20 files changed, 176 insertions, 154 deletions

diff --git a/lauxlib.c b/lauxlib.c
index 014e7052..5a040ac6 100644
--- a/lauxlib.c
+++ b/lauxlib.c
@@ -249,7 +249,7 @@ LUALIB_API int luaL_fileresult (lua_State *L, int stat, const char *fname) {
     return 1;
   }
   else {
-    lua_pushnil(L);
+    luaL_pushfail(L);
     if (fname)
       lua_pushfstring(L, "%s: %s", fname, strerror(en));
     else
@@ -291,10 +291,10 @@ LUALIB_API int luaL_execresult (lua_State *L, int stat) {
     if (*what == 'e' && stat == 0)  /* successful termination? */
       lua_pushboolean(L, 1);
     else
-      lua_pushnil(L);
+      luaL_pushfail(L);
     lua_pushstring(L, what);
     lua_pushinteger(L, stat);
-    return 3;  /* return true/nil,what,code */
+    return 3;  /* return true/fail,what,code */
   }
 }
 
diff --git a/lauxlib.h b/lauxlib.h
index f68f6af1..c50cdf4d 100644
--- a/lauxlib.h
+++ b/lauxlib.h
@@ -153,6 +153,10 @@ LUALIB_API void (luaL_requiref) (lua_State *L, const char *modname,
 #define luaL_loadbuffer(L,s,sz,n)       luaL_loadbufferx(L,s,sz,n,NULL)
 
 
+/* push the value used to represent failure/error */
+#define luaL_pushfail(L)        lua_pushnil(L)
+
+
 /*
 ** {======================================================
 ** Generic Buffer manipulation
diff --git a/lbaselib.c b/lbaselib.c
index c68e6d38..747fd45a 100644
--- a/lbaselib.c
+++ b/lbaselib.c
@@ -106,7 +106,7 @@ static int luaB_tonumber (lua_State *L) {
       return 1;
     }  /* else not a number */
   }  /* else not a number */
-  lua_pushnil(L);  /* not a number */
+  luaL_pushfail(L);  /* not a number */
   return 1;
 }
 
@@ -308,9 +308,9 @@ static int load_aux (lua_State *L, int status, int envidx) {
     return 1;
   }
   else {  /* error (message is on top of the stack) */
-    lua_pushnil(L);
+    luaL_pushfail(L);
     lua_insert(L, -2);  /* put before error message */
-    return 2;  /* return nil plus error message */
+    return 2;  /* return fail plus error message */
   }
 }
 
diff --git a/ldblib.c b/ldblib.c
index 64158395..9f7aad51 100644
--- a/ldblib.c
+++ b/ldblib.c
@@ -65,7 +65,7 @@ static int db_setmetatable (lua_State *L) {
 static int db_getuservalue (lua_State *L) {
   int n = (int)luaL_optinteger(L, 2, 1);
   if (lua_type(L, 1) != LUA_TUSERDATA)
-    lua_pushnil(L);
+    luaL_pushfail(L);
   else if (lua_getiuservalue(L, 1, n) != LUA_TNONE) {
     lua_pushboolean(L, 1);
     return 2;
@@ -80,7 +80,7 @@ static int db_setuservalue (lua_State *L) {
   luaL_checkany(L, 2);
   lua_settop(L, 2);
   if (!lua_setiuservalue(L, 1, n))
-    lua_pushnil(L);
+    luaL_pushfail(L);
   return 1;
 }
 
