julia> Pkg.add("TypeParameterAccessors")

using Test: @test
using TypeParameterAccessors

@test type_parameters(Array{Float64}, 1) == Float64
@test type_parameters(Matrix{Float64}, 2) == 2
@test type_parameters(Matrix{Float64}) == (Float64, 2)
@test type_parameters(Array{Float64}, eltype) == Float64
@test type_parameters(Matrix{Float64}, ndims) == 2
@test type_parameters(Matrix{Float64}, (eltype, ndims)) == (Float64, 2)

@test set_type_parameters(Array, 1, Float32) == Array{Float32}
@test set_type_parameters(Array, (1,), (Float32,)) == Array{Float32}
@test set_type_parameters(Array, (1, 2), (Float32, 2)) == Matrix{Float32}
@test set_type_parameters(Array, (eltype,), (Float32,)) == Array{Float32}
@test set_type_parameters(Array, (eltype, ndims), (Float32, 2)) == Matrix{Float32}

@test specify_type_parameters(Array{Float64}, (eltype, ndims), (Float32, 2)) ==
    Matrix{Float64}
@test specify_type_parameters(Array{Float64}, ndims, 2) == Matrix{Float64}
@test specify_type_parameters(Array{Float64}, eltype, Float32) == Array{Float64}

@test unspecify_type_parameters(Matrix{Float32}) == Array
@test unspecify_type_parameters(Matrix{Float32}, 1) == Matrix
@test unspecify_type_parameters(Matrix{Float32}, (eltype,)) == Matrix
@test unspecify_type_parameters(Matrix{Float32}, (ndims,)) == Array{Float32}

@test default_type_parameters(Array) == (Float64, 1)
@test default_type_parameters(Array, eltype) == Float64
@test default_type_parameters(Array, 2) == 1
@test default_type_parameters(Array, (eltype, ndims)) == (Float64, 1)

@test set_default_type_parameters(Array) == Vector{Float64}
@test set_default_type_parameters(Array, (eltype,)) == Array{Float64}
@test set_default_type_parameters(Array, 2) == Vector

@test specify_default_type_parameters(Matrix, (eltype, ndims)) == Matrix{Float64}
@test specify_default_type_parameters(Matrix, eltype) == Matrix{Float64}
@test specify_default_type_parameters(Array{Float32}, (eltype, ndims)) == Vector{Float32}