Module `Caml.Genlex`

A generic lexical analyzer.

This module implements a simple 'standard' lexical analyzer, presented as a function from character streams to token streams. It implements roughly the lexical conventions of OCaml, but is parameterized by the set of keywords of your language.

Example: a lexer suitable for a desk calculator is obtained by

let lexer = make_lexer ["+";"-";"*";"/";"let";"="; "("; ")"]

The associated parser would be a function from token stream to, for instance, int, and would have rules such as:

let rec parse_expr = parser
  | [< n1 = parse_atom; n2 = parse_remainder n1 >] -> n2
and parse_atom = parser
  | [< 'Int n >] -> n
  | [< 'Kwd "("; n = parse_expr; 'Kwd ")" >] -> n
and parse_remainder n1 = parser
  | [< 'Kwd "+"; n2 = parse_expr >] -> n1+n2
  | [< >] -> n1

One should notice that the use of the parser keyword and associated notation for streams are only available through camlp4 extensions. This means that one has to preprocess its sources e. g. by using the "-pp" command-line switch of the compilers.

type token = | Kwd of string | Ident of string | Int of int | Float of float | String of string | Char of char: The type of tokens. The lexical classes are: Int and Float for integer and floating-point numbers; String for string literals, enclosed in double quotes; Char for character literals, enclosed in single quotes; Ident for identifiers (either sequences of letters, digits, underscores and quotes, or sequences of 'operator characters' such as +, *, etc); and Kwd for keywords (either identifiers or single 'special characters' such as (, }, etc).

val make_lexer : string list -> char Stream.t -> token Stream.t: Construct the lexer function. The first argument is the list of keywords. An identifier s is returned as Kwd s if s belongs to this list, and as Ident s otherwise. A special character s is returned as Kwd s if s belongs to this list, and cause a lexical error (exception Stream.Error with the offending lexeme as its parameter) otherwise. Blanks and newlines are skipped. Comments delimited by (* and *) are skipped as well, and can be nested. A Stream.Failure exception is raised if end of stream is unexpectedly reached.