Extension

An entry can be extended using the function ``Grammar.extend''. But its interface being quite complicated and, as it must be used with appropriate type constraints, the Camlp4 library provides a file, named ``pa_extend.cmo'', compatible with ``pa_o.cmo'' and ``pa_r.cmo'' which creates a new instruction doing this work.

This instruction is ``EXTEND'' which has the following format:

EXTEND, GLOBAL and END are keywords. There are some other keywords in this instruction, all in uppercase.

The entry is a name of an entry variable (simple identifier or prefixed by a module name). It is extended according to the rules given by extension. The optional position tells how to insert the extension among the entry precedence levels: by default, the extension is done at the first precedence level (creating this level if the entry is empty).

The optional global-list restricts the list of the global entries (separated by spaces) extended in the ``EXTEND'' instruction. The other entries are created locally. By default, all entries are global and must correspond to entry variables visible at the ``EXTEND'' instruction point.

Syntax of a position

A position can be:

• FIRST: The extension is inserted at the beginning of the precedence levels.
• LAST: The extension is inserted as the end of the precedence levels.
• BEFORE label: The extension is inserted before the precedence level so labelled.
• AFTER label: The extension is inserted after the precedence level so labelled.
• LEVEL label: The extension is inserted at the precedence level so labelled.

Only the case LEVEL extends already existing levels: the other cases create new levels.

Syntax of an extension

The syntax of an entry extension is:

[ { label } { associativity } level-rules

| ...

| { label } { associativity } level-rules ]

The optional label identifies the current level for possible future extensions. The optional associativity can be LEFTA, RIGHTA or NONA for respectively left, right and no associativity: the default is left associativity.

Syntax of a level-rules

The level-rules have the format:

[ { pattern = } symbol ; ... { pattern = } symbol { -> action }

| ...

| { pattern = } symbol ; ... { pattern = } symbol { -> action } ]

The pattern is a language simple pattern, limited to identifiers, wildcard (character ``underscore'') and tuples of patterns. If not present, it is equivalent to the wildcard.

The action is a language expression where the patterns are bound to the result of their corresponding symbol; in addition, the variable ``loc'' is bound to the source location of the rule. The action part is optional; by default it is the value ``()''.

A symbol is either:

A string

meaning: a keyword. If this keyword does not exist in the current grammar, it is added. The syntax of the keyword must respect the rules of the lexer. Its type is string.

An entry name

(simple identifier or prefixed by a module name) meaning a call to this entry. Its type is the entry result type.

The keyword SELF

meaning the entry being extended (first level).

The keyword NEXT

meaning the entry being extended at the next level.

A token pattern

among the following:

• LIDENT or UIDENT optionally followed by a string: its type is string.
• INT optionally followed by a string: its type is string.
• FLOAT of type string.
• STRING of type string.
• CHAR of type char.
• QUOTATION of type (string * string).
• ANTIQUOT followed by a string: its type is string.
• LOCATE of type (int * string).
• EOI of type unit.

They are associated with the constructors of the type ``Token.pattern'' (section 6.7). Their meaning is suggested by their name, but in fact, depends on what is generated by the lexer used. The provided lexer ``Plexer.make'' interprets them canonically according to the rules of Objective Caml and Righteous syntax (section 6.4).

A metasymbol

among:

• LIST0 and LIST1 whose syntax is:
  LISTx symbol1 { SEP symbol2 }
  meaning a list (potentially empty for LIST0 and with at least one element for LIST1) of symbol1, whose elements are optionally separated by symbol2. The type is t1 list where t1 is the type of symbol1 (the result of the optional symbol2 is lost).
• OPT followed by a symbol, meaning this symbol or nothing. The type is t option where t is the type of symbol.

A level-rules

meaning an anonymous entry with only one level.

A symbol between parentheses