Prefer small unions when generating types from json objects/arrays (#144

)

* Prefer small unions when generating types from json objects/arrays

Fixes #141. In general, the Julia compiler handles Union types really
well these days, so let's prefer them when generating types. Users are
of course always free to modify the generated file if the generated
Union field is too strict.

* fix test

src/gentypes.jl

@@ -12,8 +12,13 @@ struct Top end
 get_type(NT, k) = hasfield(NT, k) ? fieldtype(NT, k) : Nothing
 
 # unify two types to a single type
+function promoteunion(T, S)
+    new = promote_type(T, S)
+    return isabstracttype(new) ? Union{T, S} : new
+end
+
 unify(a, b) = unify(b, a)
-unify(a::Type{T}, b::Type{S}) where {T,S} = Base.promote_typejoin(T, S)
+unify(a::Type{T}, b::Type{S}) where {T,S} = promoteunion(T, S)
 unify(a::Type{T}, b::Type{S}) where {T,S<:T} = T
 unify(a::Type{Top}, b::Type{T}) where {T} = T
 

test/gentypes.jl

@@ -252,6 +252,7 @@
 
         @test !(JSONTypes.Root.mutable)
         @test parsed[1].c.d == 4
+        @test fieldtype(JSONTypes.Root, 1) == Union{Int64, String}
     end
 
     @testset "Raw Types" begin