diff --git a/Project.toml b/Project.toml
index e17b62f..f5c066a 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,7 +1,7 @@
 name = "ParameterHandling"
 uuid = "2412ca09-6db7-441c-8e3a-88d5709968c5"
 authors = ["Invenia Technical Computing Corporation"]
-version = "0.3.8"
+version = "0.4.0"
 
 [deps]
 Bijectors = "76274a88-744f-5084-9051-94815aaf08c4"
diff --git a/src/flatten.jl b/src/flatten.jl
index 717b27b..5779a66 100644
--- a/src/flatten.jl
+++ b/src/flatten.jl
@@ -21,20 +21,20 @@ flatten(x) = flatten(Float64, x)
 
 function flatten(::Type{T}, x::Integer) where {T<:Real}
     v = T[]
-    unflatten_to_Integer(v::Vector{T}) = x
+    unflatten_to_Integer(v::AbstractVector{<:Real}) = x
     return v, unflatten_to_Integer
 end
 
 function flatten(::Type{T}, x::R) where {T<:Real,R<:Real}
     v = T[x]
-    unflatten_to_Real(v::Vector{T}) = convert(R, only(v))
+    unflatten_to_Real(v::AbstractVector{<:Real}) = only(v)
     return v, unflatten_to_Real
 end
 
-flatten(::Type{T}, x::Vector{R}) where {T<:Real,R<:Real} = (Vector{T}(x), Vector{R})
+flatten(::Type{T}, x::Vector{R}) where {T<:Real,R<:Real} = (Vector{T}(x), identity)
 
 function _flatten_vector_integer(::Type{T}, x::AbstractVector{<:Integer}) where {T<:Real}
-    unflatten_to_Vector_Integer(x_vec) = x
+    unflatten_to_Vector_Integer(::AbstractVector{<:Real}) = x
     return T[], unflatten_to_Vector_Integer
 end
 
@@ -44,28 +44,32 @@ function flatten(::Type{T}, x::AbstractVector{<:Integer}) where {T<:Real}
     return _flatten_vector_integer(T, x)
 end
 
-function flatten(::Type{T}, x::AbstractVector) where {T<:Real}
+function flatten(::Type{T}, x::Vector) where {T<:Real}
     x_vecs_and_backs = map(val -> flatten(T, val), x)
     x_vecs, backs = first.(x_vecs_and_backs), last.(x_vecs_and_backs)
-    function Vector_from_vec(x_vec)
+    function Vector_from_vec(x_vec::AbstractVector{<:Real})
         sz = _cumsum(map(length, x_vecs))
         x_Vec = [
             backs[n](x_vec[(sz[n] - length(x_vecs[n]) + 1):sz[n]]) for n in eachindex(x)
         ]
-        return oftype(x, x_Vec)
+        return collect(x_Vec)
     end
     return reduce(vcat, x_vecs), Vector_from_vec
 end
 
-function flatten(::Type{T}, x::AbstractArray) where {T<:Real}
+function flatten(::Type{T}, x::Array) where {T<:Real}
     x_vec, from_vec = flatten(T, vec(x))
-    Array_from_vec(x_vec) = oftype(x, reshape(from_vec(x_vec), size(x)))
+    function Array_from_vec(x_vec::AbstractVector{<:Real})
+        return collect(reshape(from_vec(x_vec), size(x)))
+    end
     return x_vec, Array_from_vec
 end
 
 function flatten(::Type{T}, x::SparseMatrixCSC) where {T<:Real}
     x_vec, from_vec = flatten(T, x.nzval)
-    Array_from_vec(x_vec) = SparseMatrixCSC(x.m, x.n, x.colptr, x.rowval, from_vec(x_vec))
+    function Array_from_vec(x_vec::AbstractVector{<:Real})
+        return SparseMatrixCSC(x.m, x.n, x.colptr, x.rowval, from_vec(x_vec))
+    end
     return x_vec, Array_from_vec
 end
 
@@ -74,7 +78,7 @@ function flatten(::Type{T}, x::Tuple) where {T<:Real}
     x_vecs, x_backs = first.(x_vecs_and_backs), last.(x_vecs_and_backs)
     lengths = map(length, x_vecs)
     sz = _cumsum(lengths)
-    function unflatten_to_Tuple(v::Vector{T})
+    function unflatten_to_Tuple(v::AbstractVector{<:Real})
         map(x_backs, lengths, sz) do x_back, l, s
             return x_back(v[(s - l + 1):s])
         end
@@ -82,18 +86,18 @@ function flatten(::Type{T}, x::Tuple) where {T<:Real}
     return reduce(vcat, x_vecs), unflatten_to_Tuple
 end
 
