data Lan : (j : Type -> Type) -> (g : Type -> Type) -> (x : Type) -> Type where 
  MkLan : {a : Type} -> (j a -> x) -> g a -> Lan j g x

Lan Tuple (CrossProduct f g) a
~ exists x y. (Tuple x y -> a, CrossProduct f g x y)
~ exists x y. (x -> y -> a, f x, g y)
~ Day f g a

call ∷ String → Parser (Tuple String (Array String))
call name = do
  fn ← string name
  params ← parens args
  pure $ Tuple fn []

args ∷ Parser (NonEmptyList String)
args = ??? `sepBy1` string ","

import Data.Foldable (foldMap)
import Data.String.CodeUnits (singleton)

args ∷ Parser (List String)
args = do
  xs ← many alphaNum `sepBy` string ","
  pure (foldMap singleton <$> xs)

args ∷ Parser (List String)
args = quoted arg `sepBy` (char ',' <* skipSpaces)