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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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21 changes: 19 additions & 2 deletions base/iterators.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -15,7 +15,8 @@ using .Base:
AbstractRange, AbstractUnitRange, UnitRange, LinearIndices, TupleOrBottom,
(:), |, +, -, *, !==, !, ==, !=, <=, <, >, >=, =>, missing,
any, _counttuple, eachindex, ntuple, zero, prod, reduce, in, firstindex, lastindex,
tail, fieldtypes, min, max, minimum, zero, oneunit, promote, promote_shape, LazyString
tail, fieldtypes, min, max, minimum, zero, oneunit, promote, promote_shape, LazyString,
tuple_type_tail
using Core: @doc

using .Base:
Expand Down Expand Up @@ -1202,7 +1203,23 @@ 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.
eltype(::Type{Flatten{I}}) where {I<:Tuple} = _flatten_eltype(Union{}, I)

function eltype(::Type{Flatten{I}}) where {I<:NamedTuple{<:Any, T}} where T
_flatten_eltype(Union{}, T)
end

function _flatten_eltype(T::Type, I::Type{<:Tuple{E, Vararg{Any}}}) where E
T2 = promote_typejoin(T, eltype(E))
T2 === Any && return Any
_flatten_eltype(T2, tuple_type_tail(I))
end

_flatten_eltype(T::Type, I::Type{Tuple{}}) = T

eltype(::Type{Flatten{Tuple{}}}) = eltype(Tuple{})
IteratorEltype(::Type{Flatten{I}}) where {I} = _flatteneltype(I, IteratorEltype(I))
IteratorEltype(::Type{Flatten{Tuple{}}}) = IteratorEltype(Tuple{})
Expand Down
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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