Alfonso Acosta wrote:

class Synchronous s f1 f2 | s -> f1, s -> f2 where mapSY :: f1 a b -> s a -> s b delaySY :: a -> s a -> s a zipWithSY :: f2 a b c-> s a -> s b -> s c

The goal of this class is to extend the name of the following functions (which BTW are already present in a working library and for that reason _it is a must_ that their types remain untouched) ...

mapSY :: (a->b) -> Signal a -> Signal b delaySY :: a -> Signal a -> Signal b -> Signal c zipWithSY :: (a->b->c) -> Signal a -> Signal b -> Signal c

.. accepting these definitions as well

mapSY :: (HDPrimType a, HDPrimType b) => HDFun (a->b) -> HDSignal a -> HDSignal b delaySY :: HDPrimType a => a -> HDSignal a -> HDSignal a zipWithSY :: (HDPrimType a, HDPrimType b, HDPrimType c) => HDFun (a->b->c) -> HDSignal a -> HDSignal b -> HDSignal c

First of all, the design already exhibits the problem: delaySY :: HDPrimType a => a -> HDSignal a -> HDSignal a cannot be made _at all_ the member of an instantiated Synchronous class. The reason is that in the class definition

class Synchronous s f1 f2 | s -> f1, s -> f2 where delaySY :: a -> s a -> s a

the function delaySY is declared *fully* polymorphic over 'a' -- there are no constraints on a and no restrictions. However,

delaySY :: HDPrimType a => a -> HDSignal a -> HDSignal a

shows that 'a' is constrained to satisfy the HDPrimType a. That's the problem: the latter function is not generic enough. The problem is described (and solved) in a message `Restricted Datatypes Now' http://www.haskell.org/pipermail/haskell-prime/2006-February/000498.html I'm not certain if there is a compelling reason to place mapSY, delaySY and zipWithSY in the same class. If not, the following is the solution to the problem. Both sets of mapSY, delaySY and zipWithSY are unified in overloaded functions: {-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-} module SY where class SynchronousM arg1 s a b | s a b -> arg1 where mapSY :: arg1 -> s a -> s b class SynchronousD s a where delaySY :: a -> s a -> s a class SynchronousZ arg1 s a b c | s a b c -> arg1 where zipWithSY :: arg1 -> s a -> s b -> s c -- stubs newtype Signal a = Signal a newtype HDSignal a = HDSignal a newtype HDFun a = HDFun a class HDPrimType a where cnv :: a -> a; cnv = id instance HDPrimType Int instance HDPrimType Bool -- (not so) Grand Unification instance SynchronousM (a->b) Signal a b where mapSY f (Signal x) = Signal (f x) instance (HDPrimType a, HDPrimType b) => SynchronousM (HDFun (a->b)) HDSignal a b where mapSY (HDFun f) (HDSignal x) = HDSignal (cnv . f . cnv $ x) instance SynchronousD Signal a where delaySY _ = id instance HDPrimType a => SynchronousD HDSignal a where delaySY _ (HDSignal x) = HDSignal (cnv x) instance SynchronousZ (a->b->c) Signal a b c where zipWithSY f (Signal x) (Signal y) = Signal (f x y) instance (HDPrimType a, HDPrimType b, HDPrimType c) => SynchronousZ (HDFun (a->b->c)) HDSignal a b c where zipWithSY (HDFun f) (HDSignal x) (HDSignal y) = HDSignal (cnv (f (cnv x) (cnv y)))