@@ -159,7 +159,7 @@ static int db_getinfo (lua_State *L) {
   }
   else {  /* stack level */
     if (!lua_getstack(L1, (int)luaL_checkinteger(L, arg + 1), &ar)) {
-      lua_pushnil(L);  /* level out of range */
+      luaL_pushfail(L);  /* level out of range */
       return 1;
     }
   }
@@ -223,7 +223,7 @@ static int db_getlocal (lua_State *L) {
       return 2;
     }
     else {
-      lua_pushnil(L);  /* no name (nor value) */
+      luaL_pushfail(L);  /* no name (nor value) */
       return 1;
     }
   }
@@ -389,8 +389,10 @@ static int db_gethook (lua_State *L) {
   char buff[5];
   int mask = lua_gethookmask(L1);
   lua_Hook hook = lua_gethook(L1);
-  if (hook == NULL)  /* no hook? */
-    lua_pushnil(L);
+  if (hook == NULL) {  /* no hook? */
+    luaL_pushfail(L);
+    return 1;
+  }
   else if (hook != hookf)  /* external hook? */
     lua_pushliteral(L, "external hook");
   else {  /* hook table must exist */
diff --git a/liolib.c b/liolib.c
index 56507d5e..d8b0a6f9 100644
--- a/liolib.c
+++ b/liolib.c
@@ -153,7 +153,7 @@ static int io_type (lua_State *L) {
   luaL_checkany(L, 1);
   p = (LStream *)luaL_testudata(L, 1, LUA_FILEHANDLE);
   if (p == NULL)
-    lua_pushnil(L);  /* not a file */
+    luaL_pushfail(L);  /* not a file */
   else if (isclosed(p))
     lua_pushliteral(L, "closed file");
   else
@@ -593,7 +593,7 @@ static int g_read (lua_State *L, FILE *f, int first) {
     return luaL_fileresult(L, 0, NULL);
   if (!success) {
     lua_pop(L, 1);  /* remove last result */
-    lua_pushnil(L);  /* push nil instead */
+    luaL_pushfail(L);  /* push nil instead */
   }
   return n - first;
 }
@@ -624,9 +624,9 @@ static int io_readline (lua_State *L) {
     lua_pushvalue(L, lua_upvalueindex(3 + i));
   n = g_read(L, p->f, 2);  /* 'n' is number of results */
   lua_assert(n > 0);  /* should return at least a nil */
-  if (!lua_isnil(L, -n))  /* read at least one value? */
+  if (lua_toboolean(L, -n))  /* read at least one value? */
     return n;  /* return them */
-  else {  /* first result is nil: EOF or error */
+  else {  /* first result is false: EOF or error */
     if (n > 1) {  /* is there error information? */
       /* 2nd result is error message */
       return luaL_error(L, "%s", lua_tostring(L, -n + 1));
@@ -782,7 +782,7 @@ static void createmeta (lua_State *L) {
 static int io_noclose (lua_State *L) {
   LStream *p = tolstream(L);
   p->closef = &io_noclose;  /* keep file opened */
-  lua_pushnil(L);
+  luaL_pushfail(L);
   lua_pushliteral(L, "cannot close standard file");
   return 2;
 }
diff --git a/lmathlib.c b/lmathlib.c
index 752647e7..f49eb318 100644
--- a/lmathlib.c
+++ b/lmathlib.c
@@ -77,7 +77,7 @@ static int math_toint (lua_State *L) {
     lua_pushinteger(L, n);
   else {
     luaL_checkany(L, 1);
-    lua_pushnil(L);  /* value is not convertible to integer */
+    luaL_pushfail(L);  /* value is not convertible to integer */
   }
   return 1;
 }
@@ -235,7 +235,7 @@ static int math_type (lua_State *L) {
     lua_pushstring(L, (lua_isinteger(L, 1)) ? "integer" : "float");
   else {
     luaL_checkany(L, 1);
-    lua_pushnil(L);
+    luaL_pushfail(L);
   }
   return 1;
 }
diff --git a/loadlib.c b/loadlib.c
index 9ef00278..d7a3fb23 100644
--- a/loadlib.c
+++ b/loadlib.c
@@ -408,10 +408,10 @@ static int ll_loadlib (lua_State *L) {
   if (stat == 0)  /* no errors? */
     return 1;  /* return the loaded function */
   else {  /* error; error message is on stack top */
-    lua_pushnil(L);
+    luaL_pushfail(L);
     lua_insert(L, -2);
     lua_pushstring(L, (stat == ERRLIB) ?  LIB_FAIL : "init");
-    return 3;  /* return nil, error message, and where */
+    return 3;  /* return fail, error message, and where */
   }
 }
 
@@ -505,9 +505,9 @@ static int ll_searchpath (lua_State *L) {
                                 luaL_optstring(L, 4, LUA_DIRSEP));
   if (f != NULL) return 1;
   else {  /* error message is on top of the stack */
-    lua_pushnil(L);
+    luaL_pushfail(L);
     lua_insert(L, -2);
-    return 2;  /* return nil + error message */
+    return 2;  /* return fail + error message */
   }
 }
 
diff --git a/lstrlib.c b/lstrlib.c
index 8c9e1a83..7f4a0184 100644
--- a/lstrlib.c
+++ b/lstrlib.c
@@ -744,7 +744,7 @@ static int str_find_aux (lua_State *L, int find) {
   const char *p = luaL_checklstring(L, 2, &lp);
   size_t init = posrelatI(luaL_optinteger(L, 3, 1), ls) - 1;
   if (init > ls) {  /* start after string's end? */
-    lua_pushnil(L);  /* cannot find anything */
+    luaL_pushfail(L);  /* cannot find anything */
     return 1;
   }
   /* explicit request or no special characters? */
@@ -779,7 +779,7 @@ static int str_find_aux (lua_State *L, int find) {
       }
     } while (s1++ < ms.src_end && !anchor);
   }
-  lua_pushnil(L);  /* not found */
+  luaL_pushfail(L);  /* not found */
   return 1;
 }
 
diff --git a/lutf8lib.c b/lutf8lib.c
index b4b787e7..e63a5a74 100644
--- a/lutf8lib.c
+++ b/lutf8lib.c
@@ -103,7 +103,7 @@ static int utflen (lua_State *L) {
   while (posi <= posj) {
     const char *s1 = utf8_decode(s + posi, NULL, !lax);
     if (s1 == NULL) {  /* conversion error? */
-      lua_pushnil(L);  /* return nil ... */
+      luaL_pushfail(L);  /* return fail ... */
       lua_pushinteger(L, posi + 1);  /* ... and current position */
       return 2;
     }
@@ -216,7 +216,7 @@ static int byteoffset (lua_State *L) {
   if (n == 0)  /* did it find given character? */
     lua_pushinteger(L, posi + 1);
   else  /* no such character */
-    lua_pushnil(L);
+    luaL_pushfail(L);
   return 1;
 }
 
diff --git a/manual/2html b/manual/2html
index 605c6e59..a300f8d4 100755
--- a/manual/2html
+++ b/manual/2html
@@ -324,6 +324,7 @@ N = function (s) return (string.gsub(s, " ", "&nbsp;")) end,
 NE = id,        -- tag"foreignphrase",
 num = id,
 ["nil"] = fixed(Tag.b"nil"),
+fail = fixed(Tag.b"fail"),
 Open = fixed"{",
 part = section("h1", true),
 Pat = compose(verbfixed, prepos("'", "'")),
diff --git a/manual/manual.of b/manual/manual.of
index bb6ae884..53073a54 100644
--- a/manual/manual.of
+++ b/manual/manual.of
@@ -4058,12 +4058,15 @@ Returns 0 if the userdata does not have that value.
 
