{-# LANGUAGE TypeFamilies, MultiParamTypeClasses, FlexibleContexts, FlexibleInstances, GADTs #-}
lets say I have a data type
data S a = S a deriving Show
and a nice simple typeclass
class Foo a where inc :: a -> S a
make Integer an instance and....
instance Foo Integer where inc x = S x
brilliant... now lets say I want my typeclass to enforce that any result of inc is also of a type that is an instance of the typeclass I'll follow my nose...which causes me a problem that makes me realise I maybe I don't understand whats going on.
class Bar a where inc' :: (Bar (S a)) => a -> S a
follow it to....
instance Bar Integer where inc' x = S x
this WORKS!...even though "S Integer" is not an instance of Bar!...maybe I don't understand.
-- x = inc' (1 :: Integer)
This would go BOOM ...."no instance of (Bar (S Integer))" ...so that makes sense.... How do I force Haskell to check the constraint before I evaluate an expression? follow my nose?
-- class (Baz (S a)) => Baz a where -- inc'' :: a -> S a
BOOM ...."Cycle in class declaration".... booo..but I suppose thats understandable.... so a) is my use of typeclasses correct?...or am I missing something? b) if so, how do I force the compiler to make sure my typeclass operations is nicely closed (under the typeclass)? CONFIDENTIALITY NOTICE This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. If you communicate with us by e-mail, you acknowledge and assume these risks, and you agree to take appropriate measures to minimise these risks when e-mailing us. MTV Networks International, MTV Networks UK & Ireland, Greenhouse, Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be Viacom, Viacom International Media Networks and VIMN and Comedy Central are all trading names of MTV Networks Europe. MTV Networks Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT.