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
>                     | ...