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Make IteratorSize(::Iterators.Cycle) not always IsInfinite and document why #54187

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Make IteratorSize(::Iterators.Cycle) not always IsInfinite and document why #54187

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make `IteratorSize(::Iterators.Cycle)` not always `IsInfinite` and do…
LilithHafner Apr 22, 2024
7b0eba0
Fix typo
LilithHafner Apr 22, 2024
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12 changes: 12 additions & 0 deletions base/generator.jl
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Expand Up @@ -83,12 +83,24 @@
This trait is generally used to select between algorithms that pre-allocate space for their
result, and algorithms that resize their result incrementally.

Most iterators' `IteratorSize` category can be determined from their type. In this case
the generic `IteratorSize(iterator) = IteratorSize(typeof(iterator))` fallback applies.
However, some iterators (e.g. [`Iterators.cycle`](@ref)) have a size category that can only
be determined at runtime. In this case the `IteratorSize(itr)` method will not be type
stable and the `IteratorSize(::Type)` method will return `SizeUnkown`.

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perhaps "Unkown" should be "Unknown".
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```jldoctest
julia> Base.IteratorSize(1:5)
Base.HasShape{1}()

julia> Base.IteratorSize((2,3))
Base.HasLength()

julia> Base.IteratorSize(Iterators.cycle(1:5))
Base.IsInfinite()

julia> Base.IteratorSize(Iterators.cycle(1:0))
Base.HasLength()
```
"""
IteratorSize(x) = IteratorSize(typeof(x))
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8 changes: 7 additions & 1 deletion base/iterators.jl
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Expand Up @@ -988,7 +988,13 @@ cycle(xs, n::Integer) = flatten(repeated(xs, n))

eltype(::Type{Cycle{I}}) where {I} = eltype(I)
IteratorEltype(::Type{Cycle{I}}) where {I} = IteratorEltype(I)
IteratorSize(::Type{Cycle{I}}) where {I} = IsInfinite() # XXX: this is false if iterator ever becomes empty
function IteratorSize(::Type{Cycle{I}}) where I
# TODO: find a better way of communicating the size of a cycle
# IsInfinite() would be false if iterator ever becomes empty
IteratorSize(I) === IsInfinite() ? IsInfinite() : SizeUnknown()
end
IteratorSize(it::Cycle) = isempty(it.xs) ? HasLength() : IsInfinite()
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Do we have other examples where IteratorSize(it) and IteratorSize(typeof(it)) (potentially) disagree? Makes me a bit uncomfortable.

And if it.xs is stateful, it seems that IsInfinite might turn out to be wrong, because isempty(it.xs) might become true.

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Do we have other examples where IteratorSize(it) and IteratorSize(typeof(it)) (potentially) disagree? Makes me a bit uncomfortable.

No, this is the first case, and changes the docs to suggest the possibility.

And if it.xs is stateful, it seems that IsInfinite might turn out to be wrong, because isempty(it.xs) might become true.

IMO that's because Iterators.cycle on stateful iterators is broken. It is explicitly documented to be infinite for non-empty arguments.

length(it::Cycle) = isempty(it.xs) ? 0 : throw(ArgumentError("Cannot compute length of infinite iterator"))

iterate(it::Cycle) = iterate(it.xs)
isdone(it::Cycle) = isdone(it.xs)
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12 changes: 12 additions & 0 deletions test/iterators.jl
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Expand Up @@ -1049,3 +1049,15 @@ end
@testset "Iterators docstrings" begin
@test isempty(Docs.undocumented_names(Iterators))
end

@testset "cycle IteratorSize (#53169)" begin
@test Base.IteratorSize(Iterators.cycle(1:3)) == Base.IsInfinite()
@test Base.IteratorSize(Iterators.cycle(1:0)) == Base.HasLength()
@test length(Iterators.cycle(1:0)) == 0
@test collect(Iterators.cycle(1:0))::Vector{Int} == Int[]
@test Base.IteratorSize(Iterators.cycle(Iterators.Repeated(6))) == Base.IsInfinite()

@test Base.IteratorSize(typeof(Iterators.cycle(1:3))) == Base.SizeUnknown()
@test Base.IteratorSize(typeof(Iterators.cycle(1:0))) == Base.SizeUnknown()
@test Base.IteratorSize(typeof(Iterators.cycle(Iterators.Repeated(6)))) == Base.IsInfinite()
end