precedence and
+ * shouldn't be shifted
+ */
+ continue;
+ if (s->precedence && is->precedence &&
+ is->precedence == s->precedence && s->assoc != Right)
+ /* This terminal has a matching precedence and is
+ * not Right-associative, so we mustn't shift it.
+ */
+ continue;
+ if (symset_find(&done, s->num) < 0) {
symset_add(&done, s->num, 0);
+ if (s->line_like)
+ is->starts_line = 1;
+ }
if (s->type != Nonterminal)
continue;
again = 1;
}
sn = save_set(g, LA);
LA = set_find(g, sn);
+ if (symset_find(&LA, TK_newline))
+ symset_add(&LAnl, TK_newline, 0);
+ snnl = save_set(g, LAnl);
+ LAnl = set_find(g, snnl);
}
+
/* Add productions for this symbol */
for (p2 = s->first_production;
p2 < g->production_count &&
int itm = item_num(p2, 0);
int pos = symset_find(&is->items, itm);
if (pos < 0) {
- symset_add(&is->items, itm, sn);
+ if (g->productions[p2]->line_like)
+ symset_add(&is->items, itm, snnl);
+ else
+ symset_add(&is->items, itm, sn);
/* Will have re-ordered, so start
* from beginning again */
i = -1;
} else if (type >= LALR) {
struct symset ss = set_find(g, is->items.data[pos]);
struct symset tmp = INIT_SYMSET;
+ struct symset *la = &LA;
+ if (g->productions[p2]->line_like)
+ la = &LAnl;
symset_union(&tmp, &ss);
- if (symset_union(&tmp, &LA)) {
+ if (symset_union(&tmp, la)) {
is->items.data[pos] = save_set(g, tmp);
i = -1;
- }else
+ } else
symset_free(tmp);
}
}
###### derive itemsets
// Now we have a completed itemset, so we need to
- // create all the 'next' itemset and create them
+ // compute all the 'next' itemsets and create them
// if they don't exist.
for (i = 0; i < done.cnt; i++) {
int j;
unsigned short state;
+ struct symbol *sym = g->symtab[done.syms[i]];
+ enum assoc assoc = Non;
+ unsigned short precedence = 0;
struct symset newitemset = INIT_SYMSET;
if (type >= LALR)
newitemset = INIT_DATASET;
if (bp == pr->body_size)
continue;
- if (pr->body[bp]->num != done.syms[i])
+ if (pr->body[bp] != sym)
continue;
if (type >= LALR)
la = is->items.data[j];
pos = symset_find(&newitemset, pr->head->num);
+ if (bp + 1 == pr->body_size &&
+ pr->precedence > 0 &&
+ pr->precedence > precedence) {
+ // new itemset is reducible and has a precedence.
+ precedence = pr->precedence;
+ assoc = pr->assoc;
+ }
if (pos < 0)
symset_add(&newitemset, item_num(p, bp+1), la);
else if (type >= LALR) {
}
}
}
- state = add_itemset(g, newitemset, type);
+ state = add_itemset(g, newitemset, assoc, precedence, type);
if (symset_find(&is->go_to, done.syms[i]) < 0)
symset_add(&is->go_to, done.syms[i], state);
}
-All that is left is to crate the initial itemset from production zero, and
+All that is left is to create the initial itemset from production zero, and
with `TK_eof` as the LA set.
###### functions
la = save_set(g, eof);
first = INIT_DATASET;
}
- // production 0, offset 0 (with no data)
+ // production 0, offset 0 (with no data)
symset_add(&first, item_num(0, 0), la);
- add_itemset(g, first, type);
+ add_itemset(g, first, Non, 0, type);
for (again = 0, is = g->items;
is;
is = is->next ?: again ? (again = 0, g->items) : NULL) {
if (is->completed)
continue;
is->completed = 1;
+ again = 1;
## complete itemset
## derive itemsets
symset_free(done);
for (s = g->syms; s; s = s->next)
g->symtab[s->num] = s;
- if (type >= SLR) {
- set_nullable(g);
+ set_nullable(g);
+ set_line_like(g);
+ if (type >= SLR)
build_first(g);
- }
+
if (type == SLR)
build_follow(g);
The purpose of the report is to give the grammar developer insight into
how the grammar parser will work. It is basically a structured dump of
-all the tables that have been generated, plus an description of any conflicts.
+all the tables that have been generated, plus a description of any conflicts.
###### grammar_report
static int grammar_report(struct grammar *g, enum grammar_type type)
return report_conflicts(g, type);
}
-Firstly we have the complete list of symbols, together with the "FIRST"
-set if that was generated.
+Firstly we have the complete list of symbols, together with the
+"FIRST" set if that was generated. We add a mark to each symbol to
+show if it can end in a newline (`>`), if it is considered to be
+"line-like" (`<`), or if it is nullable (`.`).
###### functions
if (!s)
continue;
- printf(" %c%3d%c: ",
- s->nullable ? '*':' ',
+ printf(" %c%c%3d%c: ",
+ s->nullable ? '.':' ',
+ s->line_like ? '<':' ',
s->num, symtypes[s->type]);
prtxt(s->name);
if (s->precedence)
}
}
-Then we have to follow sets if they were computed.
+Then we have the follow sets if they were computed.
static void report_follow(struct grammar *g)
{
if (dot == pr->body_size)
printf(" .");
printf(" [%d]", p);
- if (pr->precedence)
+ if (pr->precedence && dot == pr->body_size)
printf(" (%d%s)", pr->precedence,
assoc_names[pr->assoc]);
+ if (dot < pr->body_size &&
+ pr->body[dot]->precedence) {
+ struct symbol *s = pr->body[dot];
+ printf(" [%d%s]", s->precedence,
+ assoc_names[s->assoc]);
+ }
+ if (pr->line_like == 1)
+ printf(" $$NEWLINE");
+ else if (pr->line_like)
+ printf(" $$OUT");
printf("\n");
}
Then the go to sets:
-
static void report_goto(struct grammar *g, struct symset gt)
{
int i;
for (s = 0; s < g->states; s++) {
int j;
struct itemset *is = g->statetab[s];
- printf(" Itemset %d:\n", s);
+ printf(" Itemset %d:%s min prefix=%d",
+ s, is->starts_line?" (startsline)":"", is->min_prefix);
+ if (is->precedence)
+ printf(" %d%s", is->precedence, assoc_names[is->assoc]);
+ printf("\n");
for (j = 0; j < is->items.cnt; j++) {
report_item(g, is->items.syms[j]);
if (is->items.data != NO_DATA)
}
LR0 conflicts are any state which have both a reducible item and
-a shiftable item.
