Lua相关问题整理（4） – 让Lua的eval函数支持赋值语句

上一篇文章提到了在Lua中实现类似于JavaScript中的eval函数，遗憾是该eval函数不支持赋值语句，原因是Lua的赋值运算符是不支持返回值。所以如果要让该eval函数也支持赋值语句，就需要一个额外的工作，让它鉴别一个语句是不是赋值语句，如果是，则return的是被赋值后变量的值。为此，我写了一个isAssignmentExpression函数，比较粗糙，不过够用了，基本思想是检测语句中第一个出现的”=“操作符，且该”=“不能在一对引号当中。

[js collapse=”true”]
— Lua code
function isAssignmentExpression( str )
local i = 1;
local curChar;
local quotesType = "none" — none, single or double
local isEscaping = false

curChar = string.sub( str, 1, 1 )
while ( curChar ~= "" ) do
if ( curChar == "’" and
isEscaping == false and
quotesType ~= "double" )
then
if ( quotesType == "single" )
then quotesType = "none"
elseif ( quotesType == "none" )
then quotesType = "single"
end
end

if ( curChar == """ and
isEscaping == false and
quotesType ~= "single" )
then
if ( quotesType == "double" )
then quotesType = "none"
elseif ( quotesType == "none" )
then quotesType = "double"
end
end

if ( curChar == "\" and isEscaping == false )
then isEscaping = true
else isEscaping = false
end

if ( curChar == "=" and quotesType == "none" )
then
if ( string.sub( str, i+1, i+1 ) ~= "=" )
then
return true, string.sub( str, 1, i – 1 )
else
return false
end
end

i = i + 1
curChar = string.sub( str, i, i )
end

return false
end

function eval( str )
local bAssign
local var
bAssign, var = isAssignmentExpression( str )
if ( bAssign )
then
print( "Assignment, var=" .. var )
loadstring( str )()
return loadstring( "return " .. var )()
else
return loadstring( "return " .. str )()
end
end

— 以下是一组测试
print( eval( "3+4" ) )
— 7
function Multiply( a, b )
return a*b
end
print( eval( "Multiply( 3, 4 )" ) )
— 12
print( eval( "i" ) )
— nil
print( eval( "i = 1" ) )
— Assignment, var=i
— 1
print( eval( "i = i + 1" ) )
— Assignment, var=i
— 2
print( eval( "i" ) )
— 2
print( eval( "i+1" ) )
— 3
print( eval( "i, j = 4, 5" ) )
— Assignment, var=i, j
— 4 5
print( eval( "i = {}" ) )
— Assignment, var=i
— table: 003CD818
print( eval( "i[ "0" ] = 0" ) )
— Assignment, var=i[ "0" ]
— 0
print( eval( "i[ "\"0=" ] = 1" ) )
— Assignment, var=i[ ""0=" ]
— 1
print( eval( "i.name="hello"" ) )
— Assignment, var=i.name
— hello
print( eval( "i[0], i.name = 4" ) )
— Assignment, var=i[0], i.name
— 4 nil
print( eval( "i == 10" ) )
— false
[/js]