Lexing(3o) OCaml library Lexing(3o)
NAME
Lexing - The run-time library for lexers generated by ocamllex.
Module
Module Lexing
Documentation
Module Lexing
: sig end
The run-time library for lexers generated by ocamllex .
Positions
type position = {
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 lex-
ing engine will manage positions.
val dummy_pos : position
A value of type position , guaranteed to be different from any valid
position.
Lexer buffers
type lexbuf = {
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 : int array ;
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 corre-
sponds to passing ~with_position:true to functions that create lexer
buffers. In this mode, the lexing engine and lexer actions are co-re-
sponsible 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 en-
gine 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 ini-
tialised before the first use of the lexbuf, and updated by the rele-
vant lexer actions (i.e. at each end of line -- see also new_line ).
val from_channel : ?with_positions:bool -> in_channel -> lexbuf
Create a lexer buffer on the given input channel. Lexing.from_channel
inchan returns a lexer buffer which reads from the input channel inchan
, at the current reading position.
val from_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.
val from_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 func-
tion, 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.
val set_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
val set_filename : lexbuf -> string -> unit
Set filename in the initial tracked position to file in lexbuf .
Since 4.11
val with_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.
Functions for lexer semantic actions
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.
val lexeme : lexbuf -> string
Lexing.lexeme lexbuf returns the string matched by the regular expres-
sion.
val lexeme_char : lexbuf -> int -> char
Lexing.lexeme_char lexbuf i returns character number i in the matched
string.
val lexeme_start : lexbuf -> int
Lexing.lexeme_start lexbuf returns the offset in the input stream of
the first character of the matched string. The first character of the
stream has offset 0.
val lexeme_end : lexbuf -> int
Lexing.lexeme_end lexbuf 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.
val lexeme_start_p : lexbuf -> position
Like lexeme_start , but return a complete position instead of an off-
set. When position tracking is disabled, the function returns
dummy_pos .
val lexeme_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 .
val new_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 3.11.0
Miscellaneous functions
val flush_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 2023-02-12 Lexing(3o)
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