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Fix eltype of flatten of tuple with non-2 length #56802

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Fix eltype of flatten of tuple with non-2 length #56802

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aeb3473
Fix eltype of flatten of tuple with non-2 length
jakobnissen Dec 11, 2024
d084db4
Use foldable meta?
jakobnissen Dec 11, 2024
184153e
Delete dead code
jakobnissen Dec 11, 2024
6eae199
Use recursion to encourage constant folding
jakobnissen Dec 12, 2024
a989a70
Back to recursive implementation
jakobnissen Dec 12, 2024
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Fix eltype of flatten of tuple with non-2 length 
In 4c076c8, eltype of flatten of tuple was improved by computing a refined
eltype at compile time. However, this implementation only worked for length-2
tuples, and errored for all others.
Generalize this to all tuples.
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jakobnissen committed Dec 11, 2024
commit aeb3473e2eba4df4537d33f852337191c44b8acc
16 changes: 15 additions & 1 deletion base/iterators.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -1202,7 +1202,21 @@ julia> [(x,y) for x in 0:1 for y in 'a':'c'] # collects generators involving It
flatten(itr) = Flatten(itr)

eltype(::Type{Flatten{I}}) where {I} = eltype(eltype(I))
eltype(::Type{Flatten{I}}) where {I<:Union{Tuple,NamedTuple}} = promote_typejoin(map(eltype, fieldtypes(I))...)

# For tuples, we statically know the element type of each index, so we can compute
# this at compile time.
Base.@assume_effects :foldable function eltype(
::Type{Flatten{I}}
) where {I<:Union{Tuple,NamedTuple}}
T = Union{}
for i in fieldtypes(I)
T = Base.promote_typejoin(T, eltype(i))
T === Any && return Any
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end
T
end

promote_typejoin(map(eltype, fieldtypes(I))...)
eltype(::Type{Flatten{Tuple{}}}) = eltype(Tuple{})
IteratorEltype(::Type{Flatten{I}}) where {I} = _flatteneltype(I, IteratorEltype(I))
IteratorEltype(::Type{Flatten{Tuple{}}}) = IteratorEltype(Tuple{})
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3 changes: 3 additions & 0 deletions test/iterators.jl
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Expand Up @@ -515,6 +515,9 @@ end
@test eltype(flatten(UnitRange{Int8}[1:2, 3:4])) == Int8
@test eltype(flatten(([1, 2], [3.0, 4.0]))) == Real
@test eltype(flatten((a = [1, 2], b = Int8[3, 4]))) == Signed
@test eltype(flatten((Int[], Nothing[], Int[]))) == Union{Int, Nothing}
@test eltype(flatten((String[],))) == String
@test eltype(flatten((Int[], UInt[], Int8[],))) == Integer
@test length(flatten(zip(1:3, 4:6))) == 6
@test length(flatten(1:6)) == 6
@test collect(flatten(Any[])) == Any[]
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