Fix type instability in unflattening Tuples

Project.toml

@@ -1,7 +1,7 @@
 name = "ParameterHandling"
 uuid = "2412ca09-6db7-441c-8e3a-88d5709968c5"
 authors = ["Invenia Technical Computing Corporation"]
-version = "0.4.8"
+version = "0.4.9"
 
 [deps]
 ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"

src/flatten.jl

@@ -79,16 +79,20 @@ function flatten(::Type{T}, x::SparseMatrixCSC) where {T<:Real}
 end
 
 function flatten(::Type{T}, x::Tuple) where {T<:Real}
-    x_vecs_and_backs = map(val -> flatten(T, val), x)
-    x_vecs, x_backs = first.(x_vecs_and_backs), last.(x_vecs_and_backs)
-    lengths = map(length, x_vecs)
-    sz = _cumsum(lengths)
+    vec1, back1 = flatten(T, first(x))
+    vec2, back2 = flatten(T, Base.tail(x))
+    l1 = length(vec1)
+    l2 = length(vec2)
     function unflatten_to_Tuple(v::Vector{T})
-        map(x_backs, lengths, sz) do x_back, l, s
-            return x_back(v[(s - l + 1):s])
-        end
+        return (back1(v[1:l1]), back2(v[(l1 + 1):(l1 + l2)])...)
     end
-    return reduce(vcat, x_vecs), unflatten_to_Tuple
+    return vcat(vec1, vec2), unflatten_to_Tuple
+end
+
+function flatten(::Type{T}, x::Tuple{}) where {T<:Real}
+    v = T[]
+    unflatten_to_empty_Tuple(::Vector{T}) = x
+    return v, unflatten_to_empty_Tuple
 end
 
 function flatten(::Type{T}, x::NamedTuple) where {T<:Real}

test/flatten.jl

@@ -39,6 +39,13 @@
         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)
+
+        # Prevent regression of PR #67
+        @testset "Type stability of unflatten" begin
+            θ = (1.0, ((2.0, 3.0), 4.0))
+            x, unflatten = flatten(θ)
+            @test (@inferred unflatten(x)) == θ
+        end
     end
 
     @testset "NamedTuple" begin