Error getting length of Iterator.flatten with tuples and arrays of different lengths

[zot]

It works for tuples of the same length:

julia> length(Iterators.flatten(((1,2),(3,4))))
4

It fails for arrays of the same or different lengths, tuples of different lengths, and generators of the same length. Here are some errors:

julia> length(Iterators.flatten(([1,2],[3,4])))
ERROR: ArgumentError: Iterates of the argument to Flatten are not known to have constant length
Stacktrace:
 [1] flatten_length(f::Base.Iterators.Flatten{Tuple{Vector{Int64}, Vector{Int64}}}, T::Type)
   @ Base.Iterators ./iterators.jl:1241
 [2] length(f::Base.Iterators.Flatten{Tuple{Vector{Int64}, Vector{Int64}}})
   @ Base.Iterators ./iterators.jl:1243
 [3] top-level scope
   @ REPL[26]:1

julia> length(Iterators.flatten(([1,2],[3])))
ERROR: ArgumentError: Iterates of the argument to Flatten are not known to have constant length
Stacktrace:
 [1] flatten_length(f::Base.Iterators.Flatten{Tuple{Vector{Int64}, Vector{Int64}}}, T::Type)
   @ Base.Iterators ./iterators.jl:1241
 [2] length(f::Base.Iterators.Flatten{Tuple{Vector{Int64}, Vector{Int64}}})
   @ Base.Iterators ./iterators.jl:1243
 [3] top-level scope
   @ REPL[22]:1

julia> length(Iterators.flatten(((1,),(2,3))))
ERROR: ArgumentError: Iterates of the argument to Flatten are not known to have constant length
Stacktrace:
 [1] flatten_length(f::Base.Iterators.Flatten{Tuple{Tuple{Int64}, Tuple{Int64, Int64}}}, T::Type)
   @ Base.Iterators ./iterators.jl:1241
 [2] length(f::Base.Iterators.Flatten{Tuple{Tuple{Int64}, Tuple{Int64, Int64}}})
   @ Base.Iterators ./iterators.jl:1243
 [3] top-level scope
   @ REPL[33]:1

julia> length(Iterators.flatten(((i for i in (1,2)), (i for i in (3,4)))))
ERROR: ArgumentError: Iterates of the argument to Flatten are not known to have constant length
Stacktrace:
 [1] flatten_length(f::Base.Iterators.Flatten{Tuple{Base.Generator{…}, Base.Generator{…}}}, T::Type)
   @ Base.Iterators ./iterators.jl:1241
 [2] length(f::Base.Iterators.Flatten{Tuple{Base.Generator{…}, Base.Generator{…}}})
   @ Base.Iterators ./iterators.jl:1243
 [3] top-level scope
   @ REPL[25]:1
Some type information was truncated. Use `show(err)` to see complete types.

[adienes]

thanks for the report; this is a known issue. I know it seems simple but there are a few questions left about how to do it the "right" way. see the linked dup issue for more discussion

[zot]

The code in base/iterators.jl did not look simple to me :).