-function flatten(::Type{T}, x::NamedTuple) where {T<:Real}
+function flatten(::Type{T}, x::NamedTuple{names}) where {T<:Real,names}
     x_vec, unflatten = flatten(T, values(x))
-    function unflatten_to_NamedTuple(v::Vector{T})
+    function unflatten_to_NamedTuple(v::AbstractVector{<:Real})
         v_vec_vec = unflatten(v)
-        return typeof(x)(v_vec_vec)
+        return NamedTuple{names}(v_vec_vec)
     end
     return x_vec, unflatten_to_NamedTuple
 end
 
 function flatten(::Type{T}, d::Dict) where {T<:Real}
     d_vec, unflatten = flatten(T, collect(values(d)))
-    function unflatten_to_Dict(v::Vector{T})
+    function unflatten_to_Dict(v::AbstractVector{<:Real})
         v_vec_vec = unflatten(v)
         return Dict(key => v_vec_vec[n] for (n, key) in enumerate(keys(d)))
     end
diff --git a/src/parameters.jl b/src/parameters.jl
index a3c1dd4..ed7f752 100644
--- a/src/parameters.jl
+++ b/src/parameters.jl
@@ -48,7 +48,7 @@ value(x::Positive) = x.transform(x.unconstrained_value) + x.ε
 function flatten(::Type{T}, x::Positive) where {T<:Real}
     v, unflatten_to_Real = flatten(T, x.unconstrained_value)
 
-    function unflatten_Positive(v_new::Vector{T})
+    function unflatten_Positive(v_new::AbstractVector{<:Real})
         return Positive(unflatten_to_Real(v_new), x.transform, x.ε)
     end
 
@@ -84,10 +84,10 @@ function bounded(val::Real, lower_bound::Real, upper_bound::Real)
     return Bounded(inv_transform(val), lb, ub, transform, ε)
 end
 
-struct Bounded{T<:Real,V<:Bijector,Tε<:Real} <: AbstractParameter
+struct Bounded{T<:Real,Tbound<:Real,V<:Bijector,Tε<:Real} <: AbstractParameter
     unconstrained_value::T
-    lower_bound::T
-    upper_bound::T
+    lower_bound::Tbound
+    upper_bound::Tbound
     transform::V
     ε::Tε
 end
@@ -97,7 +97,7 @@ value(x::Bounded) = x.transform(x.unconstrained_value)
 function flatten(::Type{T}, x::Bounded) where {T<:Real}
     v, unflatten_to_Real = flatten(T, x.unconstrained_value)
 
-    function unflatten_Bounded(v_new::Vector{T})
+    function unflatten_Bounded(v_new::AbstractVector{<:Real})
         return Bounded(
             unflatten_to_Real(v_new), x.lower_bound, x.upper_bound, x.transform, x.ε
         )
@@ -122,7 +122,7 @@ end
 value(x::Fixed) = x.value
 
 function flatten(::Type{T}, x::Fixed) where {T<:Real}
-    unflatten_Fixed(v_new::Vector{T}) = x
+    unflatten_Fixed(v_new::AbstractVector{<:Real}) = x
     return T[], unflatten_Fixed
 end
 
@@ -148,7 +148,7 @@ value(x::Deferred) = x.f(value(x.args)...)
 
 function flatten(::Type{T}, x::Deferred) where {T<:Real}
     v, unflatten = flatten(T, x.args)
-    unflatten_Deferred(v_new::Vector{T}) = Deferred(x.f, unflatten(v_new))
+    unflatten_Deferred(v_new::AbstractVector{<:Real}) = Deferred(x.f, unflatten(v_new))
     return v, unflatten_Deferred
 end
 
@@ -188,7 +188,9 @@ value(X::Orthogonal) = nearest_orthogonal_matrix(X.X)
 
 function flatten(::Type{T}, X::Orthogonal) where {T<:Real}
     v, unflatten_to_Array = flatten(T, X.X)
-    unflatten_Orthogonal(v_new::Vector{T}) = Orthogonal(unflatten_to_Array(v_new))
+    function unflatten_Orthogonal(v_new::AbstractVector{<:Real})
+        return Orthogonal(unflatten_to_Array(v_new))
+    end
     return v, unflatten_Orthogonal
 end
 
