of_ returns nullable

spago.dhall

@@ -1,6 +1,13 @@
 { name = "web-intl"
 , dependencies =
-  [ "effect", "functions", "js-date", "prelude", "unsafe-coerce" ]
+  [ "effect"
+  , "functions"
+  , "js-date"
+  , "maybe"
+  , "nullable"
+  , "prelude"
+  , "unsafe-coerce"
+  ]
 , packages = ./packages.dhall
 , sources = [ "src/**/*.purs" ]
 , license = "MIT"

src/Web/Intl/DisplayNames.purs

@@ -14,6 +14,9 @@ module Web.Intl.DisplayNames
 
 import Data.Function.Uncurried (Fn2)
 import Data.Function.Uncurried as Function.Uncurried
+import Data.Maybe (Maybe)
+import Data.Nullable (Nullable)
+import Data.Nullable as Nullable
 import Effect (Effect)
 import Effect.Uncurried (EffectFn2)
 import Effect.Uncurried as Effect.Uncurried
@@ -65,7 +68,7 @@ supportedLocalesOf locales options =
   Function.Uncurried.runFn2 _supportedLocalesOf locales (Unsafe.Coerce.unsafeCoerce options)
 
 foreign import _of
-  :: Fn2 DisplayNames String String
+  :: Fn2 DisplayNames String (Nullable String)
 
-of_ :: DisplayNames -> String -> String
-of_ = Function.Uncurried.runFn2 _of
+of_ :: DisplayNames -> String -> Maybe String
+of_ displayNames key = Nullable.toMaybe (Function.Uncurried.runFn2 _of displayNames key)

test/Main.purs

@@ -5,6 +5,7 @@ import Prelude
 import Data.Either (Either(..))
 import Data.JSDate (JSDate)
 import Data.JSDate as JSDate
+import Data.Maybe (Maybe(..))
 import Data.String (Pattern(..), Replacement(..))
 import Data.String as String
 import Effect (Effect)
@@ -289,7 +290,7 @@ test_DisplayNames = do
   Console.log "DisplayNames.of_"
   Test.assertEqual
     { actual: DisplayNames.of_ displayNames "en-US"
-    , expected: "American English"
+    , expected: Just "American English"
     }
 
 test_ListFormat :: Effect Unit