Module Lexing
: (moduleStdlib__Lexing)Positionstypeposition = {
pos_fname : string ;
pos_lnum : int ;
pos_bol : int ;
pos_cnum : int ;
}
A value of type position describes a point in a source file. pos_fname is the file name; pos_lnum is the
line number; pos_bol is the offset of the beginning of the line (number of characters between the
beginning of the lexbuf and the beginning of the line); pos_cnum is the offset of the position (number of
characters between the beginning of the lexbuf and the position). The difference between pos_cnum and
pos_bol is the character offset within the line (i.e. the column number, assuming each character is one
column wide).
See the documentation of type lexbuf for information about how the lexing engine will manage positions.
valdummy_pos : position
A value of type position , guaranteed to be different from any valid position.
Lexerbufferstypelexbuf = {
refill_buff : lexbuf->unit ;
mutable lex_buffer : bytes ;
mutable lex_buffer_len : int ;
mutable lex_abs_pos : int ;
mutable lex_start_pos : int ;
mutable lex_curr_pos : int ;
mutable lex_last_pos : int ;
mutable lex_last_action : int ;
mutable lex_eof_reached : bool ;
mutable lex_mem : intarray ;
mutable lex_start_p : position ;
mutable lex_curr_p : position ;
}
The type of lexer buffers. A lexer buffer is the argument passed to the scanning functions defined by the
generated scanners. The lexer buffer holds the current state of the scanner, plus a function to refill
the buffer from the input.
Lexers can optionally maintain the lex_curr_p and lex_start_p position fields. This "position tracking"
mode is the default, and it corresponds to passing ~with_position:true to functions that create lexer
buffers. In this mode, the lexing engine and lexer actions are co-responsible for properly updating the
position fields, as described in the next paragraph. When the mode is explicitly disabled (with
~with_position:false ), the lexing engine will not touch the position fields and the lexer actions should
be careful not to do it either; the lex_curr_p and lex_start_p field will then always hold the dummy_pos
invalid position. Not tracking positions avoids allocations and memory writes and can significantly
improve the performance of the lexer in contexts where lex_start_p and lex_curr_p are not needed.
Position tracking mode works as follows. At each token, the lexing engine will copy lex_curr_p to
lex_start_p , then change the pos_cnum field of lex_curr_p by updating it with the number of characters
read since the start of the lexbuf . The other fields are left unchanged by the lexing engine. In order
to keep them accurate, they must be initialised before the first use of the lexbuf, and updated by the
relevant lexer actions (i.e. at each end of line -- see also new_line ).
valfrom_channel : ?with_positions:bool->in_channel->lexbuf
Create a lexer buffer on the given input channel. Lexing.from_channelinchan returns a lexer buffer
which reads from the input channel inchan , at the current reading position.
valfrom_string : ?with_positions:bool->string->lexbuf
Create a lexer buffer which reads from the given string. Reading starts from the first character in the
string. An end-of-input condition is generated when the end of the string is reached.
valfrom_function : ?with_positions:bool->(bytes->int->int)->lexbuf
Create a lexer buffer with the given function as its reading method. When the scanner needs more
characters, it will call the given function, giving it a byte sequence s and a byte count n . The
function should put n bytes or fewer in s , starting at index 0, and return the number of bytes provided.
A return value of 0 means end of input.
valset_position : lexbuf->position->unit
Set the initial tracked input position for lexbuf to a custom value. Ignores pos_fname . See
Lexing.set_filename for changing this field.
Since 4.11
valset_filename : lexbuf->string->unit
Set filename in the initial tracked position to file in lexbuf .
Since 4.11
valwith_positions : lexbuf->bool
Tell whether the lexer buffer keeps track of position fields lex_curr_p / lex_start_p , as determined by
the corresponding optional argument for functions that create lexer buffers (whose default value is true
).
When with_positions is false , lexer actions should not modify position fields. Doing it nevertheless
could re-enable the with_position mode and degrade performances.
Functionsforlexersemanticactions
The following functions can be called from the semantic actions of lexer definitions (the ML code
enclosed in braces that computes the value returned by lexing functions). They give access to the
character string matched by the regular expression associated with the semantic action. These functions
must be applied to the argument lexbuf , which, in the code generated by ocamllex , is bound to the lexer
buffer passed to the parsing function.
vallexeme : lexbuf->stringLexing.lexemelexbuf returns the string matched by the regular expression.
vallexeme_char : lexbuf->int->charLexing.lexeme_charlexbufi returns character number i in the matched string.
vallexeme_start : lexbuf->intLexing.lexeme_startlexbuf returns the offset in the input stream of the first character of the matched
string. The first character of the stream has offset 0.
vallexeme_end : lexbuf->intLexing.lexeme_endlexbuf returns the offset in the input stream of the character following the last
character of the matched string. The first character of the stream has offset 0.
vallexeme_start_p : lexbuf->position
Like lexeme_start , but return a complete position instead of an offset. When position tracking is
disabled, the function returns dummy_pos .
vallexeme_end_p : lexbuf->position
Like lexeme_end , but return a complete position instead of an offset. When position tracking is
disabled, the function returns dummy_pos .
valnew_line : lexbuf->unit
Update the lex_curr_p field of the lexbuf to reflect the start of a new line. You can call this function
in the semantic action of the rule that matches the end-of-line character. The function does nothing
when position tracking is disabled.
Since 3.11
Miscellaneousfunctionsvalflush_input : lexbuf->unit
Discard the contents of the buffer and reset the current position to 0. The next use of the lexbuf will
trigger a refill.
OCamldoc 2025-06-12 Stdlib.Lexing(3o)
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