A version of flex that can produce Go code
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README.md

A version of flex that can produce Go code.

This is a modified version of flex version 2.6.0, see: http://flex.sourceforge.net/

Keywords: flex, lex, go, golang

A basic scanner

%top{

    // code to put at the very top of the file

    package main

    import (
        "io"
        "log"
        "os"
    )

    // return type of yy.Lex()
    type YYtype interface{}

    // type of Scanner.Context
    type YYcontext interface{}

}

%{

    // code to put near the top of the file

    YY_USER_DATA(
        // user data inside Lex()
    )

    YY_USER_INIT(
        // code to run inside Lex() when it is called the first time
    )

    YY_USER_ACTION(
        // code to run at the start of each action
    )

    // more code

%}

    // definitions: no definition

%%

Hi      yyout.Write([]byte("Hello world!")

%%

    // code to put at the end of the file

    func main() {
        // runs yy.Lex() on files given as arguments, or stdin if there are no arguments
        YYmain(os.Args[1:]...)
    }

See directory tests_go for more complex examples.

To compile a flex file into Go code, call one of these:

flex --go
flex -G
flexgo

Run flex without --go or -G to get the behaviour of regular flex.

Regular flex output is C code with a lot of global variables. With the option --reentrant you get a scanner with those variables encapsulated in a struct, and there are special commands to interact with the scanner.

The Go code produced with flexgo has no global variables, other then those that should never be changed. A scanner is an object you create with NewScanner() and then you call its Lex() method.

To pass in extra parameters to the scanner, do:

%top{

    // type of Scanner.Context
    type YYcontext struct {
        foo int
        bar int
    }

    // more...

}

//more ...

scanner := NewScanner()
scanner.Context.foo = 42
scanner.Context.bar = 1999
scanner.Lex()

Inside the scanner, the variable is available as yy.Context.foo

Not implemented

Multiple buffers are not supported. If you want to switch input sources temporarily, just start another scanner. There are no global variables.

Ignored/irrelevant options:

  • --array
  • --c++
  • --reentrant
  • others... TODO

Difference between C output and Go output

Variables and functions

        C                  Go
----------------------------------------------------------------
        yyin               yy.In       io.Reader
        yyout              yy.Out      io.Writer
        yylineno           yy.Lineno   int
        yytext             yytext      []byte
    int yywrap(void)       yy.Wrap     func() bool

YY_DECL yylex(void)        yy.Lex() YYtype
    int input(void)        yyinput() (byte, error)
   void unput(int)         yyunput(byte)

Inside actions, the names yyout and yylineno are also available.

The default value of yy.Wrap is a function that always returns true.

Macros

Because Go doesn't have the #define of C, flexgo uses more M4 macros than regular flex, to achieve the same functionality. Like flex, the tokens [[ and ]] have special meaning.

The following are defined as macros:

  • INITIAL
  • BEGIN
  • ECHO
  • REJECT
  • yyterminate()
  • yyless(n)
  • yymore()
  • yyinput()
  • yyunput(c)
  • YY_START
  • YYSTATE

Only those relevant to the user are listed.

The M4 macro mechanism has some particularities you have to watch out for.

BEGIN is defined as a macro without argument:

BEGIN state     // OK
BEGIN (state)   // OK
BEGIN(state)    // Error: MUST use space
BEGIN x + y     // Error: MUST use parenthesis
BEGIN (x + y)   // OK
BEGIN(x + y)    // Error: MUST use space

yyless(n) and yyinput(c) are defined as macros with argument:

yyless(n)      // OK
yyless (n)     // Error: MUST NOT use space

Furthermore, there are three macros that can be used to set data and code inside the scanner, YY_USER_DATA() (not in regular flex), YY_USER_INIT(), and YY_USER_ACTION(). If you use these macros, you must put them in the block between %{ and %}. See above, the basic scanner.

The macro YY_USER_INIT() doesn't seem very useful in Go, but it is there in regular flex.

YYmain

YYmain(filenames ...string) is a convenience function. If you call it without arguments, it is the same as this:

scanner := NewScanner()
return scanner.Lex(), nil

When there are filenames, YYmain sets up scanner.YYwrap() to iterate over all files before calling scanner.Lex().

Inside actions in the scanner, the current file name is available as yy.Filename.

IsInteractive

Regular flex checks to see if the input file is a terminal. It then reads in bytes one by one instead of in large chunks.

In flexgo, the input is a io.Reader, and there is no easy way to tell if it's interactive or not. Calling scanner := NewScanner() sets up the variable scanner.IsInteractive(io.Reader) to return true if you called flexgo with the option -I, and false if you called flexgo with the option -B. You can point scanner.IsInteractive to another function of you need more control.

To do

  • Use YY_FATAL() for critical errors?
  • More tests