 }
 
-@APIEntry{void lua_setmetatable (lua_State *L, int index);|
+@APIEntry{int lua_setmetatable (lua_State *L, int index);|
 @apii{1,0,-}
 
 Pops a table from the stack and
 sets it as the new metatable for the value at the given index.
 
+(For historical reasons, this function returns an @id{int},
+which now is always 1.)
+
 }
 
 @APIEntry{void lua_settable (lua_State *L, int index);|
@@ -5782,7 +5785,7 @@ that will be called to close the stream
 when the handle is closed or collected;
 this function receives the file handle as its sole argument and
 must return either @true, in case of success,
-or @nil plus an error message, in case of error.
+or a false value plus an error message, in case of error.
 Once Lua calls this field,
 it changes the field value to @id{NULL}
 to signal that the handle is closed.
@@ -5904,6 +5907,14 @@ to its expected parameters.
 For instance, a function documented as @T{foo(arg)}
 should not be called without an argument.
 
+The notation @fail means a return value representing
+some kind of failure or the absence of a better value to return.
+Currently, @fail is equal to @nil,
+but that may change in future versions.
+The recommendation is to test the success of these functions
+with @T{(not status)}, instead of @T{(status == nil)}.
+
+
 Currently, Lua has the following standard libraries:
 @itemize{
 
@@ -6108,8 +6119,8 @@ with previous results.
 A return of an empty string, @nil, or no value signals the end of the chunk.
 
 If there are no syntactic errors,
-returns the compiled chunk as a function;
-otherwise, returns @nil plus the error message.
+@id{load} returns the compiled chunk as a function;
+otherwise, it returns @fail plus the error message.
 
 When you load a main chunk,
 the resulting function will always have exactly one upvalue,
@@ -6301,7 +6312,7 @@ When called with no @id{base},
 If the argument is already a number or
 a string convertible to a number,
 then @id{tonumber} returns this number;
-otherwise, it returns @nil.
+otherwise, it returns @fail.
 
 The conversion of strings can result in integers or floats,
 according to the lexical conventions of Lua @see{lexical}.
@@ -6315,7 +6326,7 @@ In bases @N{above 10}, the letter @Char{A} (in either upper or lower case)
 @N{represents 10}, @Char{B} @N{represents 11}, and so forth,
 with @Char{Z} representing 35.
 If the string @id{e} is not a valid numeral in the given base,
-the function returns @nil.
+the function returns @fail.
 
 }
 
@@ -6762,7 +6773,7 @@ will try to open the files
 
 Returns the resulting name of the first file that it can
 open in read mode (after closing the file),
-or @nil plus an error message if none succeeds.
+or @fail plus an error message if none succeeds.
 (This error message lists all file names it tried to open.)
 
 }
@@ -6841,7 +6852,7 @@ Looks for the first match of
 @id{pattern} @see{pm} in the string @id{s}.
 If it finds a match, then @id{find} returns the indices @N{of @T{s}}
 where this occurrence starts and ends;
-otherwise, it returns @nil.
+otherwise, it returns @fail.
 A third, optional numeric argument @id{init} specifies
 where to start the search;
 its default value @N{is 1} and can be negative.
@@ -7034,7 +7045,7 @@ Looks for the first @emph{match} of
 the @id{pattern} @see{pm} in the string @id{s}.
 If it finds one, then @id{match} returns
 the captures from the pattern;
-otherwise it returns @nil.
+otherwise it returns @fail.
 If @id{pattern} specifies no captures,
 then the whole match is returned.
 A third, optional numeric argument @id{init} specifies
@@ -7499,7 +7510,7 @@ Returns the number of UTF-8 characters in string @id{s}
 that start between positions @id{i} and @id{j} (both inclusive).
 The default for @id{i} is @num{1} and for @id{j} is @num{-1}.
 If it finds any invalid byte sequence,
-returns a false value plus the position of the first invalid byte.
+returns @fail plus the position of the first invalid byte.
 
 }
 
@@ -7515,7 +7526,7 @@ so that @T{utf8.offset(s, -n)} gets the offset of the
 @id{n}-th character from the end of the string.
 If the specified character is neither in the subject
 nor right after its end,
-the function returns @nil.
+the function returns @fail.
 
 As a special case,
 when @id{n} is 0 the function returns the start of the encoding
@@ -7850,7 +7861,7 @@ Returns the tangent of @id{x} (assumed to be in radians).
 
 If the value @id{x} is convertible to an integer,
 returns that integer.
-Otherwise, returns @nil.
+Otherwise, returns @fail.
 
 }
 
@@ -7858,7 +7869,7 @@ Otherwise, returns @nil.
 
 Returns @St{integer} if @id{x} is an integer,
 @St{float} if it is a float,
-or @nil if @id{x} is not a number.
+or @fail if @id{x} is not a number.
 
 }
 
@@ -7897,10 +7908,10 @@ three predefined file handles with their usual meanings from C:
 The I/O library never closes these files.
 
