Module Eio.Buf_read

Buffered input and parsing.

This module provides fairly efficient non-backtracking parsers. It is modelled on Angstrom's API, and you should use that if backtracking is needed.


let r = Buf_read.of_flow flow ~max_size:1_000_000 in
Buf_read.line r
type t

An input buffer.

exception Buffer_limit_exceeded

Raised if parsing an item would require enlarging the buffer beyond its configured limit.

type 'a parser = t -> 'a

An 'a parser is a function that consumes and returns a value of type 'a.

  • raises Failure

    The flow can't be parsed as a value of type 'a.

  • raises End_of_file

    The flow ended without enough data to parse an 'a.

val parse : ?initial_size:int -> max_size:int -> 'a parser -> _ Flow.source -> ('a, [> `Msg of string ]) Stdlib.result

parse p flow ~max_size uses p to parse everything in flow.

It is a convenience function that does

let buf = of_flow flow ~max_size in
format_errors (p <* end_of_input) buf
  • parameter initial_size

    see of_flow.

val parse_exn : ?initial_size:int -> max_size:int -> 'a parser -> _ Flow.source -> 'a

parse_exn wraps parse, but raises Failure msg if that returns Error (`Msg msg).

Catching exceptions with parse and then raising them might seem pointless, but this has the effect of turning e.g. an End_of_file exception into a Failure with a more user-friendly message.

val parse_string : 'a parser -> string -> ('a, [> `Msg of string ]) Stdlib.result

parse_string p s uses p to parse everything in s. It is defined as format_errors (p <* end_of_input) (of_string s)

val parse_string_exn : 'a parser -> string -> 'a

parse_string_exn is like parse_string, but handles errors like parse_exn.

val of_flow : ?initial_size:int -> max_size:int -> _ Flow.source -> t

of_flow ~max_size flow is a buffered reader backed by flow.

  • parameter initial_size

    The initial amount of memory to allocate for the buffer.

  • parameter max_size

    The maximum size to which the buffer may grow. This must be large enough to hold the largest single item you want to parse (e.g. the longest line, if using line), plus any terminator needed to know the value is complete (e.g. the newline character(s)). This is just to prevent a run-away input from consuming all memory, and you can usually just set it much larger than you expect to need.

val of_buffer : Cstruct.buffer -> t

of_buffer buf is a reader that reads from buf. buf is used directly, without being copied. eof_seen (of_buffer buf) = true. This module will not modify buf itself, but it will expose it via peek.

val of_string : string -> t

of_string s is a reader that reads from s.

val as_flow : t -> Flow.source_ty Std.r

as_flow t is a buffered flow.

Reading from it will return data from the buffer, only reading the underlying flow if the buffer is empty.

Reading data

val line : string parser

line parses one line.

Lines can be terminated by either LF or CRLF. The returned string does not include the terminator.

If End_of_file is reached after seeing some data but before seeing a line terminator, the data seen is returned as the last line.

val lines : string Stdlib.Seq.t parser

lines returns a sequence that lazily reads the next line until the end of the input is reached.

lines = seq line ~stop:at_end_of_input

val char : char -> unit parser

char c checks that the next byte is c and consumes it.

  • raises Failure

    if the next byte is not c

val any_char : char parser

any_char parses one character.

val peek_char : char option parser

peek_char returns Some c where c is the next character, but does not consume it.

Returns None at the end of the input stream rather than raising End_of_file.

val string : string -> unit parser

string s checks that s is the next string in the stream and consumes it.

  • raises Failure

    if s is not a prefix of the stream.

val uint8 : int parser

uint8 parses the next byte as an unsigned 8-bit integer.

module BE : sig ... end

Big endian parsers

module LE : sig ... end

Little endian parsers

val take : int -> string parser

take n takes exactly n bytes from the input.

val take_all : string parser

take_all takes all remaining data until end-of-file.

Returns "" if already at end-of-file.

