JSON doesn't allow trailing commas

crates/compiler/builtins/roc/Json.roc

@@ -80,18 +80,18 @@ string = \s -> custom \bytes, @Json {} ->
 
 list = \lst, encodeElem ->
     custom \bytes, @Json {} ->
-        writeList = \{ buffer, elemIndex }, elem ->
+        writeList = \{ buffer, elemsLeft }, elem ->
-            bufferWithPrefix =
+            bufferWithElem = appendWith buffer (encodeElem elem) (@Json {})
-                if elemIndex > 0 then
+            bufferWithSuffix =
-                    List.append buffer (Num.toU8 ',')
+                if elemsLeft > 1 then
+                    List.append bufferWithElem (Num.toU8 ',')
                 else
-                    buffer
+                    bufferWithElem
-            bufferWithElem = appendWith bufferWithPrefix (encodeElem elem) (@Json {})
 
-            { buffer: bufferWithElem, elemIndex: elemIndex + 1 }
+            { buffer: bufferWithSuffix, elemsLeft: elemsLeft - 1 }
 
         head = List.append bytes (Num.toU8 '[')
-        { buffer: withList } = List.walk lst { buffer: head, elemIndex: 0 } writeList
+        { buffer: withList } = List.walk lst { buffer: head, elemsLeft: List.len lst } writeList
 
         List.append withList (Num.toU8 ']')
 
@@ -106,7 +106,7 @@ record = \fields ->
                 |> appendWith value (@Json {})
 
             bufferWithSuffix =
-                if fieldsLeft > 0 then
+                if fieldsLeft > 1 then
                     List.append bufferWithKeyValue (Num.toU8 ',')
                 else
                     bufferWithKeyValue
@@ -124,7 +124,7 @@ tag = \name, payload ->
         writePayload = \{ buffer, itemsLeft }, encoder ->
             bufferWithValue = appendWith buffer encoder (@Json {})
             bufferWithSuffix =
-                if itemsLeft > 0 then
+                if itemsLeft > 1 then
                     List.append bufferWithValue (Num.toU8 ',')
                 else
                     bufferWithValue

crates/compiler/test_gen/src/gen_abilities.rs

@@ -490,7 +490,7 @@ fn encode_derived_record_one_field_string() {
                     _ -> "<bad>"
             "#
         ),
-        RocStr::from(r#"{"a":"foo",}"#),
+        RocStr::from(r#"{"a":"foo"}"#),
         RocStr
     )
 }
@@ -513,7 +513,7 @@ fn encode_derived_record_two_fields_strings() {
                     _ -> "<bad>"
             "#
         ),
-        RocStr::from(r#"{"a":"foo","b":"bar",}"#),
+        RocStr::from(r#"{"a":"foo","b":"bar"}"#),
         RocStr
     )
 }
@@ -537,7 +537,7 @@ fn encode_derived_nested_record_string() {
                     _ -> "<bad>"
             "#
         ),
-        RocStr::from(r#"{"a":{"b":"bar",},}"#),
+        RocStr::from(r#"{"a":{"b":"bar"}}"#),
         RocStr
     )
 }
@@ -561,7 +561,7 @@ fn encode_derived_tag_one_payload_string() {
                     _ -> "<bad>"
             "#
         ),
-        RocStr::from(r#"{"A":["foo",]}"#),
+        RocStr::from(r#"{"A":["foo"]}"#),
         RocStr
     )
 }
@@ -585,7 +585,7 @@ fn encode_derived_tag_two_payloads_string() {
                     _ -> "<bad>"
             "#
         ),
-        RocStr::from(r#"{"A":["foo","bar",]}"#),
+        RocStr::from(r#"{"A":["foo","bar"]}"#),
         RocStr
     )
 }
@@ -610,7 +610,7 @@ fn encode_derived_nested_tag_string() {
                     _ -> "<bad>"
             "#
         ),
-        RocStr::from(r#"{"A":[{"B":["foo","bar",]},]}"#),
+        RocStr::from(r#"{"A":[{"B":["foo","bar"]}]}"#),
         RocStr
     )
 }
@@ -635,7 +635,7 @@ fn encode_derived_nested_record_tag_record() {
                     _ -> "<bad>"
             "#
         ),
-        RocStr::from(r#"{"a":{"B":[{"c":"foo",},]},}"#),
+        RocStr::from(r#"{"a":{"B":[{"c":"foo"}]}}"#),
         RocStr
     )
 }
@@ -683,7 +683,7 @@ fn encode_derived_list_of_records() {
                     _ -> "<bad>"
             "#
         ),
-        RocStr::from(r#"[{"a":"foo",},{"a":"bar",},{"a":"baz",}]"#),
+        RocStr::from(r#"[{"a":"foo"},{"a":"bar"},{"a":"baz"}]"#),
         RocStr
     )
 }