 Unless otherwise stated,
-all I/O functions return @nil on failure,
+all I/O functions return @fail on failure,
 plus an error message as a second result and
 a system-dependent error code as a third result,
-and some value different from @nil on success.
+and some non-false value on success.
 On non-POSIX systems,
 the computation of the error message and error code
 in case of errors
@@ -8021,7 +8032,7 @@ and it is automatically removed when the program ends.
 Checks whether @id{obj} is a valid file handle.
 Returns the string @T{"file"} if @id{obj} is an open file handle,
 @T{"closed file"} if @id{obj} is a closed file handle,
-or @nil if @id{obj} is not a file handle.
+or @fail if @id{obj} is not a file handle.
 
 }
 
@@ -8075,7 +8086,7 @@ Reads the file @id{file},
 according to the given formats, which specify what to read.
 For each format,
 the function returns a string or a number with the characters read,
-or @nil if it cannot read data with the specified format.
+or @fail if it cannot read data with the specified format.
 (In this latter case,
 the function does not read subsequent formats.)
 When called without arguments,
@@ -8094,31 +8105,32 @@ is a valid prefix for a numeral;
 if that prefix does not form a valid numeral
 (e.g., an empty string, @St{0x}, or @St{3.4e-})
 or it is too long (more than 200 characters),
-it is discarded and the format returns @nil.
+it is discarded and the format returns @fail.
 }
 
 @item{@St{a}|
 reads the whole file, starting at the current position.
-On end of file, it returns the empty string.
+On end of file, it returns the empty string;
+this format never fails.
 }
 
 @item{@St{l}|
 reads the next line skipping the end of line,
-returning @nil on end of file.
+returning @fail on end of file.
 This is the default format.
 }
 
 @item{@St{L}|
 reads the next line keeping the end-of-line character (if present),
-returning @nil on end of file.
+returning @fail on end of file.
 }
 
 @item{@emph{number}|
 reads a string with up to this number of bytes,
-returning @nil on end of file.
+returning @fail on end of file.
 If @id{number} is zero,
 it reads nothing and returns an empty string,
-or @nil on end of file.
+or @fail on end of file.
 }
 
 }
@@ -8139,7 +8151,7 @@ specified by the string @id{whence}, as follows:
 }
 In case of success, @id{seek} returns the final file position,
 measured in bytes from the beginning of the file.
-If @id{seek} fails, it returns @nil,
+If @id{seek} fails, it returns @fail,
 plus a string describing the error.
 
 The default value for @id{whence} is @T{"cur"},
@@ -8179,7 +8191,6 @@ Writes the value of each of its arguments to @id{file}.
 The arguments must be strings or numbers.
 
 In case of success, this function returns @id{file}.
-Otherwise it returns @nil plus a string describing the error.
 
 }
 
@@ -8251,7 +8262,7 @@ This function is equivalent to the @ANSI{system}.
 It passes @id{command} to be executed by an operating system shell.
 Its first result is @true
 if the command terminated successfully,
-or @nil otherwise.
+or @fail otherwise.
 After this first result
 the function returns a string plus a number,
 as follows:
@@ -8293,7 +8304,7 @@ closes the Lua state before exiting.
 @LibEntry{os.getenv (varname)|
 
 Returns the value of the process environment variable @id{varname},
-or @nil if the variable is not defined.
+or @fail if the variable is not defined.
 
 }
 
@@ -8301,7 +8312,7 @@ or @nil if the variable is not defined.
 
 Deletes the file (or empty directory, on @x{POSIX} systems)
 with the given name.
-If this function fails, it returns @nil,
+If this function fails, it returns @fail
 plus a string describing the error and the error code.
 Otherwise, it returns true.
 
@@ -8310,7 +8321,7 @@ Otherwise, it returns true.
 @LibEntry{os.rename (oldname, newname)|
 
 Renames the file or directory named @id{oldname} to @id{newname}.
-If this function fails, it returns @nil,
+If this function fails, it returns @fail,
 plus a string describing the error and the error code.
 Otherwise, it returns true.
 
@@ -8325,7 +8336,7 @@ Sets the current locale of the program.
 @T{"monetary"}, @T{"numeric"}, or @T{"time"};
 the default category is @T{"all"}.
 The function returns the name of the new locale,
-or @nil if the request cannot be honored.
+or @fail if the request cannot be honored.
 
 If @id{locale} is the empty string,
 the current locale is set to an implementation-defined native locale.
@@ -8444,6 +8455,8 @@ the current hook function, the current hook mask,
 and the current hook count,
 as set by the @Lid{debug.sethook} function.
 
+Returns @fail if there is no active hook.
+
 }
 
 @LibEntry{debug.getinfo ([thread,] f [, what])|
@@ -8458,7 +8471,7 @@ of the given thread:
 (except for tail calls, which do not count on the stack);
 and so on.
 If @id{f} is a number greater than the number of active functions,
-then @id{getinfo} returns @nil.
+then @id{getinfo} returns @fail.
 
 The returned table can contain all the fields returned by @Lid{lua_getinfo},
 with the string @id{what} describing which fields to fill in.
@@ -8496,7 +8509,8 @@ Compile-time constants may not appear in this listing,
 if they were optimized away by the compiler.
 Negative indices refer to vararg arguments;
 @num{-1} is the first vararg argument.
-The function returns @nil if there is no variable with the given index,
+The function returns @fail
+if there is no variable with the given index,
 and raises an error when called with a level out of range.
 (You can call @Lid{debug.getinfo} to check whether the level is valid.)
 
@@ -8527,7 +8541,8 @@ Returns the registry table @see{registry}.
 