@@ -217,7 +219,9 @@ value(X::PositiveDefinite) = A_At(vec_to_tril(X.L))
 
 function flatten(::Type{T}, X::PositiveDefinite) where {T<:Real}
     v, unflatten_v = flatten(T, X.L)
-    unflatten_PositiveDefinite(v_new::Vector{T}) = PositiveDefinite(unflatten_v(v_new))
+    function unflatten_PositiveDefinite(v_new::AbstractVector{<:Real})
+        return PositiveDefinite(unflatten_v(v_new))
+    end
     return v, unflatten_PositiveDefinite
 end
 
diff --git a/src/test_utils.jl b/src/test_utils.jl
index 22ce3d3..3a1c5e7 100644
--- a/src/test_utils.jl
+++ b/src/test_utils.jl
@@ -6,31 +6,30 @@ using Test
 
 using ParameterHandling: AbstractParameter, value
 
-# Handles equality of scalars, functions, tuples or arbitrary types
-function default_equality(a::T, b::T; kwargs...) where {T}
-    vals = fieldvalues(a)
-
-    # If we don't have any fields then we're probably dealing with scalars
-    if isempty(vals)
-        # Only call isapprox for numbers, otherwise we fallback to ==
-        return T <: Number ? isapprox(a, b; kwargs...) : a == b
-    else
-        return all(t -> default_equality(t...; kwargs...), zip(vals, fieldvalues(b)))
-    end
+# Handles equality of structs / mutable structs.
+function default_equality(a::Ta, b::Tb; kwargs...) where {Ta,Tb}
+    (isstructtype(Ta) && isstructtype(Tb)) || throw(error("Arguments aren't structs"))
+    return all(t -> default_equality(t...; kwargs...), zip(fieldvalues(a), fieldvalues(b)))
 end
 
+default_equality(a::Number, b::Number; kwargs...) = isapprox(a, b; kwargs...)
+
 # Handles extracting elements from arrays.
 # Needed because fieldvalues(a) are empty, but we may need to recurse depending on
 # the element type
-function default_equality(a::T, b::T; kwargs...) where {T<:AbstractArray}
+function default_equality(a::AbstractArray, b::AbstractArray; kwargs...)
     return all(t -> default_equality(t...; kwargs...), zip(a, b))
 end
 
 # Handles extracting values for any dictionary types
-function default_equality(a::T, b::T; kwargs...) where {T<:AbstractDict}
+function default_equality(a::AbstractDict, b::AbstractDict; kwargs...)
     return all(t -> default_equality(t...; kwargs...), zip(values(a), values(b)))
 end
 
+struct MyReal{T} <: Real
+    v::T
+end
+
 # NOTE: May want to make the equality function a kwarg in the future.
 function test_flatten_interface(x::T; check_inferred::Bool=true) where {T}
     @testset "flatten($T)" begin
@@ -50,7 +49,9 @@ function test_flatten_interface(x::T; check_inferred::Bool=true) where {T}
             @test typeof(_v) === Vector{Float64}
             @test _v == v
             @test default_equality(x, unflatten(_v))
-            @test _unflatten(_v) isa T
+
+            # Check that unflattening works with different reals.
+            _unflatten(map(MyReal, randn(length(_v))))
 
             # Check that everything infers properly.
             check_inferred && @inferred flatten(Float64, x)
@@ -59,7 +60,9 @@ function test_flatten_interface(x::T; check_inferred::Bool=true) where {T}
             _v, _unflatten = flatten(Float32, x)
             @test typeof(_v) === Vector{Float32}
             @test default_equality(x, _unflatten(_v); atol=1e-5)
-            @test _unflatten(_v) isa T
+
+            # Check that unflattening works with different precisions.
+            _unflatten(map(MyReal, randn(length(_v))))
 
             # Check that everything infers properly.
             check_inferred && @inferred flatten(Float32, x)
@@ -68,7 +71,9 @@ function test_flatten_interface(x::T; check_inferred::Bool=true) where {T}
             _v, _unflatten = flatten(Float16, x)
             @test typeof(_v) === Vector{Float16}
             @test default_equality(x, _unflatten(_v); atol=1e-2)
-            @test _unflatten(_v) isa T
+
+            # Check that unflattening works with different precisions.
+            _unflatten(map(MyReal, randn(length(_v))))
 
             # Check that everything infers properly.
             check_inferred && @inferred flatten(Float16, x)
diff --git a/test/flatten.jl b/test/flatten.jl
index 4a34c99..0d6a5b2 100644
--- a/test/flatten.jl
+++ b/test/flatten.jl
@@ -26,7 +26,6 @@
 
     @testset "Tuple" begin
         test_flatten_interface((1.0, 2.0); check_inferred=tuple_infers)
-
         test_flatten_interface((1.0, (2.0, 3.0), randn(5)); check_inferred=tuple_infers)
     end