OCaml library : Genlex

# Module Genlex

    module Genlex: sig .. end

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.