+a shiftable item, or two reducible items.
-LR05 conflicts only occurs if two possibly reductions exist,
+LR05 conflicts only occur if two possible reductions exist,
as shifts always over-ride reductions.
###### conflict functions
}
SLR, LALR, and LR1 conflicts happen if two reducible items have over-lapping
-look ahead, or if a symbol in a look-ahead can be shifted. The differ only
+look ahead, or if a symbol in a look-ahead can be shifted. They differ only
in the source of the look ahead set.
-We build a dataset mapping terminal to item for possible SHIFTs and then
-another for possible REDUCE operations. We report when we get conflicts
-between the two.
+We build two datasets to reflect the "action" table: one which maps
+terminals to items where that terminal could be shifted and another
+which maps terminals to items that could be reduced when the terminal
+is in look-ahead. We report when we get conflicts between the two.
+
+As a special case, if we find a SHIFT/REDUCE conflict, on the NEWLINE
+terminal, we ignore it. NEWLINES are handled specially with its own
+rules for when to shift and when to reduce. Conflicts are expected,
+but handled internally.
static int conflicts_slr(struct grammar *g, enum grammar_type type)
{
int p = item_prod(itm);
int bp = item_index(itm);
struct production *pr = g->productions[p];
+ struct symbol *s;
- if (bp < pr->body_size &&
- pr->body[bp]->type == Terminal) {
- /* shiftable */
- int sym = pr->body[bp]->num;
- if (symset_find(&shifts, sym) < 0)
- symset_add(&shifts, sym, itm);
- }
+ if (bp >= pr->body_size ||
+ pr->body[bp]->type != Terminal)
+ /* not shiftable */
+ continue;
+
+ s = pr->body[bp];
+ if (s->precedence && is->precedence)
+ /* Precedence resolves this, so no conflict */
+ continue;
+
+ if (symset_find(&shifts, s->num) < 0)
+ symset_add(&shifts, s->num, itm);
}
- /* Now look for reduction and conflicts */
+ /* Now look for reductions and conflicts */
for (j = 0; j < is->items.cnt; j++) {
unsigned short itm = is->items.syms[j];
int p = item_prod(itm);
int k;
for (k = 0; k < la.cnt; k++) {
int pos = symset_find(&shifts, la.syms[k]);
- if (pos >= 0) {
+ if (pos >= 0 && la.syms[k] != TK_newline) {
printf(" State %d has SHIFT/REDUCE conflict on ", i);
- prtxt(g->symtab[la.syms[k]]->name);
+ cnt++;
+ prtxt(g->symtab[la.syms[k]]->name);
printf(":\n");
report_item(g, shifts.data[pos]);
report_item(g, itm);
- cnt++;
}
pos = symset_find(&reduce, la.syms[k]);
if (pos < 0) {
return cnt;
}
-
## Generating the parser
-The export part of the parser is the `parse_XX` function, where the name
+The exported part of the parser is the `parse_XX` function, where the name
`XX` is based on the name of the parser files.
This takes a `code_node`, a partially initialized `token_config`, and an
known words added and then is used with the `code_node` to initialize the
scanner.
-`parse_XX` then call the library function `parser_run` to actually complete
-the parse, This needs the `states` table and function to call the various
+`parse_XX` then calls the library function `parser_run` to actually complete
+the parse. This needs the `states` table and function to call the various
pieces of code provided in the grammar file, so they are generated first.
###### parser_generate
- static void gen_parser(FILE *f, struct grammar *g, char *file, char *name)
+ static void gen_parser(FILE *f, struct grammar *g, char *file, char *name,
+ struct code_node *pre_reduce)
{
gen_known(f, g);
+ gen_non_term(f, g);
gen_goto(f, g);
gen_states(f, g);
- gen_reduce(f, g, file);
+ gen_reduce(f, g, file, pre_reduce);
gen_free(f, g);
fprintf(f, "#line 0 \"gen_parser\"\n");
fprintf(f, "\tconfig->known_count = sizeof(known)/sizeof(known[0]);\n");
fprintf(f, "\tconfig->ignored |= (1 << TK_line_comment) | (1 << TK_block_comment);\n");
fprintf(f, "\ttokens = token_open(code, config);\n");
- fprintf(f, "\tvoid *rv = parser_run(tokens, states, do_reduce, do_free, trace);\n");
+ fprintf(f, "\tvoid *rv = parser_run(tokens, states, do_reduce, do_free, trace, non_term, config);\n");
fprintf(f, "\ttoken_close(tokens);\n");
fprintf(f, "\treturn rv;\n");
fprintf(f, "}\n\n");
}
-### Table words table
+### Known words table
-The know words is simply an array of terminal symbols.
+The known words table is simply an array of terminal symbols.
+The table of nonterminals used for tracing is a similar array. We
+include virtual symbols in the table of non_terminals to keep the
+numbers right.
###### functions
fprintf(f, "};\n\n");
}
+ static void gen_non_term(FILE *f, struct grammar *g)
+ {
+ int i;
+ fprintf(f, "#line 0 \"gen_non_term\"\n");
+ fprintf(f, "static const char *non_term[] = {\n");
+ for (i = TK_reserved;
+ i < g->num_syms;
+ i++)
+ if (g->symtab[i]->type != Terminal)
+ fprintf(f, "\t\"%.*s\",\n", g->symtab[i]->name.len,
+ g->symtab[i]->name.txt);
+ fprintf(f, "};\n\n");
+ }
+
### States and the goto tables.
-For each state we record the goto table and the reducible production if
-there is one.
