Lua: When is it possible to use colon syntax? -

while understand basic difference between . , :, haven't figured out when lua allows use colon syntax. instance, work:

s = "test" -- type(s) string. -- can write colon function type function string:myfunc()    return #self  end  -- , colon function calls possible s:myfunc() 

however same pattern not seem work other types. instance, when have table instead of string:

t = {} -- type(t) table. -- can write colon function type function table:myfunc()    return #self  end  -- surprisingly, colon function call not not possible! t:myfunc() -- error: attempt call method 'myfunc' (a nil value) -- verbose dot call works table.myfunc(t) 

moving on type:

x = 1 -- type(x) number. -- expecting can write colon function  -- type well. however, in case -- fails:  function number:myfunc()    return self  end -- error: attempt index global 'number' (a nil value) 

i'm trying make sense of this. correct conclude

• certain types string allow both colon-function-definitions and colon-function-calls.
• other types table allow colon-function-definitions not colon-function-calls.
• yet other types number allow neither.

what reason these differences? there list of types, showing type of colon-syntax support? there maybe workaround number case, allowing write e.g. x:abs()?

the first example on string works because, strings share same metatable, , it's stored in table named string.

from string:

the string library provides functions inside table string. sets metatable strings __index field points string table. therefore, can use string functions in object-oriented style. instance, string.byte(s,i) can written s:byte(i).

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the second example on table doesn't work because, every table has own metatable, table named table collection of functions of table library.

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types numbers don't support metatable default.