feat: distinguish missing and nothing, prefer nothing

Author: mcmcgrath13

src/gentypes.jl

@@ -9,7 +9,7 @@ end
 struct Top end
 
 # get the type from a named tuple, given a name
-get_type(NT, k) = hasfield(NT, k) ? fieldtype(NT, k) : Nothing
+get_type(NT, k) = hasfield(NT, k) ? fieldtype(NT, k) : Missing
 
 # unify two types to a single type
 function promoteunion(T, S)
@@ -36,7 +36,7 @@ function unify(
     for (k, v) in zip(B, fieldtypes(b))
         if !(k in ks)
             push!(ks, k)
-            push!(ts, unify(v, Nothing))
+            push!(ts, unify(v, Missing))
         end
     end
 
@@ -161,13 +161,27 @@ function generate_expr!(
     mutable::Bool = true,
 ) where {N,T<:Tuple}
     sub_exprs = []
+    missing_fields = []
     for (n, t) in zip(N, fieldtypes(nt))
         push!(sub_exprs, generate_field_expr!(exprs, t, n; mutable = mutable))
+        if Missing <: t
+            push!(missing_fields, n)
+        end
     end
 
     struct_name = pascalcase(root_name)
+    missing_kw_args = (length(missing_fields) > 0 ? "; " : "") * join(map(n -> "n=missing", missing_fields), ", ")
     if mutable
-        push!(sub_exprs, Meta.parse("$struct_name() = new()"))
+        if length(missing_fields) == 0
+            push!(sub_exprs, Meta.parse("$struct_name() = new()"))
+        else
+            missing_assigns = join(map(n -> "x.$n = missing", missing_fields), "\n")
+            push!(sub_exprs, Meta.parse("""function $struct_name()
+                                            x = new()
+                                            $missing_assigns
+                                            return x
+                                        end"""))
+        end
     end
 
     push!(exprs, Expr(:struct, mutable, struct_name, Expr(:block, sub_exprs...)))

src/structs.jl

@@ -52,7 +52,8 @@ function read(::Struct, buf, pos, len, b, U::Union; kw...)
     # Julia implementation detail: Unions are sorted :)
     # This lets us avoid the below try-catch when U <: Union{Missing,T}
     if U.a === Nothing || U.a === Missing
-        if buf[pos] == UInt8('n')
+        # fallback to nothing if Union{Missing, Nothing}
+        if buf[pos] == UInt8('n') && !(Nothing <: U.b)
             return read(StructType(U.a), buf, pos, len, b, U.a)
         else
             return read(StructType(U.b), buf, pos, len, b, U.b; kw...)

test/gentypes.jl

@@ -313,4 +313,22 @@
         @test length(json_arr) == 2
         @test json_arr[1].menu.header == "SVG Viewer"
     end
+
+    @testset "Missingness" begin
+        json = """
+            [
+                {"a": "w", "b": 5, "c": 9, "d": null},
+                {"a": 3, "b": 4, "c": 2},
+                {"a": 7, "b": 7, "c": 0, "d": 10}
+            ]
+        """
+
+        JSON3.@generatetypes(json)
+        res = JSON3.read(json, Vector{JSONTypes.Root})
+
+        @test res[3].d == 10
+        @test ismissing(res[2].d)
+        @test res[1].d === nothing
+
+    end
 end

test/runtests.jl

@@ -456,6 +456,7 @@ obj = JSON3.read("""
 @test JSON3.read("\"1\"", Union{String, Int}) == "1"
 @test JSON3.read("null", Union{Int, String, Nothing}) === nothing
 @test JSON3.read("1.0", Union{Float64, Int}) === 1.0
+@test JSON3.read("null", Union{Missing, Nothing, Int}) === nothing
 
 @test JSON3.read("1", Any) == 1
 @test JSON3.read("3.14", Any) == 3.14