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Allow Iterators.flatten for empty tuple #29112

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Sep 12, 2018
Merged

Allow Iterators.flatten for empty tuple #29112

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046be32
allow Iterators.flatten for empty tuple
KlausC Sep 10, 2018
61f1256
added testcase for `Iterators.flatten(())`
KlausC Sep 10, 2018
fcbea5f
moved method of `IteratorEltype` to `generator.jl`
KlausC Sep 11, 2018
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1 change: 1 addition & 0 deletions base/generator.jl
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Expand Up @@ -126,3 +126,4 @@ IteratorEltype(::Type) = HasEltype() # HasEltype is the default
IteratorEltype(::Type{Generator{I,T}}) where {I,T} = EltypeUnknown()

IteratorEltype(::Type{Any}) = EltypeUnknown()
IteratorEltype(::Type{Union{}}) = EltypeUnknown()
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Should this perhaps have known element type Union{}? I could imagine this pessimizing something.

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eltype(Union{}}) throws an error, so I think this is fine.

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This is unfortunate in combination with

julia/base/generator.jl

Line 123 in 1968b23

IteratorEltype(x) = IteratorEltype(typeof(x))

If x cannot be inferred exactly, we get typeof(x)::Type{<:Something}, which includes Type{Union{}}, although x will of course never be ::Union{}. E.g.

julia> code_warntype(Base.IteratorEltype, Tuple{Array{Int}})
Body::Union{EltypeUnknown, HasEltype}
123 1 ─ %1 = (Base.typeof)(x)::Type{#s55} where #s55<:Array{Int64,N} where N                                    │
    │   %2 = (isa)(%1, Type{Union{}})::Bool                                                                     │
    └──      goto #3 if not %2                                                                                  │
    2 ─      goto #4                                                                                            │
    3 ─ %5 = (Base.IteratorEltype)(%1)::Union{EltypeUnknown, HasEltype}                                         │
    └──      goto #4                                                                                            │
    4 ┄ %7 = φ (#2 => $(QuoteNode(Base.EltypeUnknown())), #3 => %5)::Union{EltypeUnknown, HasEltype}            │
    └──      return %7

Compare with this method removed:

julia> code_warntype(Base.IteratorEltype, Tuple{Array{Int}})
Body::Base.HasEltype
123 1 ─     return $(QuoteNode(Base.HasEltype()))

This causes the regression noted in #29464.

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@JeffreySarnoff JeffreySarnoff Oct 5, 2018 •
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fwiw

julia> code_warntype(Base.IteratorEltype, Tuple{Array{Int}})
Body::Base.HasEltype
123 1 ─     return $(QuoteNode(Base.HasEltype()))

julia> versioninfo()
Julia Version 1.0.0
Commit 5d4eaca0c9 (2018-08-08 20:58 UTC)
OS: Linux (x86_64-pc-linux-gnu)
LLVM: libLLVM-6.0.0 (ORCJIT, skylake)

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Yes, the point here is that there is a regression in 1.0.1 vs 1.0.0.

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This method should be changed to throw an error instead, like eltype.

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Ah, that won't quite work. Instead we can add a special case for flatten when eltype(I) is Union{}.

1 change: 1 addition & 0 deletions base/iterators.jl
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Expand Up @@ -880,6 +880,7 @@ _flatteneltype(I, et) = EltypeUnknown()
flatten_iteratorsize(::Union{HasShape, HasLength}, ::Type{<:NTuple{N,Any}}) where {N} = HasLength()
flatten_iteratorsize(::Union{HasShape, HasLength}, ::Type{<:Tuple}) = SizeUnknown()
flatten_iteratorsize(::Union{HasShape, HasLength}, ::Type{<:Number}) = HasLength()
flatten_iteratorsize(::Union{HasShape, HasLength}, ::Type{Union{}}) = SizeUnknown()
flatten_iteratorsize(a, b) = SizeUnknown()

_flatten_iteratorsize(sz, ::EltypeUnknown, I) = SizeUnknown()
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1 change: 1 addition & 0 deletions test/iterators.jl
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Expand Up @@ -365,6 +365,7 @@ end
@test length(flatten(zip(1:3, 4:6))) == 6
@test length(flatten(1:6)) == 6
@test collect(flatten(Any[])) == Any[]
@test collect(flatten(())) == Union{}[]
@test_throws ArgumentError length(flatten(NTuple[(1,), ()])) # #16680
@test_throws ArgumentError length(flatten([[1], [1]]))

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