feature/issue-7-codec-optional

tests/ObjectCodec.fs

@@ -31,6 +31,8 @@ type Large =
     Baz : Guid
   }
 
+type Baz = { Baz : string option }
+
 module Codec =
 
   let fooBar : Codec<FooBar> =
@@ -73,6 +75,13 @@ module Codec =
         }
     }
 
+  let baz : Codec<Baz> =
+    objectCodec {
+      let! baz = Codec.optional "baz" (fun x -> x.Baz) Codec.string
+
+      return { Baz = baz }
+    }
+
 let tests =
   testList
     "ObjectCodec"
@@ -118,4 +127,19 @@ let tests =
 
         Expect.equal actual expected "The decoded value must match the original"
       }
+
+      test "objectCodec optional field works" {
+        let withValue = { Baz = Some "abc" }
+
+        let withoutValue = { Baz = None }
+
+        let actualWithValue =
+          roundTrip Codec.baz withValue
+
+        let actualWithoutValue =
+          roundTrip Codec.baz withoutValue
+
+        Expect.equal actualWithValue withValue "The decoded Some value must match the original"
+        Expect.equal actualWithoutValue withoutValue "The decoded None value must match the original"
+      }
     ]

thoth-json-codec/Object.fs

@@ -49,3 +49,17 @@ module ObjectCodecComputationExpression =
         Decoder = Decode.field fieldName fieldCodec.Decoder
         Picker = picker
       }
+
+    let optional
+      (fieldName : string)
+      (picker : 'u -> 't option)
+      (fieldCodec : Codec<'t>)
+      : ObjectCodecFieldSet<'t option, 'u> =
+      {
+        Values =
+          function
+          | Some i -> [ fieldName, fieldCodec.Encoder i ]
+          | None -> []
+        Decoder = Decode.optional fieldName fieldCodec.Decoder
+        Picker = picker
+      }