DataFrames.jl forward fill

ffill.jl

ffill(a::AbstractArray) = copy(a)
function ffill(a::AbstractArray{Union{Missing, T}}) where {T}
    res = similar(a, T)
    if ismissing(first(a))
        error("first value in array is missing")
    else
        fillval::T = first(a)
    end

    for i in eachindex(a)
        v = a[i]
        if !ismissing(v)
            fillval = v
        end
        res[i] = fillval
    end
    res
end

Would be great to extend to dataframes :)