Haskell type of specific data constructor -

suppose have following haskell code:

data option     =     | opt1 int double string     -- more options here in real case  handleoption :: option -> io () handleoption option = case option of     -> handlehelp     opt1 n f s -> handleopt1 n f s  handlehelp :: io () handlehelp = print "help"  handleopt1 :: int -> double -> string -> io () handleopt1 n f s = print (n, f, s) 

in above code, seems me waste deconstruct object ahead of time in sense keep data bundled neatly together. have pass each part of opt1 individually or create single separate data type haul them along. possible pass in entire opt1 handleopt1 while not allowing general option instance being passed in, such making handleopt1 help compile error?

example pseudo code below:

---

data option     =     | opt1 int double string  handleoption :: option -> io () handleoption option = case option of     -> handlehelp     opt1 @ opt1{} -> handleopt1 opt1  handlehelp :: io () handlehelp = print "help"  handleopt1 :: option:opt1 -> io () handleopt1 (opt1 n f s) = print (n, f, s) 

you can use gadts this.

{-# language gadts #-}  data option     :: option ()     opt1 :: int -> double -> string -> option (int, double, string)  handleoption :: option -> io () handleoption option = case option of              -> handlehelp     opt1 @ opt1{} -> handleopt1 opt1  handlehelp :: io () handlehelp = print "help"  handleopt1 :: option (int, double, string) -> io () handleopt1 (opt1 n f s) = print (n, f, s) 

with gadts, give more type information compiler. handleopt1, since accepts option (int, double, string), compiler knows option () (i.e. help) never passed in.

that said, using gadts makes quite few other things harder. instance, automatic deriving (e.g. deriving (eq, show)) doesn't work them. should consider pros , cons of using them in case.