First-class types

[Simon Peyton-Jones]

| 1) Coerce a a can be defined as coerce=3Did for all a.  However, this
|    may of course lead to overlap in the type structure, so we must
|    write a separate instance definition for Coerce Int Int, Coerce
|    Double Double, etc. if we want types to be decidable.  I'd love for
|    some clever person to solve this little difficulty.

Indeed so.  And=20
	instance Coerce a a where coerce =3D id
does not overlap with
	instance Coerce Int Bool where ...
	instance Coerce Bool Int where ...

There's no overlap here.  (Overlap =3D there is a Coerce t1 t2, which
matches more than one instance decl.)  Furthermore, it works fine; I
just tried it.

| 2) When we define D a b c, we know that D b a c is also allowed.
|    Again, decidability prevents us from asserting this directly.
|    Again, a clever solution could save us a lot of code and even more
|    debugging.  I suspect this may be a marginally easier nut to crack
|    than the previous one.

Much harder, I think.  Commutativity is always tricky!

Simon