19.10 Module 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 Caml, but is parameterized by the set of keywords of your language.
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 Parse_error) otherwise. Blanks and newlines are skipped. Comments delimited by (* and *) are skipped as well, and can be nested.
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 parse_expr = parser
                  [< 'Int n >] -> n
                | [< 'Kwd "("; n = parse_expr; 'Kwd ")" >] -> n
                | [< n1 = parse_expr; n2 = parse_remainder n1 >] -> n2
           and parse_remainder n1 = parser
                  [< 'Kwd "+"; n2 = parse_expr >] -> n1+n2
                | ...