semicolon insertion ala google go with flex

Question

I'm interested in adding semi-colon insertion ala Google Go to my flex file.

From the Go documentation:

Semicolons

Like C, Go's formal grammar uses semicolons to terminate statements; unlike C, those semicolons do not appear in the source. Instead the lexer uses a simple rule to insert semicolons automatically as it scans, so the input text is mostly free of them.

The rule is this. If the last token before a newline is an identifier (which includes words like int and float64), a basic literal such as a number or string constant, or one of the tokens

break continue fallthrough return ++ -- ) }

the lexer always inserts a semicolon after the token. This could be summarized as, “if the newline comes after a token that could end a statement, insert a semicolon”.

A semicolon can also be omitted immediately before a closing brace, so a statement such as

go func() { for { dst <- <-src } }()

needs no semicolons. Idiomatic Go programs have semicolons only in places such as for loop clauses, to separate the initializer, condition, and continuation elements. They are also necessary to separate multiple statements on a line, should you write code that way.

One caveat. You should never put the opening brace of a control structure (if, for, switch, or select) on the next line. If you do, a semicolon will be inserted before the brace, which could cause unwanted effects. Write them like this

if i < f() {
    g()
}

not like this

if i < f()  // wrong! 
{           // wrong!
    g()     // wrong!
}           // wrong!

How would I go about doing this (how can I insert tokens in the stream, how can I see the last token that was matched to see if it is a good idea, etc etc etc)?

I am using bison too, but Go seems to just use their lexer for semicolon insertion.

Answer 1

You could pass lexer result tokens through a function that inserts semicolons where necessary. Upon detection of the need to insert, the next token can be put back to the input stream, basically lexing it again in the next turn.

Below is an example that inserts a SEMICOLON before a newline, when it follows a WORD. The bison file "insert.y" is this:

%{
#include <stdio.h>

void yyerror(const char *str) {
  printf("ERROR: %s\n", str);
}

int main() {
  yyparse();
  return 0;
}
%} 
%union {
  char *string;
}
%token <string> WORD
%token SEMICOLON NEWLINE
%%
input: 
     | input WORD          {printf("WORD: %s\n", $2); free($2);}
     | input SEMICOLON     {printf("SEMICOLON\n");}
     ;
%%

and the lexer is generated by flex from this:

%{
#include <string.h>
#include "insert.tab.h"
int f(int token);
%}
%option noyywrap
%%
[ \t]          ;
[^ \t\n;]+     {yylval.string = strdup(yytext); return f(WORD);}
;              {return f(SEMICOLON);}
\n             {int token = f(NEWLINE); if (token != NEWLINE) return token;}
%%
int insert = 0;

int f(int token) {
  if (insert && token == NEWLINE) {
    unput('\n');
    insert = 0;
    return SEMICOLON;
  } else {
    insert = token == WORD;
    return token;
  }
}

For input

abc def
ghi
jkl;

it prints

WORD: abc
WORD: def
SEMICOLON
WORD: ghi
SEMICOLON
WORD: jkl
SEMICOLON

Unputting a non-constant token requires a little extra work - I have tried to keep the example simple, just to give the idea.

Answer 2

Alter the lexer rules for \n and } to look at the last token returned by the lexer. This will require that your lexer record the last token returned for every rule.

Then your newline rule will look like this:

\n   { if (newline_is_semi(last_token)) {
          return SEMICOLON;
       }
     }

newline_is_semi will check if last_token is in the list of tokens you listed.

To handle the optional semicolon before a closing brace: when matching '}' check if last_token was SEMICOLON and if not unput the '}' and return SEMICOLON

'}'  { if (last_token != SEMICOLON) {
          unput('}');
          return SEMICOLON;
       }
     }