Implementation of our command line parsing combinators

Implementation using existential data types

• data P res
      = Nil res
      | forall arg . Ap (P (arg->res)) (P arg)
      | P res :-- String
      | Token (String->Maybe res) String
      | P res :! P res
      | forall item . Many (res->[item]) ([item]->res) (P item)
  
  many :: P a -> P [a]
  many = Many id id
  

• The extra arguments to Many express the fact that the result type is expected to be isomorphic to some list type.