+For each state we record the goto table, the reducible production if
+there is one, or a symbol to shift for error recovery.
Some of the details of the reducible production are stored in the
`do_reduce` function to come later. Here we store the production number,
the body size (useful for stack management) and the resulting symbol (useful
short reduce_prod;
short reduce_size;
short reduce_sym;
+ char starts_line;
+ char newline_only;
+ short min_prefix;
};
-
###### functions
static void gen_goto(FILE *f, struct grammar *g)
fprintf(f, "static const struct state states[] = {\n");
for (i = 0; i < g->states; i++) {
struct itemset *is = g->statetab[i];
- int j, prod = -1;
+ int j, prod = -1, prod_len;
+
for (j = 0; j < is->items.cnt; j++) {
int itm = is->items.syms[j];
int p = item_prod(itm);
if (bp < pr->body_size)
continue;
/* This is what we reduce */
- prod = p;
- break;
+ if (prod < 0 || prod_len < pr->body_size) {
+ prod = p;
+ prod_len = pr->body_size;
+ }
}
- fprintf(f, "\t[%d] = { %d, goto_%d, %d, %d, %d },\n",
- i, is->go_to.cnt, i, prod,
- prod < 0 ? -1 : g->productions[prod]->body_size,
- prod < 0 ? -1 : g->productions[prod]->head->num);
+ if (prod >= 0)
+ fprintf(f, "\t[%d] = { %d, goto_%d, %d, %d, %d, %d, %d, %d },\n",
+ i, is->go_to.cnt, i, prod,
+ g->productions[prod]->body_size,
+ g->productions[prod]->head->num,
+ is->starts_line,
+ g->productions[prod]->line_like,
+ is->min_prefix);
+ else
+ fprintf(f, "\t[%d] = { %d, goto_%d, -1, -1, -1, %d, 0, %d },\n",
+ i, is->go_to.cnt, i,
+ is->starts_line, is->min_prefix);
}
fprintf(f, "};\n\n");
}
`do_reduce` to `malloc` that "somewhere", we pass in a large buffer and have
`do_reduce` return the size to be saved.
+In order for the code to access "global" context, we pass in the
+"config" pointer that was passed to parser function. If the `struct
+token_config` is embedded in some larger structure, the reducing code
+can access the larger structure using pointer manipulation.
+
The code fragment requires translation when written out. Any `$N` needs to
be converted to a reference either to that buffer (if `$0`) or to the
-structure returned by a previous reduction. These pointer need to be cast
-to the appropriate type for each access. All this is handling in
+structure returned by a previous reduction. These pointers need to be cast
+to the appropriate type for each access. All this is handled in
`gen_code`.
+`gen_code` also allows symbol references to contain a '`<`' as in '`$<2`'.
+This applied only to symbols with references (or pointers), not those with structures.
+The `<` implies that the reference it being moved out, so the object will not be
+automatically freed. This is equivalent to assigning `NULL` to the pointer.
###### functions
static void gen_code(struct production *p, FILE *f, struct grammar *g)
{
char *c;
+ char *used = calloc(1, p->body_size);
+ int i;
+
fprintf(f, "\t\t\t");
for (c = p->code.txt; c < p->code.txt + p->code.len; c++) {
int n;
+ int use = 0;
if (*c != '$') {
fputc(*c, f);
if (*c == '\n')
continue;
}
c++;
+ if (*c == '<') {
+ use = 1;
+ c++;
+ }
if (*c < '0' || *c > '9') {
+ if (use)
+ fputc('<', f);
fputc(*c, f);
continue;
}
n = n * 10 + *c - '0';
}
if (n == 0)
- fprintf(f, "(*(struct %.*s*)ret)",
+ fprintf(f, "(*(struct %.*s*%s)ret)",
p->head->struct_name.len,
- p->head->struct_name.txt);
+ p->head->struct_name.txt,
+ p->head->isref ? "*":"");
else if (n > p->body_size)
fprintf(f, "$%d", n);
else if (p->body[n-1]->type == Terminal)
n-1);
else if (p->body[n-1]->struct_name.txt == NULL)
fprintf(f, "$%d", n);
- else
- fprintf(f, "(*(struct %.*s*)body[%d])",
+ else {
+ fprintf(f, "(*(struct %.*s*%s)body[%d])",
p->body[n-1]->struct_name.len,
- p->body[n-1]->struct_name.txt, n-1);
+ p->body[n-1]->struct_name.txt,
+ p->body[n-1]->isref ? "*":"", n-1);
+ used[n-1] = use;
+ }
}
fputs("\n", f);
+ for (i = 0; i < p->body_size; i++) {
+ if (p->body[i]->struct_name.txt &&
+ used[i]) {
+ // assume this has been copied out
+ if (p->body[i]->isref)
+ fprintf(f, "\t\t*(void**)body[%d] = NULL;\n", i);
+ else
+ fprintf(f, "\t\tmemset(body[%d], 0, sizeof(struct %.*s));\n", i, p->body[i]->struct_name.len, p->body[i]->struct_name.txt);
+ }
+ }
+ free(used);
}
###### functions
- static void gen_reduce(FILE *f, struct grammar *g, char *file)
+ static void gen_reduce(FILE *f, struct grammar *g, char *file,
+ struct code_node *code)
{
- int i, j;
- fprintf(f, "#line 0 \"gen_reduce\"\n");
- fprintf(f, "static int do_reduce(int prod, int depth, void **body,\n");
- fprintf(f, " void *ret, FILE *trace)\n");
+ int i;
+ fprintf(f, "#line 1 \"gen_reduce\"\n");
+ fprintf(f, "static int do_reduce(int prod, void **body, struct token_config *config, void *ret)\n");
fprintf(f, "{\n");
fprintf(f, "\tint ret_size = 0;\n");
+ if (code)
+ code_node_print(f, code, file);
+ fprintf(f, "#line 4 \"gen_reduce\"\n");
fprintf(f, "\tswitch(prod) {\n");
for (i = 0; i < g->production_count; i++) {
struct production *p = g->productions[i];
fprintf(f, "\tcase %d:\n", i);
- if (p->code.txt)
+ if (p->code.txt) {
+ fprintf(f, "#line %d \"%s\"\n", p->code_line, file);
gen_code(p, f, g);
-
- fprintf(f, "\t\tif (trace) {\n");
- fprintf(f, "\t\t\tfprintf(trace, \"[%%2d]%.*s ->\", depth);\n",
- p->head->name.len, p->head->name.txt);
- for (j = 0; j < p->body_size; j++) {
- if (p->body[j]->type == Terminal) {
- fputs("\t\t\tfputs(\" \", trace);\n", f);
- fprintf(f, "\t\t\ttext_dump(trace, (*(struct token*)body[%d]).txt, 20);\n", j);
- } else {
- fprintf(f, "\t\t\tfprintf(trace, \" %.*s\");\n",
- p->body[j]->name.len,
- p->body[j]->name.txt);
- }
}
- fprintf(f, "\t\t}\n");
if (p->head->struct_name.txt)
- fprintf(f, "\t\tret_size = sizeof(struct %.*s);\n",
+ fprintf(f, "\t\tret_size = sizeof(struct %.*s%s);\n",
p->head->struct_name.len,
- p->head->struct_name.txt);
+ p->head->struct_name.txt,
+ p->head->isref ? "*":"");
fprintf(f, "\t\tbreak;\n");
}
It is particularly important to have fine control over freeing during error
recovery where individual stack frames might need to be freed.
