Re: idea: __iter generic-for metamethod
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- Subject: Re: idea: __iter generic-for metamethod
- From: spir ☣ <denis.spir@...>
- Date: 2010年5月25日 12:28:24 +0200
On 2010年5月24日 16:38:56 -0700
"Stuart P. Bentley" <stuart@testtrack4.com> wrote:
> Depending on how common iterator factories like pairs() are (especially
> among the metatable-using crowd), this proposal might make more sense:
>
> --proposed addition
> if type(f)~= "function" then
> local meta_factory = metatable(f).__iter
> if not meta_factory and type(f)=="table" then meta_factory =
> pairs end
> if meta_factory then f, s, var = meta_factory(f) end
> end
Yes, the first version was OK, I guess; but this one is even clearer, by showing pairs() is just the standard version of iterator factory.
Denis
PS: Just to compare:
For a toy language project, I introduced the notion of traversal object for traversal loops (foreach). A traversal thing must have a 'start' method to store or setup parameter data, and a 'next' method to produce every item. Traversal objects can be used to traverse enumerated collections (iterator) or defined series (generator). Below 2 examples (untested!).
grammar:
There is a strict distinction between actions & functions.
'@' denotes the current object (this, self).
'#' is the operator returning an item from an index (getItem).
Here is how to traverse a sequence in reverse order:
Reverse : Type
start : action(sequence)
@.sequence : sequence
@.index : sequence.count
next : function
if @.index > 0
item : @.sequence#@.index
@.index :: @.index - 1
item
Here is how to generate the cubes of integers in a given interval:
Cubes : Type
start : action(interval)
@.number : interval.first
@.last : interval.last
next : function
if @.number <= @.last
cube : @.number ^ 3
@.number :: @.number + 1
cube
Any kind of traversal can be defined as "function" method in custom collections or collection types, provided the result object does the job. By default, the language looks for a method called 'traversal'. Traversals would be used as expected, I guess ('!' is the "execute" operator):
foreach item in myColl ' default 'traversal' method
foreach item in Reverse(myColl)! ' externally defined
foreach item in myColl.reverse! ' defined on myColl or its type
foreach cube in Cubes(3,9)!
________________________________
vit esse estrany ☣
spir.wikidot.com