  • raises Buffer_limit_exceeded

    if the remaining data exceeds or equals the buffer limit (it needs one extra byte to confirm it has reached end-of-file).

val take_while : (char -> bool) -> string parser

take_while p finds the first byte for which p is false and consumes and returns all bytes before that.

If p is true for all remaining bytes, it returns everything until end-of-file.

It will return the empty string if there are no matching characters (and therefore never raises End_of_file).

val take_while1 : (char -> bool) -> string parser

take_while1 p is like take_while. However, the parser fails with "take_while1" if at least one character of input hasn't been consumed by the parser.

val skip_while : (char -> bool) -> unit parser

skip_while p skips zero or more bytes for which p is true.

skip_while p t does the same thing as ignore (take_while p t), except that it is not limited by the buffer size.

val skip_while1 : (char -> bool) -> unit parser

skip_while1 p is like skip_while. However, the parser fails with "skip_while1" if at least one character of input hasn't been skipped.

val skip : int -> unit parser

skip n discards the next n bytes.

skip n = map ignore (take n), except that the number of skipped bytes may be larger than the buffer (it will not grow).

Note: if End_of_file is raised, all bytes in the stream will have been consumed.

val at_end_of_input : bool parser

at_end_of_input returns true when at the end of the stream, or false if there is at least one more byte to be read.

val end_of_input : unit parser

end_of_input checks that there are no further bytes in the stream.

  • raises Failure

    if there are further bytes


val seq : ?stop:bool parser -> 'a parser -> 'a Stdlib.Seq.t parser

seq p is a sequence that uses p to get the next item.

A sequence node can only be used while the stream is at the expected position, and will raise Invalid_argument if any bytes have been consumed in the meantime. This also means that each node can only be used once; use Seq.memoize to make the sequence persistent.

It is not necessary to consume all the elements of the sequence.

  • parameter stop

    This is used before parsing each item. The sequence ends if this returns true. The default is at_end_of_input.

val pair : 'a parser -> 'b parser -> ('a * 'b) parser

pair a b is a parser that first uses a to parse a value x, then uses b to parse a value y, then returns (x, y).

Note that this module does not support backtracking, so if b fails then the bytes consumed by a are lost.

val return : 'a -> 'a parser

return x is a parser that consumes nothing and always returns x. return is just Fun.const.

val map : ('a -> 'b) -> 'a parser -> 'b parser

map f a is a parser that parses the stream with a to get v, and then returns f v.

val bind : 'a parser -> ('a -> 'b parser) -> 'b parser

bind a f is a parser that first uses a to parse a value v, then uses f v to select the next parser, and then uses that.

val format_errors : 'a parser -> ('a, [> `Msg of string ]) Stdlib.result parser

format_errors p catches Failure, End_of_file and Buffer_limit_exceeded exceptions and returns them as a formatted error message.

module Syntax : sig ... end

Convenient syntax for some of the combinators.

Low-level API

val buffered_bytes : t -> int

buffered_bytes t is the number of bytes that can be read without reading from the underlying flow.

val peek : t -> Cstruct.t

peek t returns a view onto the active part of t's internal buffer.

Performing any operation that might add to the buffer may invalidate this, so it should be used immediately and then forgotten.

Cstruct.length (peek t) = buffered_bytes t.

val ensure : t -> int -> unit

ensure t n ensures that the buffer contains at least n bytes of data.

If not, it reads from the flow until there is.

buffered_bytes (ensure t n) >= n.

  • raises End_of_file

    if the flow ended before n bytes were available

val consume : t -> int -> unit

consume t n discards the first n bytes from t's buffer.

Use this after peek to mark some bytes as consumed.

buffered_bytes t' = buffered_bytes t - n

Note: unlike skip, this will not read data from the underlying flow.

val consumed_bytes : t -> int

consumed_bytes t is the total number of bytes consumed.

i.e. it is the offset into the stream of the next byte to be parsed.

val eof_seen : t -> bool

eof_seen t indicates whether we've received End_of_file from the underlying flow.

If so, there will never be any further data beyond what peek already returns.

Note that this returns false if we're at the end of the stream but don't know it yet. Use at_end_of_input to be sure.