 This function returns the name and the value of the upvalue
 with index @id{up} of the function @id{f}.
-The function returns @nil if there is no upvalue with the given index.
+The function returns @fail
+if there is no upvalue with the given index.
 
 (For Lua functions,
 upvalues are the external local variables that the function uses,
@@ -8615,7 +8630,7 @@ and @N{level 1} is the hook function.)
 
 This function assigns the value @id{value} to the local variable
 with index @id{local} of the function at level @id{level} of the stack.
-The function returns @nil if there is no local
+The function returns @fail if there is no local
 variable with the given index,
 and raises an error when called with a @id{level} out of range.
 (You can call @id{getinfo} to check whether the level is valid.)
@@ -8638,7 +8653,7 @@ Returns @id{value}.
 
 This function assigns the value @id{value} to the upvalue
 with index @id{up} of the function @id{f}.
-The function returns @nil if there is no upvalue
+The function returns @fail if there is no upvalue
 with the given index.
 Otherwise, it returns the name of the upvalue.
 
@@ -8653,7 +8668,7 @@ the @id{n}-th user value associated to the given @id{udata}.
 @id{udata} must be a full userdata.
 
 Returns @id{udata},
-or @nil if the userdata does not have that value.
+or @fail if the userdata does not have that value.
 
 }
 
diff --git a/testes/api.lua b/testes/api.lua
index 4f9d6717..b2680633 100644
--- a/testes/api.lua
+++ b/testes/api.lua
@@ -698,7 +698,7 @@ for k, v in ipairs(t) do
   assert(v1 == v and p)
 end
 
-assert(debug.getuservalue(4) == nil)
+assert(not debug.getuservalue(4))
 
 debug.setuservalue(b, function () return 10 end, 10)
 collectgarbage()   -- function should not be collected
diff --git a/testes/db.lua b/testes/db.lua
index a64a1130..c43243a6 100644
--- a/testes/db.lua
+++ b/testes/db.lua
@@ -351,12 +351,12 @@ assert(g(0,0) == 30)
  
 
 debug.sethook(nil);
-assert(debug.gethook() == nil)
+assert(not debug.gethook())
 
 
 -- minimal tests for setuservalue/getuservalue
 do
-  assert(debug.setuservalue(io.stdin, 10) == nil)
+  assert(not debug.setuservalue(io.stdin, 10))
   local a, b = debug.getuservalue(io.stdin, 10)
   assert(a == nil and not b)
 end
@@ -414,7 +414,7 @@ end, "c")
 a:f(1,2,3,4,5)
 assert(X.self == a and X.a == 1   and X.b == 2 and X.c == nil)
 assert(XX == 12)
-assert(debug.gethook() == nil)
+assert(not debug.gethook())
 
 
 -- testing access to local variables in return hook (bug in 5.2)
diff --git a/testes/errors.lua b/testes/errors.lua
index 6e7b8004..f9623b1d 100644
--- a/testes/errors.lua
+++ b/testes/errors.lua
@@ -18,7 +18,7 @@ end
 
 local function doit (s)
   local f, msg = load(s)
-  if f == nil then return msg end
+  if not f then return msg end
   local cond, msg = pcall(f)
   return (not cond) and msg
 end
@@ -312,8 +312,8 @@ end
 
 local function lineerror (s, l)
   local err,msg = pcall(load(s))
-  local line = string.match(msg, ":(%d+):")
-  assert(tonumber(line) == l)
+  local line = tonumber(string.match(msg, ":(%d+):"))
+  assert(line == l or (not line and not l))
 end
 
 lineerror("local a\n for i=1,'a' do \n print(i) \n end", 2)
@@ -359,7 +359,7 @@ local p = [[
 g()
 ]]
 X=3;lineerror((p), 3)
-X=0;lineerror((p), nil)
+X=0;lineerror((p), false)
 X=1;lineerror((p), 2)
 X=2;lineerror((p), 1)
 
@@ -510,7 +510,7 @@ checksyntax("a\1a = 1", "", "<\\1>", 1)
 checksyntax("\255a = 1", "", "<\\255>", 1)
 
 doit('I = load("a=9+"); a=3')
-assert(a==3 and I == nil)
+assert(a==3 and not I)
 print('+')
 
 lim = 1000
diff --git a/testes/files.lua b/testes/files.lua
index 585e5948..677c0dc2 100644
--- a/testes/files.lua
+++ b/testes/files.lua
@@ -184,7 +184,7 @@ three
   local f <close> = assert(io.open(file, "r"))
   -- second item failing
   l1, n1, n2, dummy = f:read("l", "n", "n", "l")
-  assert(l1 == "a line" and n1 == nil)
+  assert(l1 == "a line" and not n1)
 end
 assert(os.remove(file))
 
