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GHC/Stand-alone deriving declarations

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Bjorn Bringert has recently implemented "stand-alone deriving" declarations, documented briefly here [1]. There are a few loose ends which I summarise here:

Contents

1 Syntax

The current syntax is

  deriving Show for T

There seems to be a consensus that this would be better:

  derive instance Show T
so that it looks more like a regular instance declaration. Here
derive
is not a new keyword; it's a "special-id", distinguished by the following
instance
keyword. That means that
derive
can still be used freely as a regular varid. (Perhaps
derived
would sound more declarative than
derive
.)

2 Context on the declaration

Because it looks like a regular instance declaration, it would arguably be reasonable to require the programmer to supply the context. It seems odd to say:

  derive instance Show (T a)

and perhaps cleaner to say

  derive instance Show a => Show (T a)

(At the moment, the compiler figures out the appropriate context, but at some point that automation may run out of steam.)

Alternatively the syntax could be:

  instance deriving Show a => Show (T a)
with the advantage that we don't need a complicated rule to determine whether or not
derive
is an identifier or a keyword (all other reserved ids always occur inside the construct not before it), or even just
  instance Show a => Show (T a)
where the fact that the compiler is automatically deriving the content of the instance declaration is implicit in the absence of a
where
clause.

3 Interaction with "newtype-deriving"

GHC's "newtype deriving mechanism" (see [2]) should obviously work in a standalone deriving setting too. But perhaps it can be generalised a little. Currently you can only say

  deriving C a for Foo
(where Foo is the newtype), and get an instance for
(C a Foo)
. But what if you want and instance for
C Foo a
, where the new type is not the last parameter. You can't do that at the moment. However, even with the new instance-like syntax, it's not clear to me how to signal the type to be derived. Consider
  newtype Foo = F Int
  newtype Bar = B Bool
  derive instance C Foo Bar

Which of these thee instances do we want?

  instance C Foo Bool => C Foo Bar
  instance C Int Bar  => C Foo Bar
  instance C Int Bool => C Foo Bar

The obvious way to signal this is to give the instance context (just as above). This is perhaps another reason for having an explicit instance context in a standalone deriving declaration.

Incidentally, notice that the third of the alternatives in the previous bullet unwraps two newtypes simultaneously. John Meacham suggested this example:

  class SetLike m k  where 
  instance SetLike IntSet Int where
 
  newtype Id = Id Int
  newtype IdSet = IdSet IntSet
 
  derive instance SetLike IntSet Int => SetLike IdSet Id

4 Duplicate instances

Suppose two modules, M1 and M2 both contain an identical standalone deriving declaration

  derive Show T

Then, can you import M1 and M2 into another module X and use show on values of type T, or will you get an overlapping instance error? Since both instances are derived in the very same way, their code must be identical, so arguably we can choose either. (There is some duplicated code of course.)

This situation is expected to be common, as the main use of the standalone feature is to obtain derived instances that were omitted when the data type was defined.