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RFC: absolutely minimal way to close #18823 #25169

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RFC: absolutely minimal way to close #18823 #25169

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6 changes: 4 additions & 2 deletions src/julia-syntax.scm
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Original file line number Diff line number Diff line change
Expand Up @@ -1630,6 +1630,7 @@
;; (for (= lhs X) body)
(let* ((coll (make-ssavalue))
(state (gensy))
(nxt (make-ssavalue))
(outer? (and (pair? lhs) (eq? (car lhs) 'outer)))
(lhs (if outer? (cadr lhs) lhs)))
`(block (= ,coll ,(expand-forms X))
Expand All @@ -1646,8 +1647,9 @@
,@(if (and (not outer?) (or *depwarn* *deperror*))
(map (lambda (v) `(warn-if-existing ,v)) (lhs-vars lhs))
'())
,(lower-tuple-assignment (list lhs state)
`(call (top next) ,coll ,state))
(= ,nxt (call (top next) ,coll ,state))
(if (call (core ===) ,nxt (core nothing)) (break))
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I assume this check gets optimized out in the common case of a next function that always returns a tuple?

Hopefully it gets optimized out even for iteration over a type-unstable container, e.g. where next returns Tuple{ ...inference fails... , MyState}?

,(lower-tuple-assignment (list lhs state) nxt)
,body))))))

;; convert an operator parsed as (op a b) to (call op a b)
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19 changes: 19 additions & 0 deletions test/iterators.jl
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Expand Up @@ -447,3 +447,22 @@ end
@test Iterators.reverse(Iterators.reverse(t)) === t
end
end

struct Itr18823
pred::Function
rng::AbstractRNG
Itr18823(pred::Function) = new(pred, MersenneTwister())
end

Base.start(itr::Itr18823) = nothing
Base.done(itr::Itr18823, state::Void) = false

function Base.next(itr::Itr18823, state::Void)
x = rand(itr.rng)
itr.pred(x) || return nothing
return x, nothing
end

@testset "zero look-ahead iterators" begin
@test all(x -> x ≥ 0.01, Itr18823(x -> x ≥ 0.01))
end