# @parallel vcat throws TypeError but @parallel hcat does not

**URL:** <https://discourse.julialang.org/t/parallel-vcat-throws-typeerror-but-parallel-hcat-does-not/5334>\
**Category:** General Usage\
**Tags:** question, parallel\
**Created:** [August 11, 2017, 4:02pm UTC](https://discourse.julialang.org/t/parallel-vcat-throws-typeerror-but-parallel-hcat-does-not/5334 "2017-08-11T16:02:15Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![pearlzli](https://avatars.discourse-cdn.com/v4/letter/p/76d3ee/32.png) [@pearlzli](https://discourse.julialang.org/u/pearlzli)\
**Post date:** [August 11, 2017, 4:02pm UTC](https://discourse.julialang.org/t/parallel-vcat-throws-typeerror-but-parallel-hcat-does-not/5334/1 "2017-08-11T16:02:15Z")

</div>

I think I’m having the same issue as [this post](https://discourse.julialang.org/t/a-function-got-exactly-the-type-it-expected-so-why-is-it-throwing-a-typeerror/3236): I get an error when I use `@parallel vcat` where each iteration returns a `Dict{Symbol, Array{Float64}}` , but not when I use `@parallel hcat`.

Suppose I define the following functions:

```julia
function parallel_hcat(make_dict::Function)
    res = @sync @parallel hcat for i = 1:10 
        make_dict()
    end
    println(typeof(res))
end

function parallel_vcat(make_dict::Function)
    res = @sync @parallel vcat for i = 1:10 
        make_dict()
    end
    println(typeof(res))
end

```

Both `parallel_vcat` and `parallel_hcat` work as expected when I specify that the dictionary values are 1-dimensional arrays:

```julia
julia> addprocs(3);

julia> @everywhere make_dict_spec() = Dict{Symbol, Array{Float64, 1}}()

julia> parallel_hcat(make_dict_spec)
Array{Dict{Symbol,Array{Float64,1}},2}

julia> parallel_vcat(make_dict_spec)
Array{Dict{Symbol,Array{Float64,1}},1}

```

However, only `parallel_hcat` works when I don’t specify the array dimension:

```julia
julia @everywhere make_dict_unspec() = Dict{Symbol, Array{Float64}}()

julia> parallel_hcat(make_dict_unspec)
Array{Dict{Symbol,Array{Float64,N}},2}

julia> parallel_vcat(make_dict_unspec)
ERROR: TypeError: collect_to!: in typeassert, expected Array{Dict{Symbol,Array{Float64,N}},1}, got Array{Dict{Symbol,Array{Float64,N}},1}
 in collect_to!(::Array{Array{Dict{Symbol,Array{Float64,N}},1},1}, ::Base.Generator{Array{Task,1},Base.#wait}, ::Int64, ::Int64) at ./array.jl:343
 in collect_to_with_first!(::Array{Array{Dict{Symbol,Array{Float64,N}},1},1}, ::Array{Dict{Symbol,Array{Float64,N}},1}, ::Base.Generator{Array{Task,1},Base.#wait}, ::Int64) at ./array.jl:327
 in collect(::Base.Generator{Array{Task,1},Base.#wait}) at ./array.jl:308
 in preduce(::Function, ::Function, ::UnitRange{Int64}) at ./multi.jl:2013
 in macro expansion at ./task.jl:326 [inlined]
 in parallel_vcat(::#make_dict_unspec) at ./REPL[11]:2

```

Is there a reason for this asymmetric behavior between `@parallel hcat` and `@parallel vcat`? What does the error

```julia
ERROR: TypeError: collect_to!: in typeassert, expected Array{Dict{Symbol,Array{Float64,N}},1}, got Array{Dict{Symbol,Array{Float64,N}},1}

```

mean?