| 1) Coerce a a can be defined as coerce=id 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
instance Coerce a a where coerce = id
does not overlap with
instance Coerce Int Bool where ...
instance Coerce Bool Int where ...
There's no overlap here. (Overlap = 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
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