Command Line compilation Using Fan¶

Hello world

Create a file hello.ml as follows:
```
print_endline "hello, Fan"
```
The compile is quite simple, make sure fan.byte or fan.native is in your search path:
```
$ ocamlc -pp 'fan.native' hello.ml -o test $ ./test
```
As you may notice, adding -pp 'fan.native' flag is enough to switching to Fan. Using fan.byte or fan.native is up to you, for the time being, only the performance matters here. So, compiling with the following command line does also work:
```
$ ocamlc -pp 'fan.native' hello.ml -o test
```
First class lexer

Writing hello world is not very interesting, for the following example, we show you how DDSL fits into Fan. Suppose we want to write a lexical analyzier to filter nested comments in OCaml, the traditional way is to write a complex regex expression, or start a new file to write a lexer. The first way is hackish, inefficient, unmaintainable in the long run while the second way is too heavy weight, since lexer generator is a standard alone external DDSL which introduces another staging phase.

Within Fan, we show how easy it is now:
```
let depth = ref 0

let rec f  = %lex{
 | "(*" -> comment lexbuf
 | '"' -> (print_char '"'; string lexbuf)
 | _ as c -> (print_char c; f lexbuf)
 | ! -> exit 0
}
and comment  = %lex{
 | "*)" ->
     if !depth = 0 then f lexbuf
     else begin
       decr depth;
       comment lexbuf
     end
 | "(*" -> incr depth
 | _ -> comment lexbuf
 | ! -> failwith "unterminated comment"
}
and string = %lex{
 | '"' -> (print_char '"'; f lexbuf)
 | _ as c -> (print_char c; string lexbuf)
 | ! -> failwith "unterminated string"
}

let _ = f (Lexing.from_channel (open_in "comment.ml"));;
```
Compiling is the same as the previous example hello:
```
$ ocamlc -pp 'fan.native' comment.ml -o comment
```
Here we see the lexer DDSL is first class construct in Fan, the user don’t need to create a new file to isolate their lexer, it’s as convenient as regex expression in perl. So it works in toplevel, it works with module system, and objects, that said, the user could make lexer reusable by using objects instead of functions.

Abot the internal of lexer DDSL, see Lexer DDSL