Is it possible to type a variadic function in Haskell? -


mind following haskell term:

callntimes :: forall . int -> (a -> a) -> -> callntimes n f 0 = x callntimes n f x = f (callntimes (n-1) f x)  firstof :: ?????? firstof n = callntimes n (\ x y -> x)

if ignore types , normalize functions hand, firstof function receives n, n arguments, discards first , returns it. firstof 3 10 20 30 returns 3. possible type function in ghc 8.0 new dependent typing features?

i managed working version - it's not asked demonstrates commenting , think it's quite close

{-# language flexiblecontexts #-} {-# language typefamilies #-} {-# language multiparamtypeclasses #-}  module variadic  data z = z data s t = s (t a)  class var n   type el n :: *   type res n :: *   firstof :: n -> el n -> res n  instance var (z a)   type el (z a) =   type res (z a) =   firstof z =  instance (el (n a) ~ a, var (n a)) => var (s n a)   type el (s n a) =   type res (s n a) = -> res (n a)   firstof (s n) _ = firstof n

here few examples:

λ> firstof z 5 5 λ> firstof (s z) 5 9 5 λ> firstof (s (s z)) 5 8 9 5 λ> firstof (s (s z)) "hi" "world" "uhu" "hi"

if interested in how got there can check edit-history

remarks

  • it uses s , z poor-mansreplacement* type-level literals , can working this
  • the version firstof (s (s z)) expect want 2 arguments waiting 3 - that's because start z = 1 argument
  • i saw wanted firstof 3 10 20 30 = 3 not (this give 10) - doable type-level literals , obvious overload too

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