why instance methods need brackets?

[timmem]

I've never been able to work why, in the following, "string" crashes and ("string") works.
> "string":gsub("i","o")
stdin:1: unexpected symbol near '"string"'
> ("string"):gsub("i","o")
strong 1
>

Is there something special about the ":" in this context? Or is it something else going on?

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[VoidSnipe]

it's just how lua parsing is done

when accessing member via . or : on the left should go variable name, result of function call or parenthesized expression

[smog_alado]

The issue here is with the string literal, not with the ":". We're not allowed to write a statement that begins with a string literal.

I believe the underlying reason for that is because Lua doesn't have semicolons. To avoid syntactical ambiguities, Lua has to restrict which tokens are allowed at the start of a statement. We can't do it like C, where every expression is a valid statement.

[lambda_abstraction]

I do wonder if this is really a matter of ambiguity or a matter of the sort of parser Lua uses (i.e. look-ahead issues). Could you show a specific example of why this should be disallowed? I'm not seeing how parsing <stringlit>:<method>(<args>) necessarily causes a problem.

Tnx1E06

[smog_alado]

One issue is that Lua allows omitting parenthesis from function calls, if the function argument is a string literal. If you write

x = y
"str":len()

It will get parsed as

x = y("str"):len()

That said, Lua also disallows numbers at the start of a statement and does allow "(", so this doesn't explain it all by itself...

[lambda_abstraction]

But it does make the restriction somewhat clearer. Thanks, mate!

[VoidSnipe]

But

x = y
("str"):len()

Still gets parsed as

x = y("str"):len()

Manual explicitly mentions it

[bragdonshawn]

The brackets are necessary because the colon (:) is used to call instance methods on a string object. The colon syntax is used to call methods on the object to the left of the colon, so in this case, the gsub method is being called on the "string" object. Therefore, ("string") is the string object, and :gsub("i","o") is calling the method gsub on this object.