write out C code files which can be compiled to make a parser.
"2D support" means that indentation and line breaks can be significant.
+Indent tokens (IN and OUT) and end-of-line (EOL) tokens can be used to
+describe the grammar and the parser can selectively ignore these where
+they aren't relevant.
There are several distinct sections.
before the beginning. As we need to store some value, `TK_eof` is used
to mark the beginning of the file as well as the end.
+Indents (IN) are sometimes shifted and sometimes only accounted.
+Whatever decision is made must apply equally to the matching OUT. To
+ensure this we keep a stack of bits in `ignored_indents` and
+`indent_depth`. When we process an IN, we record if it was ignored.
+When we see an out, we pop of the relavant bit and use it to decide how
+to handle the OUT.
+
###### parser functions
struct parser {
void **asn_stack;
int stack_size;
int tos;
+
+ ## parser state
};
#### Shift and pop
### The heart of the parser.
Now we have the parser. For each token we might shift it, trigger a
-reduction, or start error handling. 2D tokens (IN, OUT, EOL) also need
-to be handled.
+reduction, or start error handling. 2D tokens (IN, OUT, NEWLINE) might
+also be ignored. Ignoring tokens is combined with shifting.
###### parser vars
## handle error
}
+Indents are ignored unless they can be shifted onto the stack
+immediately. The end of an indented section - the OUT token - is
+ignored precisely when the indent was ignored. To keep track of this we
+need a small stack of flags, which is easily stored as bits in an
+`unsigned long`. This will never overflow and the scanner only allows
+20 levels of indentation.
-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
+###### parser state
+ unsigned long ignored_indents;
+ int indent_depth;
- 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.
+NEWLINE is ignored when in an indented section of text which was not
+explicitly expected by the grammar. So if the most recent indent is
+ignored, so is any EOL token.
For other tokens, we shift the next token if that is possible, otherwise
we try to reduce a production.
###### try shift or ignore
- if (tk->num == TK_in) {
+ if ((tk->num == TK_newline || tk->num == TK_out) &&
+ (p.ignored_indents & (1 << p.indent_depth))) {
+ /* indented, so ignore OUT and NEWLINE */
+ if (tk->num == TK_out)
+ p.indent_depth -= 1;
free(tk);
tk = NULL;
- parser_trace_action(trace, "Record");
+ parser_trace_action(trace, "Ignore");
continue;
}
- if (tk->num == TK_out) {
- if (1) {
- // OK to cancel
- 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 (1) {
free(tk);
}
}
if (shift(&p, tk->num, tk, states)) {
+ if (tk->num == TK_out)
+ p.indent_depth -= 1;
+ if (tk->num == TK_in) {
+ p.indent_depth += 1;
+ p.ignored_indents &= ~(1 << p.indent_depth);
+ }
tk = NULL;
parser_trace_action(trace, "Shift");
## did shift
continue;
}
+ if (tk->num == TK_in) {
+ /* No indent expected here, so ignore IN */
+ free(tk);
+ tk = NULL;
+ p.indent_depth += 1;
+ p.ignored_indents |= (1 << p.indent_depth);
+ parser_trace_action(trace, "Ignore");
+ continue;
+ }
+
We have already discussed the bulk of the handling of a "reduce" action,
with the `pop()` and `shift()` functions doing much of the work. There
is a little more complexity needed to manage storage for the asn (Abstract
code / ocean / commitdiff