@@ -228,7 +228,7 @@ assert(f:read("n") == 0Xdeadbeefdeadbeef); assert(f:read(2) == "x\n")
 assert(f:read("n") == 0x1.13aP3); assert(f:read(1) == "e")
 
 do   -- attempt to read too long number
-  assert(f:read("n") == nil)  -- fails
+  assert(not f:read("n"))  -- fails
   local s = f:read("L")   -- read rest of line
   assert(string.find(s, "^00*\n$"))  -- lots of 0's left
 end
@@ -314,13 +314,13 @@ assert(io.read() == "fourth_line")
 assert(io.read() == "")  -- empty line
 assert(io.read('n') == 3450)
 assert(io.read(1) == '\n')
-assert(io.read(0) == nil)  -- end of file
-assert(io.read(1) == nil)  -- end of file
-assert(io.read(30000) == nil)  -- end of file
+assert(not io.read(0))  -- end of file
+assert(not io.read(1))  -- end of file
+assert(not io.read(30000))  -- end of file
 assert(({io.read(1)})[2] == undef)
-assert(io.read() == nil)  -- end of file
+assert(not io.read())  -- end of file
 assert(({io.read()})[2] == undef)
-assert(io.read('n') == nil)  -- end of file
+assert(not io.read('n'))  -- end of file
 assert(({io.read('n')})[2] == undef)
 assert(io.read('a') == '')  -- end of file (OK for 'a')
 assert(io.read('a') == '')  -- end of file (OK for 'a')
@@ -356,7 +356,7 @@ assert(io.read(string.len(t)) == t)
 assert(io.read(1) == ' ')
 assert(io.read(0))
 assert(io.read('a') == ';end of file\n')
-assert(io.read(0) == nil)
+assert(not io.read(0))
 assert(io.close(io.input()))
 
 
@@ -364,7 +364,7 @@ assert(io.close(io.input()))
 do
   local function ismsg (m)
     -- error message is not a code number
-    return (type(m) == "string" and tonumber(m) == nil)
+    return (type(m) == "string" and not tonumber(m))
   end
 
   -- read
@@ -393,7 +393,7 @@ assert(io.read"L" == "\n")
 assert(io.read"L" == "\n")
 assert(io.read"L" == "line\n")
 assert(io.read"L" == "other")
-assert(io.read"L" == nil)
+assert(not io.read"L")
 io.input():close()
 
 local f = assert(io.open(file))
@@ -462,7 +462,7 @@ end
 -- test for multipe arguments in 'lines'
 io.output(file); io.write"0123456789\n":close()
 for a,b in io.lines(file, 1, 1) do
-  if a == "\n" then assert(b == nil)
+  if a == "\n" then assert(not b)
   else assert(tonumber(a) == tonumber(b) - 1)
   end
 end
@@ -473,13 +473,13 @@ end
 
 for a,b,c in io.lines(file, "a", 0, 1) do
   if a == "" then break end
-  assert(a == "0123456789\n" and b == nil and c == nil)
+  assert(a == "0123456789\n" and not b and not c)
 end
 collectgarbage()   -- to close file in previous iteration
 
 io.output(file); io.write"00\n10\n20\n30\n40\n":close()
 for a, b in io.lines(file, "n", "n") do
-  if a == 40 then assert(b == nil)
+  if a == 40 then assert(not b)
   else assert(a == b - 10)
   end
 end
@@ -654,7 +654,7 @@ and the rest of the file
 io.input(file)
 local _,a,b,c,d,e,h,__ = io.read(1, 'n', 'n', 'l', 'l', 'l', 'a', 10)
 assert(io.close(io.input()))
-assert(_ == ' ' and __ == nil)
+assert(_ == ' ' and not __)
 assert(type(a) == 'number' and a==123.4 and b==-56e-2)
 assert(d=='second line' and e=='third line')
 assert(h==[[
@@ -706,7 +706,7 @@ if not _soft then
   io.input():seek('set', 0)
   y = io.read()  -- huge line
   assert(x == y..'\n'..io.read())
-  assert(io.read() == nil)
+  assert(not io.read())
   io.close(io.input())
   assert(os.remove(file))
   x = nil; y = nil
diff --git a/testes/literals.lua b/testes/literals.lua
index 27f9377d..e101eabf 100644
--- a/testes/literals.lua
+++ b/testes/literals.lua
@@ -281,7 +281,7 @@ if os.setlocale("pt_BR") or os.setlocale("ptb") then
 
   assert(" 0x.1 " + " 0x,1" + "-0X.1\t" == 0x0.1)
 
-  assert(tonumber"inf" == nil and tonumber"NAN" == nil)
+  assert(not tonumber"inf" and not tonumber"NAN")
 
   assert(assert(load(string.format("return %q", 4.51)))() == 4.51)
 
diff --git a/testes/math.lua b/testes/math.lua
index bad43901..c7dc8285 100644
--- a/testes/math.lua
+++ b/testes/math.lua
@@ -39,7 +39,7 @@ do
 end
 
 assert(math.type(0) == "integer" and math.type(0.0) == "float"
-       and math.type("10") == nil)
+       and not math.type("10"))
 
 
 local function checkerror (msg, f, ...)
@@ -381,17 +381,17 @@ assert(tonumber(1/0) == 1/0)
 
 -- 'tonumber' with strings
 assert(tonumber("0") == 0)
-assert(tonumber("") == nil)
-assert(tonumber("  ") == nil)
-assert(tonumber("-") == nil)
-assert(tonumber("  -0x ") == nil)
-assert(tonumber{} == nil)
+assert(not tonumber(""))
+assert(not tonumber("  "))
+assert(not tonumber("-"))
+assert(not tonumber("  -0x "))
+assert(not tonumber{})
 assert(tonumber'+0.01' == 1/100 and tonumber'+.01' == 0.01 and
        tonumber'.01' == 0.01    and tonumber'-1.' == -1 and
        tonumber'+1.' == 1)
-assert(tonumber'+ 0.01' == nil and tonumber'+.e1' == nil and
-       tonumber'1e' == nil     and tonumber'1.0e+' == nil and
-       tonumber'.' == nil)
+assert(not tonumber'+ 0.01' and not tonumber'+.e1' and
+       not tonumber'1e'     and not tonumber'1.0e+' and
+       not tonumber'.')
 assert(tonumber('-012') == -010-2)
 assert(tonumber('-1.2e2') == - - -120)
 
