Add the update function to the Dict module

This resolves #4239

crates/compiler/builtins/roc/Dict.roc

@@ -9,6 +9,7 @@ interface Dict
         insert,
         len,
         remove,
+        update,
         contains,
         keys,
         values,
@@ -122,6 +123,34 @@ remove = \@Dict list, key ->
             |> List.dropLast
             |> @Dict
 
+## Insert or remove a value in a Dict based on its presence
+update :
+    Dict k v,
+    k,
+    ([Present v, Missing] -> [Present v, Missing])
+    -> Dict k v
+update = \dict, key, alter ->
+    possibleValue =
+        get dict key
+        |> Result.map Present
+        |> Result.withDefault Missing
+
+    when alter possibleValue is
+        Present value -> insert dict key value
+        Missing -> remove dict key
+
+## Internal for testing only
+alterValue : [Present Bool, Missing] -> [Present Bool, Missing]
+alterValue = \possibleValue ->
+    when possibleValue is
+        Missing -> Present Bool.false
+        Present value if Bool.not value -> Present Bool.true
+        Present _ -> Missing
+
+expect update empty "a" alterValue == single "a" Bool.false
+expect update (single "a" Bool.false) "a" alterValue == single "a" Bool.true
+expect update (single "a" Bool.true) "a" alterValue == empty
+
 contains : Dict k v, k -> Bool
 contains = \@Dict list, needle ->
     step = \_, Pair key _val ->

crates/compiler/module/src/symbol.rs

@@ -1410,6 +1410,7 @@ define_builtins! {
 
         15 DICT_WITH_CAPACITY: "withCapacity"
         16 DICT_CAPACITY: "capacity"
+        17 DICT_UPDATE: "update"
     }
     9 SET: "Set" => {
         0 SET_SET: "Set" exposed_type=true // the Set.Set type alias