Added `continue` keyword to Lua 5.4 [code review requested].

Hi all,

I have patched Lua 5.4.5 to add the `continue` keyword to the for loop, while loop, and repeat loop, and I've attached the patch file and some test code. Use it like so:

for i = 1, 10 do

if i % 2 == 0 then continue end

print( i )

end

This should really help to reduce unnecessary if-statement nesting inside loops.

Since this is my first time patching Lua, can someone help to review my patch?

It was surprisingly easy since the `continue` keyword can be implemented entirely at the parser level as an implicit `goto`, similar to `break`. The only part I wasn't sure of was the part in the test_then_block function -- not sure what the purpose of that is, as my tests seem to work whether I update it or not.

Note that the patch is applied to the current Lua master from github (https://github.com/lua/lua).

Thanks in advance,

David

diff --git a/llex.c b/llex.c index b0dc0acc..f96a0239 100644 --- a/llex.c +++ b/llex.c @@ -38,7 +38,7 @@ /* ORDER RESERVED */ static const char *const luaX_tokens [] = { - "and", "break", "do", "else", "elseif", + "and", "break", "continue", "do", "else", "elseif", "end", "false", "for", "function", "goto", "if", "in", "local", "nil", "not", "or", "repeat", "return", "then", "true", "until", "while", diff --git a/llex.h b/llex.h index 389d2f86..bb55dafe 100644 --- a/llex.h +++ b/llex.h @@ -31,7 +31,7 @@ */ enum RESERVED { /* terminal symbols denoted by reserved words */ - TK_AND = FIRST_RESERVED, TK_BREAK, + TK_AND = FIRST_RESERVED, TK_BREAK, TK_CONTINUE, TK_DO, TK_ELSE, TK_ELSEIF, TK_END, TK_FALSE, TK_FOR, TK_FUNCTION, TK_GOTO, TK_IF, TK_IN, TK_LOCAL, TK_NIL, TK_NOT, TK_OR, TK_REPEAT, TK_RETURN, TK_THEN, TK_TRUE, TK_UNTIL, TK_WHILE, diff --git a/lparser.c b/lparser.c index 24668c24..773dc070 100644 --- a/lparser.c +++ b/lparser.c @@ -661,6 +661,10 @@ static l_noret undefgoto (LexState *ls, Labeldesc *gt) { msg = "break outside loop at line %d"; msg = luaO_pushfstring(ls->L, msg, gt->line); } + else if (eqstr(gt->name, luaS_newliteral(ls->L, "continue"))) { + msg = "continue outside loop at line %d"; + msg = luaO_pushfstring(ls->L, msg, gt->line); + } else { msg = "no visible label '%s' for <goto> at line %d"; msg = luaO_pushfstring(ls->L, msg, getstr(gt->name), gt->line); @@ -669,6 +673,12 @@ static l_noret undefgoto (LexState *ls, Labeldesc *gt) { } +static void endloopblock (FuncState *fs) { + LexState *ls = fs->ls; + createlabel(ls, luaS_newliteral(ls->L, "continue"), 0, 0); +} + + static void leaveblock (FuncState *fs) { BlockCnt *bl = fs->bl; LexState *ls = fs->ls; @@ -1442,6 +1452,16 @@ static void breakstat (LexState *ls) { } +/* +** Continue statement. Semantically equivalent to "goto continue". +*/ +static void contstat (LexState *ls) { + int line = ls->linenumber; + luaX_next(ls); /* skip continue */ + newgotoentry(ls, luaS_newliteral(ls->L, "continue"), line, luaK_jump(ls->fs)); +} + + /* ** Check whether there is already a label with the given 'name'. */ @@ -1477,6 +1497,7 @@ static void whilestat (LexState *ls, int line) { enterblock(fs, &bl, 1); checknext(ls, TK_DO); block(ls); + endloopblock(fs); /* inside loop scope ('continue' jumps to this point) */ luaK_jumpto(fs, whileinit); check_match(ls, TK_END, TK_WHILE, line); leaveblock(fs); @@ -1494,6 +1515,7 @@ static void repeatstat (LexState *ls, int line) { enterblock(fs, &bl2, 0); /* scope block */ luaX_next(ls); /* skip REPEAT */ statlist(ls); + endloopblock(fs); /* inside loop scope ('continue' jumps to this point) */ check_match(ls, TK_UNTIL, TK_REPEAT, line); condexit = cond(ls); /* read condition (inside scope block) */ leaveblock(fs); /* finish scope */ @@ -1554,6 +1576,7 @@ static void forbody (LexState *ls, int base, int line, int nvars, int isgen) { adjustlocalvars(ls, nvars); luaK_reserveregs(fs, nvars); block(ls); + endloopblock(fs); /* inside loop scope ('continue' jumps to this point) */ leaveblock(fs); /* end of scope for declared variables */ fixforjump(fs, prep, luaK_getlabel(fs), 0); if (isgen) { /* generic for? */ @@ -1644,12 +1667,16 @@ static void test_then_block (LexState *ls, int *escapelist) { luaX_next(ls); /* skip IF or ELSEIF */ expr(ls, &v); /* read condition */ checknext(ls, TK_THEN); - if (ls->t.token == TK_BREAK) { /* 'if x then break' ? */ + if (ls->t.token == TK_BREAK || + ls->t.token == TK_CONTINUE) { /* 'if x then (break | continue)' ? */ int line = ls->linenumber; luaK_goiffalse(ls->fs, &v); /* will jump if condition is true */ luaX_next(ls); /* skip 'break' */ enterblock(fs, &bl, 0); /* must enter block before 'goto' */ - newgotoentry(ls, luaS_newliteral(ls->L, "break"), line, v.t); + if (ls->t.token == TK_BREAK) + newgotoentry(ls, luaS_newliteral(ls->L, "break"), line, v.t); + else /* ls->t.token == TK_CONTINUE */ + newgotoentry(ls, luaS_newliteral(ls->L, "continue"), line, v.t); while (testnext(ls, ';')) {} /* skip semicolons */ if (block_follow(ls, 0)) { /* jump is the entire block? */ leaveblock(fs); @@ -1658,7 +1685,7 @@ static void test_then_block (LexState *ls, int *escapelist) { else /* must skip over 'then' part if condition is false */ jf = luaK_jump(fs); } - else { /* regular case (not a break) */ + else { /* regular case (not a break or continue) */ luaK_goiftrue(ls->fs, &v); /* skip over block if condition is false */ enterblock(fs, &bl, 0); jf = v.f; @@ -1898,6 +1925,10 @@ static void statement (LexState *ls) { breakstat(ls); break; } + case TK_CONTINUE: { /* stat -> contstat */ + contstat(ls); + break; + } case TK_GOTO: { /* stat -> 'goto' NAME */ luaX_next(ls); /* skip 'goto' */ gotostat(ls);

lst = { 'one', 'two', 'three', 'four', 'five', 'six' } local function closable() return setmetatable( {}, { __close = function() print( '', '==> I was closed.' ) end } ) end print( '-------- repeat loop -----------' ) local idx = 1 repeat local word = lst[idx] idx = idx + 1 local _<close> = closable() if word:sub( 1, 1 ) == 't' then continue end print( idx-1, word ) until idx > 6 print( '-------- while loop -----------' ) local idx = 1 while idx <= 6 do local word = lst[idx] idx = idx + 1 local _<close> = closable() if word:sub( 1, 1 ) == 't' then continue end print( idx-1, word ) end print( '-------- for loop -----------' ) for idx, word in ipairs( lst ) do local _<close> = closable() if word:sub( 1, 1 ) == 't' then continue end print( idx, word ) end print( '-------- multiple statements -----------' ) for idx, word in ipairs( lst ) do local _<close> = closable() if word:sub( 1, 1 ) == 't' then print( '', '==> I am continuing.' ) continue end print( idx, word ) end