@@ -445,45 +445,45 @@ local function f (...)
   end
 end
 
-assert(f(tonumber('fFfa', 15)) == nil)
-assert(f(tonumber('099', 8)) == nil)
-assert(f(tonumber('1\0', 2)) == nil)
-assert(f(tonumber('', 8)) == nil)
-assert(f(tonumber('  ', 9)) == nil)
-assert(f(tonumber('  ', 9)) == nil)
-assert(f(tonumber('0xf', 10)) == nil)
+assert(not f(tonumber('fFfa', 15)))
+assert(not f(tonumber('099', 8)))
+assert(not f(tonumber('1\0', 2)))
+assert(not f(tonumber('', 8)))
+assert(not f(tonumber('  ', 9)))
+assert(not f(tonumber('  ', 9)))
+assert(not f(tonumber('0xf', 10)))
 
-assert(f(tonumber('inf')) == nil)
-assert(f(tonumber(' INF ')) == nil)
-assert(f(tonumber('Nan')) == nil)
-assert(f(tonumber('nan')) == nil)
+assert(not f(tonumber('inf')))
+assert(not f(tonumber(' INF ')))
+assert(not f(tonumber('Nan')))
+assert(not f(tonumber('nan')))
 
-assert(f(tonumber('  ')) == nil)
-assert(f(tonumber('')) == nil)
-assert(f(tonumber('1  a')) == nil)
-assert(f(tonumber('1  a', 2)) == nil)
-assert(f(tonumber('1\0')) == nil)
-assert(f(tonumber('1 \0')) == nil)
-assert(f(tonumber('1\0 ')) == nil)
-assert(f(tonumber('e1')) == nil)
-assert(f(tonumber('e  1')) == nil)
-assert(f(tonumber(' 3.4.5 ')) == nil)
+assert(not f(tonumber('  ')))
+assert(not f(tonumber('')))
+assert(not f(tonumber('1  a')))
+assert(not f(tonumber('1  a', 2)))
+assert(not f(tonumber('1\0')))
+assert(not f(tonumber('1 \0')))
+assert(not f(tonumber('1\0 ')))
+assert(not f(tonumber('e1')))
+assert(not f(tonumber('e  1')))
+assert(not f(tonumber(' 3.4.5 ')))
 
 
 -- testing 'tonumber' for invalid hexadecimal formats
 
-assert(tonumber('0x') == nil)
-assert(tonumber('x') == nil)
-assert(tonumber('x3') == nil)
-assert(tonumber('0x3.3.3') == nil)   -- two decimal points
-assert(tonumber('00x2') == nil)
-assert(tonumber('0x 2') == nil)
-assert(tonumber('0 x2') == nil)
-assert(tonumber('23x') == nil)
-assert(tonumber('- 0xaa') == nil)
-assert(tonumber('-0xaaP ') == nil)   -- no exponent
-assert(tonumber('0x0.51p') == nil)
-assert(tonumber('0x5p+-2') == nil)
+assert(not tonumber('0x'))
+assert(not tonumber('x'))
+assert(not tonumber('x3'))
+assert(not tonumber('0x3.3.3'))   -- two decimal points
+assert(not tonumber('00x2'))
+assert(not tonumber('0x 2'))
+assert(not tonumber('0 x2'))
+assert(not tonumber('23x'))
+assert(not tonumber('- 0xaa'))
+assert(not tonumber('-0xaaP '))   -- no exponent
+assert(not tonumber('0x0.51p'))
+assert(not tonumber('0x5p+-2'))
 
 
 -- testing hexadecimal numerals
@@ -705,19 +705,19 @@ do   -- testing floor & ceil
   assert(eqT(math.tointeger(maxint), maxint))
   assert(eqT(math.tointeger(maxint .. ""), maxint))
   assert(eqT(math.tointeger(minint + 0.0), minint))
-  assert(math.tointeger(0.0 - minint) == nil)
-  assert(math.tointeger(math.pi) == nil)
-  assert(math.tointeger(-math.pi) == nil)
+  assert(not math.tointeger(0.0 - minint))
+  assert(not math.tointeger(math.pi))
+  assert(not math.tointeger(-math.pi))
   assert(math.floor(math.huge) == math.huge)
   assert(math.ceil(math.huge) == math.huge)
-  assert(math.tointeger(math.huge) == nil)
+  assert(not math.tointeger(math.huge))
   assert(math.floor(-math.huge) == -math.huge)
   assert(math.ceil(-math.huge) == -math.huge)
-  assert(math.tointeger(-math.huge) == nil)
+  assert(not math.tointeger(-math.huge))
   assert(math.tointeger("34.0") == 34)
-  assert(math.tointeger("34.3") == nil)
-  assert(math.tointeger({}) == nil)
-  assert(math.tointeger(0/0) == nil)    -- NaN
+  assert(not math.tointeger("34.3"))
+  assert(not math.tointeger({}))
+  assert(not math.tointeger(0/0))    -- NaN
 end
 
