undropdims: type stable inverse of dropdims

undropdims.jl

function mix_dims(dims1, dims2, t)
    if in(t[1], dims2)
        return (1, mix_dims(dims1, Base.tail(dims2), Base.tail(t))...)
    else
        return (dims1[1], mix_dims(Base.tail(dims1), dims2, Base.tail(t))...)
    end
end
function mix_dims(dims1, dims2, t::Tuple{})
    return ()
end
@generated function ntup(::Val{N}) where N
    return ntuple(i -> i, Val(N))
end
function undropdims(A::AbstractArray{T,N}; dims::NTuple{M,Int}=()) where {T,N,M}
    newdims = mix_dims(size(A), dims, ntup(Val(N+M)))
    return reshape(A, newdims)
end
a = [1 2 3; 4 5 6]
undropdims(a; dims= (1, 3))

Source: https://julialang.slack.com/archives/C6GHSTN4T/p1588602233081400?thread_ts=1588598751.078000&cid=C6GHSTN4T