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editing: README.md

This directory contains data needed by Bison.

# Directory Content
## Skeletons
Bison skeletons: the general shapes of the different parser kinds, that are
specialized for specific grammars by the bison program.

Currently, the supported skeletons are:

- yacc.c
  It used to be named bison.simple: it corresponds to C Yacc
  compatible LALR(1) parsers.

- lalr1.cc
  Produces a C++ parser class.

- lalr1.java
  Produces a Java parser class.

- glr.c
  A Generalized LR C parser based on Bison's LALR(1) tables.

- glr.cc
  A Generalized LR C++ parser.  Actually a C++ wrapper around glr.c.

These skeletons are the only ones supported by the Bison team.  Because the
interface between skeletons and the bison program is not finished, *we are
not bound to it*.  In particular, Bison is not mature enough for us to
consider that "foreign skeletons" are supported.

## m4sugar
This directory contains M4sugar, sort of an extended library for M4, which
is used by Bison to instantiate the skeletons.

## xslt
This directory contains XSLT programs that transform Bison's XML output into
various formats.

- bison.xsl
  A library of routines used by the other XSLT programs.

- xml2dot.xsl
  Conversion into GraphViz's dot format.

- xml2text.xsl
  Conversion into text.

- xml2xhtml.xsl
  Conversion into XHTML.

# Implementation Notes About the Skeletons

"Skeleton" in Bison parlance means "backend": a skeleton is fed by the bison
executable with LR tables, facts about the symbols, etc. and they generate
the output (say parser.cc, parser.hh, location.hh, etc.).  They are only in
charge of generating the parser and its auxiliary files, they do not
generate the XML output, the parser.output reports, nor the graphical
rendering.

The bits of information passing from bison to the backend is named
"muscles".  Muscles are passed to M4 via its standard input: it's a set of
m4 definitions.  To see them, use `--trace=muscles`.

Except for muscles, whose names are generated by bison, the skeletons have
no constraint at all on the macro names: there is no technical/theoretical
limitation, as long as you generate the output, you can do what you want.
However, of course, that would be a bad idea if, say, the C and C++
skeletons used different approaches and had completely different
implementations.  That would be a maintenance nightmare.

Below, we document some of the macros that we use in several of the
skeletons.  If you are to write a new skeleton, please, implement them for
your language.  Overall, be sure to follow the same patterns as the existing
skeletons.

## Vocabulary

We use "formal arguments", or "formals" for short, to denote the declared
parameters of a function (e.g., `int argc, const char **argv`).  Yes, this
is somewhat contradictory with `param` in the `%param` directives.

We use "effective arguments", or "args" for short, to denote the values
passed in function calls (e.g., `argc, argv`).

## Symbols

### `b4_symbol(NUM, FIELD)`
In order to unify the handling of the various aspects of symbols (tag, type
name, whether terminal, etc.), bison.exe defines one macro per (token,
field), where field can `has_id`, `id`, etc.: see
`prepare_symbol_definitions()` in `src/output.c`.

NUM can be:
- `empty` to denote the "empty" pseudo-symbol when it exists,
- `eof`, `error`, or `undef`
- a symbol number.

FIELD can be:

- `has_id`: 0 or 1
  Whether the symbol has an `id`.

- `id`: string (e.g., `exp`, `NUM`, or `TOK_NUM` with api.token.prefix)
  If `has_id`, the name of the token kind (prefixed by api.token.prefix if
  defined), otherwise empty.  Guaranteed to be usable as a C identifier.
  This is used to define the token kind (i.e., the enum used by the return
  value of yylex).  Should be named `token_kind`.

- `tag`: string
  A human readable representation of the symbol.  Can be `'foo'`,
  `'foo.id'`, `'"foo"'` etc.

- `code`: integer
  The token code associated to the token kind `id`.
  The external number as used by yylex.  Can be ASCII code when a character,
  some number chosen by bison, or some user number in the case of `%token
  FOO <NUM>`.  Corresponds to `yychar` in `yacc.c`.

- `is_token`: 0 or 1
  Whether this is a terminal symbol.

- `kind_base`: string (e.g., `YYSYMBOL_exp`, `YYSYMBOL_NUM`)
  The base of the symbol kind, i.e., the enumerator of this symbol (token or
  nonterminal) which is mapped to its `number`.

- `kind`: string
  Same as `kind_base`, but possibly with a prefix in some languages.  E.g.,
  EOF's `kind_base` and `kind` are `YYSYMBOL_YYEOF` in C, but are
  `S_YYEMPTY` and `symbol_kind::S_YYEMPTY` in C++.

- `number`: integer
  The code associated to the `kind`.
  The internal number (computed from the external number by yytranslate).
  Corresponds to yytoken in yacc.c.  This is the same number that serves as
  key in b4_symbol(NUM, FIELD).

In bison, symbols are first assigned increasing numbers in order of
  appearance (but tokens first, then nterms).  After grammar reduction,
  unused nterms are then renumbered to appear last (i.e., first tokens, then
  used nterms and finally unused nterms).  This final number NUM is the one
  contained in this field, and it is the one used as key in `b4_symbol(NUM,
  FIELD)`.

The code of the rule actions, however, is emitted before we know what
  symbols are unused, so they use the original numbers.  To avoid confusion,
  they actually use "orig NUM" instead of just "NUM".  bison also emits
  definitions for `b4_symbol(orig NUM, number)` that map from original
  numbers to the new ones.  `b4_symbol` actually resolves `orig NUM` in the
  other case, i.e., `b4_symbol(orig 42, tag)` would return the tag of the
  symbols whose original number was 42.

- `has_type`: 0, 1
  Whether has a semantic value.

- `type_tag`: string
  When api.value.type=union, the generated name for the union member.
  yytype_INT etc. for symbols that has_id, otherwise yytype_1 etc.

- `type`: string
  If it has a semantic value, its type tag, or, if variant are used,
  its type.
  In the case of api.value.type=union, type is the real type (e.g. int).

- `slot`: string
  If it has a semantic value, the name of the union member (i.e., bounces to
  either `type_tag` or `type`).  It would be better to fix our mess and
  always use `type` for the true type of the member, and `type_tag` for the
  name of the union member.

- `has_printer`: 0, 1
- `printer`: string
- `printer_file`: string
- `printer_line`: integer
- `printer_loc`: location
  If the symbol has a printer, everything about it.

- `has_destructor`, `destructor`, `destructor_file`, `destructor_line`, `destructor_loc`
  Likewise.

### `b4_symbol_value(VAL, [SYMBOL-NUM], [TYPE-TAG])`
Expansion of $$, $1, $<TYPE-TAG>3, etc.

The semantic value from a given VAL.
- `VAL`: some semantic value storage (typically a union).  e.g., `yylval`
- `SYMBOL-NUM`: the symbol number from which we extract the type tag.
- `TYPE-TAG`, the user forced the `<TYPE-TAG>`.

The result can be used safely, it is put in parens to avoid nasty precedence
issues.

### `b4_lhs_value(SYMBOL-NUM, [TYPE])`
Expansion of `$$` or `$<TYPE>$`, for symbol `SYMBOL-NUM`.

### `b4_rhs_data(RULE-LENGTH, POS)`
The data corresponding to the symbol `#POS`, where the current rule has
`RULE-LENGTH` symbols on RHS.

### `b4_rhs_value(RULE-LENGTH, POS, SYMBOL-NUM, [TYPE])`
Expansion of `$<TYPE>POS`, where the current rule has `RULE-LENGTH` symbols
on RHS.

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