Is the same Empty returned when it matched in a function?

Question

we have mapOptional from the NICTA course:

mapOptional :: (a -> b) -> Optional a -> Optional b
mapOptional _ Empty    = Empty
mapOptional f (Full a) = Full (f a)

When matching f we obviously use that function that was passed, what about the Empty? and what about Full?

Answer 1

There is nothing in Haskell that lets you observe whether the two Emptys are the same Empty or not, and no guarantees about what an implementation must do with that code in that regard.

That said, in GHC, nullary constructors for a given parameterized type are shared across all parameterizations; so there is just one Empty in the whole program, and just one [], and so forth.

Answer 2

They can't be the same Empty, the argument has the type Optional a and the output has the type Optional b. When I try to force some sort of reuse, I will typically use something of the type

mapOptional _ a@Empty    = a

This won't compile, and I don't think that's implementation dependent.