-For this, the grammar author required to defined a `free_XX` function for
-each structure that is used by a non-terminal. `do_free` all call whichever
+For this, the grammar author is required to defined a `free_XX` function for
+each structure that is used by a non-terminal. `do_free` will call whichever
is appropriate given a symbol number, and will call `free` (as is
-appropriate for tokens` on any terminal symbol.
+appropriate for tokens) on any terminal symbol.
###### functions
continue;
fprintf(f, "\tcase %d:\n", s->num);
- fprintf(f, "\t\tfree_%.*s(asn);\n",
- s->struct_name.len,
- s->struct_name.txt);
+ if (s->isref) {
+ fprintf(f, "\t\tfree_%.*s(*(void**)asn);\n",
+ s->struct_name.len,
+ s->struct_name.txt);
+ fprintf(f, "\t\tfree(asn);\n");
+ } else
+ fprintf(f, "\t\tfree_%.*s(asn);\n",
+ s->struct_name.len,
+ s->struct_name.txt);
fprintf(f, "\t\tbreak;\n");
}
fprintf(f, "\t}\n}\n\n");
{ "SLR", 0, NULL, 'S' },
{ "LALR", 0, NULL, 'L' },
{ "LR1", 0, NULL, '1' },
+ { "tag", 1, NULL, 't' },
{ "report", 0, NULL, 'R' },
{ "output", 1, NULL, 'o' },
{ NULL, 0, NULL, 0 }
};
- const char *options = "05SL1Ro:";
+ const char *options = "05SL1t:Ro:";
###### process arguments
int opt;
char *outfile = NULL;
char *infile;
char *name;
+ char *tag = NULL;
int report = 1;
enum grammar_type type = LR05;
while ((opt = getopt_long(argc, argv, options,
report = 2; break;
case 'o':
outfile = optarg; break;
+ case 't':
+ tag = optarg; break;
default:
fprintf(stderr, "Usage: parsergen ...\n");
exit(1);
To be able to run `mdcode` and `scanner` on the grammar we need to memory
map it.
-One we have extracted the code (with `mdcode`) we expect to file three
-sections: header, code, and grammar. Anything else is an error.
+Once we have extracted the code (with `mdcode`) we expect to find three
+sections: header, code, and grammar. Anything else that is not
+excluded by the `--tag` option is an error.
"header" and "code" are optional, though it is hard to build a working
parser with neither. "grammar" must be provided.
struct code_node *hdr = NULL;
struct code_node *code = NULL;
struct code_node *gram = NULL;
+ struct code_node *pre_reduce = NULL;
for (s = table; s; s = s->next) {
- if (text_is(s->section, "header"))
+ struct text sec = s->section;
+ if (tag && !strip_tag(&sec, tag))
+ continue;
+ if (text_is(sec, "header"))
hdr = s->code;
- else if (text_is(s->section, "code"))
+ else if (text_is(sec, "code"))
code = s->code;
- else if (text_is(s->section, "grammar"))
+ else if (text_is(sec, "grammar"))
gram = s->code;
+ else if (text_is(sec, "reduce"))
+ pre_reduce = s->code;
else {
fprintf(stderr, "Unknown content section: %.*s\n",
s->section.len, s->section.txt);
rv |= 1;
}
-If a headers section is defined, we write it out and include a declaration
+If a "headers" section is defined, we write it out and include a declaration
for the `parse_XX` function so it can be used from separate code.
if (rv == 0 && hdr && outfile) {
if (f) {
if (code)
code_node_print(f, code, infile);
- gen_parser(f, g, infile, name);
+ gen_parser(f, g, infile, name, pre_reduce);
fclose(f);
} else {
fprintf(stderr, "Cannot create %s.c\n",
}
And that about wraps it up. We need to set the locale so that UTF-8 is
-recognised properly, and link with `libicuuc` is `libmdcode` requires that.
+recognised properly, and link with `libicuuc` as `libmdcode` requires that.
###### File: parsergen.mk
parsergen : parsergen.o libscanner.o libmdcode.o
## The SHIFT/REDUCE parser
-Having analysed the grammar and generated all the table, we only need the
+Having analysed the grammar and generated all the tables, we only need the
shift/reduce engine to bring it all together.
### Goto table lookup
symbol number. We need a binary search function to find a symbol in the
table.
-###### parser
+###### parser functions
static int search(const struct state *l, int sym)
{
### The state stack.
-The core data structure for the parser is the stack. This track all the
+The core data structure for the parser is the stack. This tracks all the
symbols that have been recognised or partially recognised.
