Comprehension ranging over empty sequence gives error if that variable is not the last one

[oantolin]

I'm using Julia Version 0.5.0 (2016-09-19 18:14 UTC) and noticed that in a comprehension you can't iterate over empty sequences unless that variable is the last one. (EDIT: I just tested in the binaries of the nightly builds and the same thing happens.)

I'd expect both of these comprehensions to result in empty arrays:

julia> [a for a in [1] for b in []]
0-element Array{Int64,1}

julia> [a for a in [] for b in [1]]
ERROR: ArgumentError: argument to Flatten must contain at least one iterator
 in start(::Base.Flatten{Base.Generator{Array{Any,1},##193#195}}) at ./iterator.jl:599
 in grow_to!(::Array{Any,1}, ::Base.Flatten{Base.Generator{Array{Any,1},##193#195}}) at ./array.jl:357
 in collect(::Base.Flatten{Base.Generator{Array{Any,1},##193#195}}) at ./array.jl:273

Or a list of triples (a,b,a+b) where 1<=a<b<5 and a+b is prime:

julia> [(a,b,c) for a=1:5 for b=a+1:5 for c in [a+b] if isprime(c)]
ERROR: ArgumentError: argument to Flatten must contain at least one iterator
 in start(::Base.Flatten{Base.Generator{UnitRange{Int64},##18#21{Int64}}}) at ./iterator.jl:599
 in next at ./iterator.jl:614 [inlined]
 in grow_to!(::Array{Tuple{Int64,Int64,Int64},1}, ::Base.Flatten{Base.Generator{UnitRange{Int64},##17#20}}, ::Tuple{Int64,Base.Flatten{Base.Generator{UnitRange{Int64},##18#21{Int64}}},Tuple{Int64,Base.Generator{Filter{#isprime,Array{Int64,1}},##19#22{Int64,Int64}},Tuple{Bool,Int64,Int64}}}) at ./array.jl:364
 in grow_to!(::Array{Union{},1}, ::Base.Flatten{Base.Generator{UnitRange{Int64},##17#20}}, ::Tuple{Int64,Base.Flatten{Base.Generator{UnitRange{Int64},##18#21{Int64}}},Tuple{Int64,Base.Generator{Filter{#isprime,Array{Int64,1}},##19#22{Int64,Int64}},Tuple{Bool,Int64,Int64}}}) at ./array.jl:372
 in grow_to!(::Array{Tuple{Int64,Int64,Int64},1}, ::Base.Flatten{Base.Generator{UnitRange{Int64},##17#20}}) at ./array.jl:357
 in collect(::Base.Flatten{Base.Generator{UnitRange{Int64},##17#20}}) at ./array.jl:273

Oops, that last value of a let to an empty range for b, better skip it:

julia> [(a,b,c) for a=1:4 for b=a+1:5 for c in [a+b] if isprime(c)]
5-element Array{Tuple{Int64,Int64,Int64},1}:
 (1,2,3)
 (1,4,5)
 (2,3,5)
 (2,5,7)
 (3,4,7)

[oantolin]

Sorry, it seems this is just a duplicate of #18852, where it is explained that this behavior is currently intentional as a hacky way of getting type stability.

[KristofferC]

Closing as dup of #18852