From 00e171f4d1cbe3785c290b070a2bd967a4af5056 Mon Sep 17 00:00:00 2001
From: Wynand Badenhorst <wynand.badenhorst@invenia.ca>
Date: Fri, 19 Nov 2021 16:20:47 -0600
Subject: [PATCH] Add Singleton writing

---
 src/Writer.jl      | 14 ++++++++++----
 test/serializer.jl |  6 +++++-
 2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/src/Writer.jl b/src/Writer.jl
index 6ebea04..c1a9ea6 100644
--- a/src/Writer.jl
+++ b/src/Writer.jl
@@ -22,6 +22,10 @@ end
 CompositeTypeWrapper(x, syms) = CompositeTypeWrapper(x, collect(syms))
 CompositeTypeWrapper(x) = CompositeTypeWrapper(x, propertynames(x))
 
+struct SingletonTypeWrapper{T}
+    wrapped::T
+end
+
 """
     lower(x)
 
@@ -40,11 +44,11 @@ Note that the return value need not be *recursively* lowered—this function may
 for instance return an `AbstractArray{Any, 1}` whose elements are not JSON
 primitives.
 """
-function lower(a)
-    if nfields(a) > 0
-        CompositeTypeWrapper(a)
+function lower(a::T) where {T}
+    if Base.issingletontype(T)
+        SingletonTypeWrapper(a)
     else
-        error("Cannot serialize type $(typeof(a))")
+        CompositeTypeWrapper(a)
     end
 end
 
@@ -287,6 +291,8 @@ function show_json(io::SC, s::CS, x::CompositeTypeWrapper)
     end_object(io)
 end
 
+show_json(io::SC, ::CS, x::SingletonTypeWrapper) = show_string(io, x.wrapped)
+
 function show_json(io::SC, s::CS, x::Union{AbstractVector, Tuple})
     begin_array(io)
     for elt in x
diff --git a/test/serializer.jl b/test/serializer.jl
index 87927fe..fa7399a 100644
--- a/test/serializer.jl
+++ b/test/serializer.jl
@@ -72,7 +72,11 @@ end
 
 # test serializing a type without any fields
 struct SingletonType end
-@test_throws ErrorException json(SingletonType())
+@test json(SingletonType()) == "\"Main.TestSerializer.SingletonType()\""
+
+struct ParamSingletonType{T} end
+@test json(ParamSingletonType{Float64}()) == "\"Main.TestSerializer.ParamSingletonType{Float64}()\""
+
 
 # test printing to stdout
 let filename = tempname()