The stack usually won't grow very large - maybe a few tens of entries. So
production, and by keeping a separate `asn` stack, we can just pass a
pointer into this stack.
-The other allocation store all other stack fields of which there are two.
+The other allocation stores all other stack fields of which there are six.
The `state` is the most important one and guides the parsing process. The
`sym` is nearly unnecessary. However when we want to free entries from the
`asn_stack`, it helps to know what type they are so we can call the right
freeing function. The symbol leads us to the right free function through
`do_free`.
-###### parser
+The `indents` count tracks the line indents with-in the symbol or
+immediately follow it. These are used to allow indent information to
+guide parsing and error recovery.
+
+`since_newline` tracks how many stack frames since the last
+start-of-line (whether indented or not). So if `since_newline` is
+zero, then this symbol is at the start of a line. Similarly
+`since_indent` counts the number of states since an indent, it is zero
+precisely when `indents` is not zero.
+
+`newline_permitted` keeps track of whether newlines should be ignored
+or not.
+
+The stack is most properly seen as alternating states and symbols -
+states, like the 'DOT' in items, are between symbols. Each frame in
+our stack holds a state and the symbol that was before it. The
+bottom of stack holds the start state but no symbol, as nothing came
+before the beginning.
+
+###### parser functions
struct parser {
- int state;
struct frame {
short state;
+ short newline_permitted;
+
short sym;
+ short indents;
+ short since_newline;
+ short since_indent;
} *stack;
void **asn_stack;
int stack_size;
#### Shift and pop
-The operations are needed on the stack - shift (which is like push) and pop.
+Two operations are needed on the stack - shift (which is like push) and pop.
-Shift applies no only to terminals but also to non-terminals. When we
-reduce a production we will pop off entries corresponding to the body
-symbols, then push on an item for the head of the production. This last is
-exactly the same process as shifting in a terminal so we use the same
-function for both.
+Shift applies not only to terminals but also to non-terminals. When
+we reduce a production we will pop off entries corresponding to the
+body symbols, then push on an item for the head of the production.
+This last is exactly the same process as shifting in a terminal so we
+use the same function for both. In both cases we provide the symbol,
+the number of indents the symbol contains (which will be zero for a
+terminal symbol) and a flag indicating the the symbol was at (or was
+reduced from a symbol which was at) the start of a line. The state is
+deduced from the current top-of-stack state and the new symbol.
To simplify other code we arrange for `shift` to fail if there is no `goto`
state for the symbol. This is useful in basic parsing due to our design
that we shift when we can, and reduce when we cannot. So the `shift`
function reports if it could.
-So `shift` finds the next state. If that succeed it extends the allocations
-if needed and pushed all the information onto the stacks.
+`shift` is also used to push state zero onto the stack, so if the
+stack is empty, it always chooses zero as the next state.
+
+So `shift` finds the next state. If that succeeds it extends the
+allocations if needed and pushes all the information onto the stacks.
-###### parser
+Newlines are permitted after a `starts_line` state until an internal
+indent. If the new frame has neither a `starts_line` state nor an
+indent, newlines are permitted if the previous stack frame permitted
+them.
+
+###### parser functions
static int shift(struct parser *p,
- int sym, void *asn,
+ short sym, short indents, short start_of_line,
+ void *asn,
const struct state states[])
{
// Push an entry onto the stack
- int newstate = search(&states[p->state], sym);
+ struct frame next = {0};
+ int newstate = p->tos
+ ? search(&states[p->stack[p->tos-1].state],
+ sym)
+ : 0;
if (newstate < 0)
return 0;
if (p->tos >= p->stack_size) {
p->asn_stack = realloc(p->asn_stack, p->stack_size
* sizeof(p->asn_stack[0]));
}
- p->stack[p->tos].state = p->state;
- p->stack[p->tos].sym = sym;
+ next.sym = sym;
+ next.indents = indents;
+ next.state = newstate;
+ if (states[newstate].starts_line)
+ next.newline_permitted = 1;
+ else if (indents)
+ next.newline_permitted = 0;
+ else if (p->tos)
+ next.newline_permitted =
+ p->stack[p->tos-1].newline_permitted;
+ else
+ next.newline_permitted = 0;
+
+ if (!start_of_line) {
+ if (p->tos)
+ next.since_newline = p->stack[p->tos-1].since_newline + 1;
+ else
+ next.since_newline = 1;
+ }
+ if (indents)
+ next.since_indent = 0;
+ else if (p->tos)
+ next.since_indent = p->stack[p->tos-1].since_indent + 1;
+ else
+ next.since_indent = 1;
+
+ p->stack[p->tos] = next;
p->asn_stack[p->tos] = asn;
p->tos++;
- p->state = newstate;
return 1;
}
-`pop` simply moves the top of stack (`tos`) back down the required amount
-and frees and `asn` entries that need to be freed. It is called _after_ we
-reduce a production, just before we `shift` the nonterminal in.
+`pop` primarily moves the top of stack (`tos`) back down the required
+amount and frees any `asn` entries that need to be freed. It also
+collects a summary of the indents and line starts in the symbols that
+are being removed. It is called _after_ we reduce a production, just
+before we `shift` the nonterminal in.
-###### parser
+###### parser functions
- static void pop(struct parser *p, int num,
- void(*do_free)(short sym, void *asn))
+ static int pop(struct parser *p, int num,
+ short *start_of_line,
+ void(*do_free)(short sym, void *asn))
{
int i;
+ short indents = 0;
+ int sol = 0;
+
p->tos -= num;
- for (i = 0; i < num; i++)
+ for (i = 0; i < num; i++) {
+ sol |= !p->stack[p->tos+i].since_newline;
+ indents += p->stack[p->tos+i].indents;
do_free(p->stack[p->tos+i].sym,
p->asn_stack[p->tos+i]);
-
- p->state = p->stack[p->tos].state;
+ }
+ if (start_of_line)
+ *start_of_line = sol;
+ return indents;
}
### Memory allocation
The `scanner` returns tokens in a local variable - we want them in allocated
-memory so they can live in the `asn_stack`. Similarly the `asn` produce by
+memory so they can live in the `asn_stack`. Similarly the `asn` produced by
a reduce is in a large buffer. Both of these require some allocation and
copying, hence `memdup` and `tokcopy`.
