Where causes code to be called twice about wren HOT 3 OPEN

Interesting! Sequences are evaluated lazily, which seems to be involved.
If you resolve the map sequence first via toList it prints twice only, since the where is only on the list

var bar = [1, 2].map{|x| Foo.new(x) }.toList.where{|x| x.num > 0}.toList

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I thought this was surprising too - as mhermier says it is because of the way WhereSequence calls iteratorValue on its 'upstream' sequence (at least) once to find a value that satisfies the filter and once to actually get the value (see below). Specifically iteratorValue calls back though the chain every time, in this case calling the map function on the way to the source list [1,2]

If you have two where clauses then it prints 'Creating foo' six times

var bar = [1, 2].
 map{|x| Foo.new(x) }.
 where{|x| x.num > 0}.
 where{|x| x.num < 10}.
 toList

An option would be to have WhereSequence's iterator memoise the last result but assuming you don't want to change the code, would it be possible to make this a bit clearer in the iterator protocol docs?
Thanks!

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