Re: '__iter', yet again!

[Mark Hamburg <mark@...>]

On Dec 16, 2009, at 6:32 AM, Francesco Abbate wrote:

> But there is an error in your arguments because the '__iter'
> metamethods does not add a new concept to the language. The feature is
> already there and is the metamethods mechanism itself. We are just
> talking about adding a new metamethod category to handle iterations
> over objects.

There's an interesting general argument (meta-argument) there about the notion that the Lua language has a certain number of syntactic/semantic constructs and of those a subset can be overridden via metamethods. The structure of that subset is guided by concerns over issues such as speed, size, and semantics but growing that subset is not a fundamental change in the language though it may introduce new capabilities.

Besides iteration, some other metamethod extensions that come up include:

* First class treatment in some form for the colon operator -- i.e., allow it to be more than just syntactic sugar
* Altered semantics for __index to handle recursive usage -- e.g., pass the base table along
* Altered semantics for __newindex and/or a new metamethod to catch all writes without needing to use a proxy table

In this case, the proposal is for the inclusion of an extra step that would execute after evaluating the expression list for a for loop but before actually invoking the loop. The primary cost there would seem to be speed -- it affects all iterations though only slightly if the iteration is large -- and of course VM size. A program that made heavy use of the construct could potentially earn back the VM size cost fairly rapidly.

The proposed semantics, if I gather correctly, are "if the value of the first expression is not a function (fast exit path) and it has an __iter metamethod then invoke that metamethod to get the iterator". What happens, however, if there are multiple expressions in the list? Does the rule just not apply? Does the __iter metamethod receive the additional values as parameters?

Mark