 
diff --git a/testes/pm.lua b/testes/pm.lua
index 4d87fad2..94bb63ca 100644
--- a/testes/pm.lua
+++ b/testes/pm.lua
@@ -28,10 +28,10 @@ a,b = string.find('a\0a\0a\0a\0\0ab', '\0ab', 2);  -- finds at the end
 assert(a == 9 and b == 11);
 a,b = string.find('a\0a\0a\0a\0\0ab', 'b')    -- last position
 assert(a == 11 and b == 11)
-assert(string.find('a\0a\0a\0a\0\0ab', 'b\0') == nil)   -- check ending
-assert(string.find('', '\0') == nil)
+assert(not string.find('a\0a\0a\0a\0\0ab', 'b\0'))   -- check ending
+assert(not string.find('', '\0'))
 assert(string.find('alo123alo', '12') == 4)
-assert(string.find('alo123alo', '^12') == nil)
+assert(not string.find('alo123alo', '^12'))
 
 assert(string.match("aaab", ".*b") == "aaab")
 assert(string.match("aaa", ".*a") == "aaa")
@@ -57,17 +57,17 @@ assert(f('aaa', 'ab*a') == 'aa')
 assert(f('aba', 'ab*a') == 'aba')
 assert(f('aaab', 'a+') == 'aaa')
 assert(f('aaa', '^.+$') == 'aaa')
-assert(f('aaa', 'b+') == nil)
-assert(f('aaa', 'ab+a') == nil)
+assert(not f('aaa', 'b+'))
+assert(not f('aaa', 'ab+a'))
 assert(f('aba', 'ab+a') == 'aba')
 assert(f('a$a', '.$') == 'a')
 assert(f('a$a', '.%$') == 'a$')
 assert(f('a$a', '.$.') == 'a$a')
-assert(f('a$a', '$$') == nil)
-assert(f('a$b', 'a$') == nil)
+assert(not f('a$a', '$$'))
+assert(not f('a$b', 'a$'))
 assert(f('a$a', '$') == '')
 assert(f('', 'b*') == '')
-assert(f('aaa', 'bb*') == nil)
+assert(not f('aaa', 'bb*'))
 assert(f('aaab', 'a-') == '')
 assert(f('aaa', '^.-$') == 'aaa')
 assert(f('aabaaabaaabaaaba', 'b.*b') == 'baaabaaabaaab')
@@ -101,7 +101,7 @@ end
 assert(f1('alo alx 123 b\0o b\0o', '(..*) %1') == "b\0o b\0o")
 assert(f1('axz123= 4= 4 34', '(.+)=(.*)=%2 %1') == '3= 4= 4 3')
 assert(f1('=======', '^(=*)=%1$') == '=======')
-assert(string.match('==========', '^([=]*)=%1$') == nil)
+assert(not string.match('==========', '^([=]*)=%1$'))
 
 local function range (i, j)
   if i <= j then
@@ -135,7 +135,7 @@ print('+');
 assert(string.match("alo xyzK", "(%w+)K") == "xyz")
 assert(string.match("254 K", "(%d*)K") == "")
 assert(string.match("alo ", "(%w*)$") == "")
-assert(string.match("alo ", "(%w+)$") == nil)
+assert(not string.match("alo ", "(%w+)$"))
 assert(string.find("(álo)", "%(á") == 1)
 local a, b, c, d, e = string.match("âlo alo", "^(((.).).* (%w*))$")
 assert(a == 'âlo alo' and b == 'âl' and c == 'â' and d == 'alo' and e == nil)
@@ -209,7 +209,7 @@ assert(s == r and t[1] == 1 and t[3] == 3 and t[7] == 4 and t[13] == 4)
 
 
 function isbalanced (s)
-  return string.find(string.gsub(s, "%b()", ""), "[()]") == nil
+  return not string.find(string.gsub(s, "%b()", ""), "[()]")
 end
 
 assert(isbalanced("(9 ((8))(\0) 7) \0\0 a b ()(c)() a"))
diff --git a/testes/strings.lua b/testes/strings.lua
index 2e0e160f..97875ec0 100644
--- a/testes/strings.lua
+++ b/testes/strings.lua
@@ -56,13 +56,13 @@ a,b = string.find("123456789", "345")
 assert(string.sub("123456789", a, b) == "345")
 assert(string.find("1234567890123456789", "345", 3) == 3)
 assert(string.find("1234567890123456789", "345", 4) == 13)
-assert(string.find("1234567890123456789", "346", 4) == nil)
+assert(not string.find("1234567890123456789", "346", 4))
 assert(string.find("1234567890123456789", ".45", -9) == 13)
-assert(string.find("abcdefg", "\0", 5, 1) == nil)
+assert(not string.find("abcdefg", "\0", 5, 1))
 assert(string.find("", "") == 1)
 assert(string.find("", "", 1) == 1)
 assert(not string.find("", "", 2))
-assert(string.find('', 'aaa', 1) == nil)
+assert(not string.find('', 'aaa', 1))
 assert(('alo(.)alo'):find('(.)', 1, 1) == 4)
 
 assert(string.len("") == 0)
diff --git a/testes/utf8.lua b/testes/utf8.lua
index acbb181d..5954f6e8 100644
--- a/testes/utf8.lua
+++ b/testes/utf8.lua
@@ -30,8 +30,8 @@ local function checksyntax (s, t)
   assert(assert(load(ts))() == s)
 end
 
-assert(utf8.offset("alo", 5) == nil)
-assert(utf8.offset("alo", -4) == nil)
+assert(not utf8.offset("alo", 5))
+assert(not utf8.offset("alo", -4))
 
 -- 'check' makes several tests over the validity of string 's'.
 -- 't' is the list of codepoints of 's'.