###### parser includes
#include <memory.h>
-###### parser
+###### parser functions
void *memdup(void *m, int len)
{
### The heart of the parser.
-Now we have the parser. If we can shift, we do. If not and we can reduce
-we do. If the production we reduced was production zero, then we have
+Now we have the parser. If we can shift we do, though newlines and
+reducing indenting may block that. If not and we can reduce we do
+that. If the production we reduced was production zero, then we have
accepted the input and can finish.
+We return whatever `asn` was returned by reducing production zero.
+
If we can neither shift nor reduce we have an error to handle. We pop
-single entries off the stack until we can shift the `TK_error` symbol, the
+single entries off the stack until we can shift the `TK_error` symbol, then
drop input tokens until we find one we can shift into the new error state.
-We return whatever `asn` was returned by reducing production zero.
+When we find `TK_in` and `TK_out` tokens which report indents we need
+to handle them directly as the grammar cannot express what we want to
+do with them.
+
+`TK_in` tokens are easy: we simply update indent count in the top stack frame to
+record how many indents there are following the previous token.
+
+`TK_out` tokens must be canceled against an indent count
+within the stack. If we can reduce some symbols that are all since
+the most recent indent, then we do that first. If the minimum prefix
+of the current state then extends back before the most recent indent,
+that indent can be cancelled. If the minimum prefix is shorter then
+the indent had ended prematurely and we must start error handling, which
+is still a work-in-progress.
+
+`TK_newline` tokens are ignored unless the top stack frame records
+that they are permitted. In that case they will not be considered for
+shifting if it is possible to reduce some symbols that are all since
+the most recent start of line. This is how a newline forcibly
+terminates any line-like structure - we try to reduce down to at most
+one symbol for each line where newlines are allowed.
+A consequence of this is that a rule like
+
+###### Example: newlines - broken
+
+ Newlines ->
+ | NEWLINE Newlines
+ IfStatement -> Newlines if ....
+
+cannot work, as the NEWLINE will never be shifted as the empty string
+will be reduced first. Optional sets of newlines need to be include
+in the thing that preceed:
+
+###### Example: newlines - works
+
+ If -> if
+ | NEWLINE If
+ IfStatement -> If ....
+
+Here the NEWLINE will be shifted because nothing can be reduced until
+the `if` is seen.
+
+When, during error handling, we discard token read in, we want to keep
+discarding until we see one that is recognised. If we had a full set
+of LR(1) grammar states, this will mean looking in the look-ahead set,
+but we don't keep a full look-ahead set. We only record the subset
+that leads to SHIFT. We can, however, deduce the look-ahead set but
+looking at the SHIFT subsets for all states that we can get to by
+reducing zero or more times. So we need a little function which
+checks if a given token is in any of these look-ahead sets.
###### parser includes
#include "parser.h"
-###### parser
+
+###### parser_run
+
+ static int in_lookahead(struct token *tk, const struct state *states, int state)
+ {
+ while (state >= 0) {
+ if (search(&states[state], tk->num) >= 0)
+ return 1;
+ if (states[state].reduce_prod < 0)
+ return 0;
+ state = search(&states[state], states[state].reduce_sym);
+ }
+ return 0;
+ }
+
void *parser_run(struct token_state *tokens,
const struct state states[],
- int (*do_reduce)(int, int, void**, void*, FILE*),
+ int (*do_reduce)(int, void**, struct token_config*, void*),
void (*do_free)(short, void*),
- FILE *trace)
+ FILE *trace, const char *non_term[],
+ struct token_config *config)
{
struct parser p = { 0 };
- struct token *tk;
+ struct token *tk = NULL;
int accepted = 0;
- void *ret;
+ void *ret = NULL;
- tk = tok_copy(token_next(tokens));
+ shift(&p, TK_eof, 0, 1, NULL, states);
while (!accepted) {
- if (shift(&p, tk->num, tk, states)) {
- if (trace) {
- fputs("Shift ", trace);
- text_dump(trace, tk->txt, 20);
- fputs("\n", trace);
- }
+ struct token *err_tk;
+ struct frame *tos = &p.stack[p.tos-1];
+ if (!tk)
tk = tok_copy(token_next(tokens));
+ parser_trace(trace, &p,
+ tk, states, non_term, config->known_count);
+
+ if (tk->num == TK_in) {
+ tos->indents += 1;
+ tos->since_newline = 0;
+ tos->since_indent = 0;
+ if (!states[tos->state].starts_line)
+ tos->newline_permitted = 0;
+ free(tk);
+ tk = NULL;
+ parser_trace_action(trace, "Record");
continue;
}
- if (states[p.state].reduce_prod >= 0) {
+ if (tk->num == TK_out) {
+ if (states[tos->state].reduce_size >= 0 &&
+ states[tos->state].reduce_size <= tos->since_indent)
+ goto force_reduce;
+ if (states[tos->state].min_prefix >= tos->since_indent) {
+ // OK to cancel
+ struct frame *in = tos - tos->since_indent;
+ in->indents -= 1;
+ if (in->indents == 0) {
+ /* Reassess since_indent and newline_permitted */
+ if (in > p.stack) {
+ in->since_indent = in[-1].since_indent + 1;
+ in->newline_permitted = in[-1].newline_permitted;
+ } else {
+ in->since_indent = 0;
+ in->newline_permitted = 0;
+ }
+ if (states[in->state].starts_line)
+ in->newline_permitted = 1;
+ while (in < tos) {
+ in += 1;
+ in->since_indent = in[-1].since_indent + 1;
+ if (states[in->state].starts_line)
+ in->newline_permitted = 1;
+ else
+ in->newline_permitted = in[-1].newline_permitted;
+ }
+ }
+ free(tk);
+ tk = NULL;
+ parser_trace_action(trace, "Cancel");
+ continue;
+ }
+ // fall through to error handling as both SHIFT and REDUCE
+ // will fail.
