Opened 6 years ago
Closed 4 years ago
#5391 closed feature request (wontfix)
Better deriving for Typeable
Reported by: | simonpj | Owned by: | |
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Priority: | normal | Milestone: | 7.6.2 |
Component: | Compiler | Version: | 7.0.4 |
Keywords: | Cc: | dterei, vogt.adam@… | |
Operating System: | Unknown/Multiple | Architecture: | Unknown/Multiple |
Type of failure: | None/Unknown | Test Case: | |
Blocked By: | Blocking: | ||
Related Tickets: | Differential Rev(s): | ||
Wiki Page: |
Description
In a message to GHC users, provoked by a thread on Haskell Cafe I proposed the following improvement to deriving Typeable
:
David Mazieres and others comment that you can't derive Typeable
for types like this:
data T f = MkT (f Int)
So he defines his own instance like this
[C] instance Typable1 f => Typeable (T f) where typeOf = ...
So why can't GHC do this? Well, here's what GHC does. Given a bog standard data type like Maybe
data Maybe a = Nothing | Just a deriving( Typeable )
GHC generates this instance
[A] instance Typeable1 Maybe where typeOf = ...
Remember that Typeable1
takes a type constructor, of kind (*->*)
, as its argument.
Now if we need (Typeable (Maybe Int))
, GHC first uses an instance from the Typeable
library:
[B] instance (Typeable1 f, Typeable a) => Typeable (f a) where typeOf = ...
And now it uses the (Typeable1 Maybe)
instance [A]. So it's kind of cool... the applications are decomposed by [B], leaving the tycon to [A].
But this doesn't work for T above. We can't make (Typeable1 T)
because T has kind ((*->*)->*)
, not (*->*
) as Typeable1
requires. Hence David defining his own instance.
GHC could do this too. Indeed it could do so for Maybe
too, thus:
instance Typeable a => Typeable (Maybe a) where typeOf = ...
But then, alas, we could not get (Typeable (T Maybe))
, because [C] needs Maybe
to be in Typeable1
.
Proposal
So here is a compromise, which would at least do better than the current story:
When deriving Typeable for a data type S of kind S :: k1 -> .. -> kn -> * -> ... -> * (where kn is not *, and there are M trailing * arguments), generate the instance instance (Typeable_x1 a1, ..., Typeable_xn an) => TypeableM (S a1 .. an)
That is, knock off all the trailing *
args, and then generate an instance for the remaining stub.
Example
Example from iterIO:
newtype Iter (t :: *) (m :: *->*) (a :: *) = Iter { runIter :: Chunk t -> IterR t m a } deriving( Typeable )
This should generate
instance (Typeable t, Typeable1 m) => Typeable1 (Iter t m)
where we knock off the trailing (a :: *
) argument.
Question
This approach is not beautiful. It does not solve the underlying problem, which is a lack of kind polymorphism, but that is a battle for another day. Until that day, this alternative way of deriving Typeable
would automate significantly more cases, I think. Of course, it also makes it more complicated to explain when "deriving Typeable
" will succeed.
State of play
As it happens, I'm working with Dimitrios and Julien on adding a proper kind system to GHC, and that will in turn affect the whole Typeable story. See our wiki page on kinds.
So I'm going to put this thread on ice for now, but I'm attaching a partial patch I did earlier so I don't lose it.
Attachments (1)
Change History (8)
Changed 6 years ago by
Attachment: | typeable-diff added |
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comment:1 Changed 6 years ago by
Type: | bug → feature request |
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comment:2 Changed 6 years ago by
Milestone: | → 7.6.1 |
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comment:3 Changed 6 years ago by
Cc: | dterei added |
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comment:4 Changed 5 years ago by
Milestone: | 7.6.1 → 7.6.2 |
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comment:5 Changed 5 years ago by
Cc: | vogt.adam@… added |
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comment:7 Changed 4 years ago by
difficulty: | → Unknown |
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Resolution: | → wontfix |
Status: | new → closed |
Yes, the polykinded Typeable is strictly better than the compromise offered here.
Diff for preliminary work on extending "deriving Typeable"