Decode for a value which has a type level default

TypeLevelFoo.purs

newtype WithDefault b a = WithDefault a

instance showWithDefault :: Show a => Show (WithDefault b a) where
  show (WithDefault a) = show a

class Reflect a b | b -> a where
  reflect :: Proxy a -> b

instance reflectSProxyString :: IsSymbol sym => Reflect (SProxy sym) String where
  reflect = const (reflectSymbol (SProxy :: SProxy sym))

instance reflectDProxyInt :: Nat n => Reflect n Int where
  reflect = toInt'

instance reflectBProxyBool :: BoolI b => Reflect b Boolean where
  reflect _ = toBool (undefined :: b)

instance decodeReflectable :: (Reflect b a, Decode a) => Decode (WithDefault b a) where
  decode f = if (F.isNull f) then pure $ WithDefault (reflect (Proxy :: Proxy b)) else WithDefault <$> decode f

y :: Either _ (WithDefault (SProxy "bar") String)
y = runExcept $ decode $ unsafeToForeign "foo"

z :: String
z = show $ unsafePartial $ fromRight y

foreign import _y1 :: Foreign -- exported null from JS

y1 :: Either _ (WithDefault (SProxy "bar") String)
y1 = runExcept $ decode $ _y1

z1 :: String
z1 = show $ unsafePartial $ fromRight y1

w :: Either _ (WithDefault D20 Int)
w = runExcept $ decode $ unsafeToForeign (1 :: Int)

v :: String
v = show $ unsafePartial $ fromRight w

w1 :: Either _ (WithDefault D20 Int)
w1 = runExcept $ decode $ _y1

v1 :: String
v1 = show $ unsafePartial $ fromRight w1

t :: Either _ (WithDefault False Boolean)
t = runExcept $ decode $ unsafeToForeign false

u :: String
u = show $ unsafePartial $ fromRight t

t1 :: Either _ (WithDefault True Boolean)
t1 = runExcept $ decode $ _y1

u1 :: String
u1 = show $ unsafePartial $ fromRight t1