+ }
+ if (tk->num == TK_newline) {
+ if (!tos->newline_permitted) {
+ free(tk);
+ tk = NULL;
+ parser_trace_action(trace, "Discard");
+ continue;
+ }
+ if (tos->since_newline > 1 &&
+ states[tos->state].reduce_size >= 0 &&
+ states[tos->state].reduce_size <= tos->since_newline)
+ goto force_reduce;
+ }
+ if (shift(&p, tk->num, 0, tk->num == TK_newline, tk, states)) {
+ tk = NULL;
+ parser_trace_action(trace, "Shift");
+ continue;
+ }
+ force_reduce:
+ if (states[tos->state].reduce_prod >= 0 &&
+ states[tos->state].newline_only &&
+ tk->num != TK_newline && tk->num != TK_eof && tk->num != TK_out) {
+ /* Anything other than newline in an error as this
+ * production must end at EOL
+ */
+ } else if (states[tos->state].reduce_prod >= 0) {
void **body;
- int prod = states[p.state].reduce_prod;
- int size = states[p.state].reduce_size;
- int sym = states[p.state].reduce_sym;
+ void *res;
+ const struct state *nextstate = &states[tos->state];
+ int prod = nextstate->reduce_prod;
+ int size = nextstate->reduce_size;
int bufsize;
static char buf[16*1024];
+ short indents, start_of_line;
- body = p.asn_stack +
- (p.tos - states[p.state].reduce_size);
+ body = p.asn_stack + (p.tos - size);
- bufsize = do_reduce(prod, p.tos, body,
- buf, trace);
- if (trace)
- fputs("\n", trace);
+ bufsize = do_reduce(prod, body, config, buf);
- pop(&p, size, do_free);
- shift(&p, sym, memdup(buf, bufsize), states);
- if (prod == 0)
+ indents = pop(&p, size, &start_of_line,
+ do_free);
+ res = memdup(buf, bufsize);
+ memset(buf, 0, bufsize);
+ if (!shift(&p, nextstate->reduce_sym,
+ indents, start_of_line,
+ res, states)) {
+ if (prod != 0) abort();
accepted = 1;
+ ret = res;
+ }
+ parser_trace_action(trace, "Reduce");
continue;
}
- /* Error. we walk up the stack until we
+ /* Error. We walk up the stack until we
* find a state which will accept TK_error.
* We then shift in TK_error and see what state
* that takes us too.
* Then we discard input tokens until
* we find one that is acceptable.
*/
- while (shift(&p, TK_error, tk, states) == 0
- && p.tos > 0)
+ parser_trace_action(trace, "ERROR");
+ short indents = 0, start_of_line;
+
+ err_tk = tok_copy(*tk);
+ while (p.tos > 0 &&
+ shift(&p, TK_error, 0, 0,
+ err_tk, states) == 0)
// discard this state
- pop(&p, 1, do_free);
- tk = tok_copy(*tk);
- while (search(&states[p.state], tk->num) < 0 &&
+ indents += pop(&p, 1, &start_of_line, do_free);
+ if (p.tos == 0) {
+ free(err_tk);
+ // no state accepted TK_error
+ break;
+ }
+ tos = &p.stack[p.tos-1];
+ while (!in_lookahead(tk, states, tos->state) &&
tk->num != TK_eof) {
free(tk);
tk = tok_copy(token_next(tokens));
+ if (tk->num == TK_in)
+ indents += 1;
+ if (tk->num == TK_out) {
+ if (indents == 0)
+ break;
+ indents -= 1;
+ // FIXME update since_indent here
+ }
}
- if (p.tos == 0 && tk->num == TK_eof)
- break;
+ tos->indents += indents;
}
free(tk);
- if (accepted)
- ret = p.asn_stack[0];
- else
- pop(&p, p.tos, do_free);
+ pop(&p, p.tos, NULL, do_free);
free(p.asn_stack);
free(p.stack);
return ret;
###### exported functions
void *parser_run(struct token_state *tokens,
const struct state states[],
- int (*do_reduce)(int, int, void**, void*, FILE*),
+ int (*do_reduce)(int, void**, struct token_config*, void*),
void (*do_free)(short, void*),
- FILE *trace);
+ FILE *trace, const char *non_term[],
+ struct token_config *config);
+
+### Tracing
+
+Being able to visualize the parser in action can be invaluable when
+debugging the parser code, or trying to understand how the parse of a
+particular grammar progresses. The stack contains all the important
+state, so just printing out the stack every time around the parse loop
+can make it possible to see exactly what is happening.
+
+This doesn't explicitly show each SHIFT and REDUCE action. However they
+are easily deduced from the change between consecutive lines, and the
+details of each state can be found by cross referencing the states list
+in the stack with the "report" that parsergen can generate.
+For terminal symbols, we just dump the token. For non-terminals we
+print the name of the symbol. The look ahead token is reported at the
+end inside square brackets.
