DataFrames.jl vcat column data from different rows

Specific Domains Data
2 
julia> df = DataFrame(x = ["a", "a", "b", "b"], y = [[1,2],[3,4],[5,6],[7,8]])
4×2 DataFrame
 Row │ x       y      
     │ String  Array… 
─────┼────────────────
   1 │ a       [1, 2]
   2 │ a       [3, 4]
   3 │ b       [5, 6]
   4 │ b       [7, 8]

julia> combine(groupby(df, :x), :y => Ref ∘ (x -> reduce(vcat, x)) => :y)
2×2 DataFrame
 Row │ x       y            
     │ String  Array…       
─────┼──────────────────────
   1 │ a       [1, 2, 3, 4]
   2 │ b       [5, 6, 7, 8]
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