[Haskell] Polymorphic types in RHS of type instances

[Steven Keuchel]

Hi list,

I was wondering why GHC doesn't allow usage of polymorphic types in
the right-hand side of type instance declarations for type families.
The GHC user guide states: "The right-hand side of a type instance
must be a monotype (i.e., it may not include foralls) [...]", but it
doesn't state the reason.

I stumbled upon this limitation when I was trying to generically
calculate Johann's and Ghani's interpreter (transformers) for nested
data types from their "Initial Algebra Semantics is Enough!" paper.

Cheers,
Steven