+
+###### parser functions
+
+ static char *reserved_words[] = {
+ [TK_error] = "ERROR",
+ [TK_in] = "IN",
+ [TK_out] = "OUT",
+ [TK_newline] = "NEWLINE",
+ [TK_eof] = "$eof",
+ };
+ static void parser_trace_state(FILE *trace, struct frame *f, const struct state states[])
+ {
+ fprintf(trace, "(%d", f->state);
+ if (states[f->state].starts_line)
+ fprintf(trace, "s");
+ if (f->newline_permitted)
+ fprintf(trace, "n%d", f->since_newline);
+ fprintf(trace, ") ");
+ }
+
+ void parser_trace(FILE *trace, struct parser *p,
+ struct token *tk, const struct state states[],
+ const char *non_term[], int knowns)
+ {
+ int i;
+ if (!trace)
+ return;
+ for (i = 0; i < p->tos; i++) {
+ struct frame *f = &p->stack[i];
+ if (i) {
+ int sym = f->sym;
+ if (sym < TK_reserved &&
+ reserved_words[sym] != NULL)
+ fputs(reserved_words[sym], trace);
+ else if (sym < TK_reserved + knowns) {
+ struct token *t = p->asn_stack[i];
+ text_dump(trace, t->txt, 20);
+ } else
+ fputs(non_term[sym - TK_reserved - knowns],
+ trace);
+ if (f->indents)
+ fprintf(trace, ".%d", f->indents);
+ if (f->since_newline == 0)
+ fputs("/", trace);
+ fputs(" ", trace);
+ }
+ parser_trace_state(trace, f, states);
+ }
+ fprintf(trace, "[");
+ if (tk->num < TK_reserved &&
+ reserved_words[tk->num] != NULL)
+ fputs(reserved_words[tk->num], trace);
+ else
+ text_dump(trace, tk->txt, 20);
+ fputs("]", trace);
+ }
+
+ void parser_trace_action(FILE *trace, char *action)
+ {
+ if (trace)
+ fprintf(trace, " - %s\n", action);
+ }
# A Worked Example
As `scanner` provides number parsing function using `libgmp` is it not much
work to perform arbitrary rational number calculations.
-This calculator takes one expression, or an equality test per line. The
-results are printed and in any equality test fails, the program exits with
+This calculator takes one expression, or an equality test, per line. The
+results are printed and if any equality test fails, the program exits with
an error.
-Embedding mdcode inside mdcode is rather horrible. I'd like to find a
-better approach, but as the grammar file must have 3 components I need
-something like this.
-
###### File: parsergen.mk
- calc.c : parsergen calc.cgm
- ./parsergen -o calc calc.cgm
+ calc.c calc.h : parsergen parsergen.mdc
+ ./parsergen --tag calc -o calc parsergen.mdc
calc : calc.o libparser.o libscanner.o libmdcode.o libnumber.o
$(CC) $(CFLAGS) -o calc calc.o libparser.o libscanner.o libmdcode.o libnumber.o -licuuc -lgmp
+ calctest : calc
+ ./calc parsergen.mdc
+ demos :: calctest
-###### demo grammar
-
- # header
- ~~~~~
+# calc: header
#include "number.h"
// what do we use for a demo-grammar? A calculator of course.
int err;
};
- ~~~~~
- # code
- ~~~~~
+# calc: code
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <malloc.h>
#include <gmp.h>
+ #include <string.h>
#include "mdcode.h"
#include "scanner.h"
#include "number.h"
free(n);
}
+ static int text_is(struct text t, char *s)
+ {
+ return (strlen(s) == t.len &&
+ strncmp(s, t.txt, t.len) == 0);
+ }
+
int main(int argc, char *argv[])
{
int fd = open(argv[1], O_RDONLY);
int len = lseek(fd, 0, 2);
char *file = mmap(NULL, len, PROT_READ, MAP_SHARED, fd, 0);
- struct section *s = code_extract(file, file+len, NULL);
+ struct section *table = code_extract(file, file+len, NULL);
+ struct section *s;
struct token_config config = {
.ignored = (1 << TK_line_comment)
| (1 << TK_block_comment)
- | (1 << TK_indent)
- | (1 << TK_undent),
+ | (1 << TK_in)
+ | (1 << TK_out),
.number_chars = ".,_+-",
.word_start = "",
.word_cont = "",
};
- parse_calc(s->code, &config, argc > 2 ? stderr : NULL);
+ for (s = table; s; s = s->next)
+ if (text_is(s->section, "example: input"))
+ parse_calc(s->code, &config, argc > 2 ? stderr : NULL);
+ while (table) {
+ struct section *t = table->next;
+ code_free(table->code);
+ free(table);
+ table = t;
+ }
exit(0);
}
- ~~~~~
- # grammar
- ~~~~~
+# calc: grammar
+
+ $LEFT + -
+ $LEFT * /
Session -> Session Line
| Line
exit(1);
}
}$
- | NEWLINE ${ printf("Blank line\n"); }$
+ | NEWLINE ${ printf("Blank line\n"); }$
| ERROR NEWLINE ${ printf("Skipped a bad line\n"); }$
$number
- Expression -> Expression + Term ${ mpq_init($0.val); mpq_add($0.val, $1.val, $3.val); }$
- | Expression - Term ${ mpq_init($0.val); mpq_sub($0.val, $1.val, $3.val); }$
- | Term ${ mpq_init($0.val); mpq_set($0.val, $1.val); }$
-
- Term -> Term * Factor ${ mpq_init($0.val); mpq_mul($0.val, $1.val, $3.val); }$
- | Term / Factor ${ mpq_init($0.val); mpq_div($0.val, $1.val, $3.val); }$
- | Factor ${ mpq_init($0.val); mpq_set($0.val, $1.val); }$
-
- Factor -> NUMBER ${ if (number_parse($0.val, $0.tail, $1.txt) == 0) mpq_init($0.val); }$
- | ( Expression ) ${ mpq_init($0.val); mpq_set($0.val, $2.val); }$
+ Expression -> Expression + Expression ${ mpq_init($0.val); mpq_add($0.val, $1.val, $3.val); }$
+ | Expression - Expression ${ mpq_init($0.val); mpq_sub($0.val, $1.val, $3.val); }$
+ | Expression * Expression ${ mpq_init($0.val); mpq_mul($0.val, $1.val, $3.val); }$
+ | Expression / Expression ${ mpq_init($0.val); mpq_div($0.val, $1.val, $3.val); }$
+ | NUMBER ${ if (number_parse($0.val, $0.tail, $1.txt) == 0) mpq_init($0.val); }$
+ | ( Expression ) ${ mpq_init($0.val); mpq_set($0.val, $2.val); }$
+
+# example: input
+
+ 355/113
+ 3.1415926535 - 355/113
+ 2 + 4 * 5
+ 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9
+ 10 * 9 / 2
+ 1 * 1000 + 2 * 100 + 3 * 10 + 4 * 1
+
+ error
code / ocean / blobdiff