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Module Genlex: a generic lexical analyzer
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<H2>17.10 Module <TT>Genlex</TT>: a generic lexical analyzer</H2><A NAME="s:Genlex"></A>
<A NAME="@manual321"></A><BLOCKQUOTE>
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.
</BLOCKQUOTE>
<PRE>
type token =
Kwd of string
| Ident of string
| Int of int
| Float of float
| String of string
| Char of char
</PRE>
<BLOCKQUOTE>
The type of tokens. The lexical classes are: <CODE>Int</CODE> and <CODE>Float</CODE>
for integer and floating-point numbers; <CODE>String</CODE> for
string literals, enclosed in double quotes; <CODE>Char</CODE> for
character literals, enclosed in single quotes; <CODE>Ident</CODE> for
identifiers (either sequences of letters, digits, underscores
and quotes, or sequences of ``operator characters'' such as
<CODE>+</CODE>, <CODE>*</CODE>, etc); and <CODE>Kwd</CODE> for keywords (either identifiers or
single ``special characters'' such as <CODE>(</CODE>, <CODE>}</CODE>, etc).
</BLOCKQUOTE>
<PRE>
val make_lexer: string list -> (char Stream.t -> token Stream.t)
</PRE>
<A NAME="@manual322"></A><BLOCKQUOTE>
Construct the lexer function. The first argument is the list of
keywords. An identifier <CODE>s</CODE> is returned as <CODE>Kwd s</CODE> if <CODE>s</CODE>
belongs to this list, and as <CODE>Ident s</CODE> otherwise.
A special character <CODE>s</CODE> is returned as <CODE>Kwd s</CODE> if <CODE>s</CODE>
belongs to this list, and cause a lexical error (exception
<CODE>Parse_error</CODE>) otherwise. Blanks and newlines are skipped.
Comments delimited by <CODE>(*</CODE> and <CODE>*)</CODE> are skipped as well,
and can be nested.
</BLOCKQUOTE>
<BLOCKQUOTE>
Example: a lexer suitable for a desk calculator is obtained by
<PRE>
let lexer = make_lexer ["+";"-";"*";"/";"let";"="; "("; ")"]
</PRE>
The associated parser would be a function from <CODE>token stream</CODE>
to, for instance, <CODE>int</CODE>, and would have rules such as:
<PRE>
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
| ...
</PRE>
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