CODEWITHSUNDEEP
lualua-table
seeker_of_bacon
seeker_of_bacon

Reputation: 2429

Syntactic sugar doesn't work in metatable declaration in Lua

I'm trying to add indexing to strings like in Python. This works:

getmetatable('').__index = function(str, i) return string.sub(str, i, i) end

str1 = 'hello'    
print(str1[1])

This doesn't:

getmetatable('').__index = function(str, i) return str:sub(i, i) end

Gives out the following error:

lua: test.lua:1: C stack overflow
stack traceback:
    test.lua:1: in function '__index'
    test.lua:1: in function '__index'
    ...
    test.lua:1: in function '__index'
    test.lua:4: in main chunk
    [C]: in ?

Is there some sort of loop happening? Why?

Upvotes: 1

Views: 150

Answers (1)

nobody
nobody

Reputation: 4264

The str:method shortcut works via __index. By re-defining __index, you break that.

Since 5.2 or 5.3, Lua defines a metatable for strings, roughly like

debug.setmetatable( "", { __index = string } )

which permits writing ("foo"):sub( i, j ).

Now you come along and say

getmetatable('').__index = function(str, i) return str:sub(i, i) end

so if you say ("foo")[2], that calls __index( "foo", 2 ), and inside that this causes the lookup ("foo")["sub"] (in str:sub(i, i)). This calls __index( "foo", "sub" ), and inside that this causes the lookup ("foo")["sub"] (in str:sub(i, i)). This calls __index( "foo", "sub" ), and…

…and the stack overflows, because you lookup ("foo")["sub"] forever.

Re-defining __index in the way that you did means that you can no longer use that shortcut, which means you either have to spell out string.method instead of str:method everywhere in your code, library code that you use, … – or you keep it compatible.

Character-indexing only makes sense for numbers, so you can get both by saying

getmetatable( "" ).__index = function( str, k )
    if type( k ) ~= "number" then  return string[k]  end -- lookup in string.*
    return str:sub( k, k )
end

This will only call string.sub for a numeric index. Method names are strings, so they will still be looked up in string.

Upvotes: 5

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