tryGet.purs

class GetLabelType (rl ∷ RowList) (s ∷ Symbol) (a ∷ Type) | rl s → a where
  labelType ∷ RLProxy rl → SProxy s → Proxy a

instance getLabelTypeFound ∷ GetLabelType (Cons s a tail) s a where
  labelType _ _ = Proxy
else instance getLabelTypeNotFound ∷ GetLabelType (Cons s a tail) s Void where
  labelType _ _ = Proxy
else instance getLabelTypeInd ∷ GetLabelType tail ss b ⇒ GetLabelType (Cons s a tail) ss b where
  labelType _ _ = labelType (RLProxy ∷ RLProxy tail) (SProxy ∷ SProxy ss)
else instance getLabelTypeVoid ∷ GetLabelType Nil s Void where
  labelType _ _ = Proxy

class IsEqual a b where
  refl ∷ Proxy a → Proxy b → Boolean
instance isEqualRefl ∷ IsEqual a a where
  refl _ _ = true
else instance isEqualFalse ∷ IsEqual a b where
  refl _ _ = false

tryGet
  ∷ ∀ r rl s a out
  . RowToList r rl
  ⇒ GetLabelType rl s out
  ⇒ IsEqual a out
  ⇒ IsSymbol s
  ⇒ SProxy s
  → Proxy a
  → Record r
  → Maybe a
tryGet sproxy aProxy r =
  if not refled then Nothing else Just $ RU.unsafeGet (reflectSymbol sproxy) r
  where
  outProxy ∷ Proxy out
  outProxy = Proxy

  refled ∷ Boolean
  refled = refl aProxy outProxy

main ∷ ∀ e. Eff _ Unit
main = do
  DT.traceAnyA $ tryGet (SProxy ∷ SProxy "foo") (Proxy ∷ Proxy Int) {foo: 1}
  DT.traceAnyA $ tryGet (SProxy ∷ SProxy "bar") (Proxy ∷ Proxy Int) {foo: 1}
  DT.traceAnyA $ tryGet (SProxy ∷ SProxy "foo") (Proxy ∷ Proxy String) {foo: 1, bar: 12}