`reify` doesn't find associated type families

Template Haskell's reify doesn't find the the associated type families of a type class. Consider the following two classes:

class C a where
  f :: a -> Int

class C' a where
  type F a :: *
  f' :: a -> Int

These produce identical, up to renaming, output when passed to reify:

-- putStrLn $(stringE . show =<< reify ''C)
ClassI (ClassD [] Ghci1.C [PlainTV a_1627398388] []
               [SigD Ghci1.f
                     (ForallT [PlainTV a_1627398388]
                              [ClassP Ghci1.C [VarT a_1627398388]]
                              (AppT (AppT ArrowT (VarT a_1627398388))
                                    (ConT GHC.Types.Int)))])
       []

-- putStrLn $(stringE . show =<< reify ''C')
ClassI (ClassD [] Ghci3.C' [PlainTV a_1627405973] []
               [SigD Ghci3.f'
                     (ForallT [PlainTV a_1627405973]
                              [ClassP Ghci3.C' [VarT a_1627405973]]
                              (AppT (AppT ArrowT (VarT a_1627405973))
                                    (ConT GHC.Types.Int)))])
       []

If I know the name of F, I can look up information about it:

-- putStrLn $(stringE . show =<< reify ''F)
FamilyI (FamilyD TypeFam
                 Ghci3.F
                 [PlainTV a_1627405973]
                 (Just StarT))
        []

But I can't find the name of F in the first place. Even if I add an instance of the type class, the InstanceD has none of the information about the definition. Given

instance C' [a] where
  type F [a] = a
  f' = length

we get

-- putStrLn $(stringE . show =<< reify ''C')
ClassI (ClassD [] Ghci3.C' [PlainTV a_1627405973] []
               [SigD Ghci3.f'
                     (ForallT [PlainTV a_1627405973]
                              [ClassP Ghci3.C' [VarT a_1627405973]]
                              (AppT (AppT ArrowT (VarT a_1627405973))
                                    (ConT GHC.Types.Int)))])
       [InstanceD []
                  (AppT (ConT Ghci3.C')
                        (AppT ListT (VarT a_1627406161)))
                  []]

This problem was confirmed to exist in the following configurations:

• GHC 7.10.2 with version 2.10.0.0 of the template-haskell package.
• GHC 7.8.3 with version 2.9.0.0 of the template-haskell package.
• GHC 7.4.2 with version 2.7.0.0 of the template-haskell package.

(I also posted this on Stack Overflow in //2013//, where it has been languishing ever since: http://stackoverflow.com/q/17247880/237428.)

mentioned in issue #10853

changed weight to 5

added Tbug TemplateHaskell Trac import labels

changed milestone to %8.0.1

Well, that's silly. My guess, without looking at the code, that this could be fixed in 2-3 lines. Newcomers?

added newcomer label

changed the description

How should default type instances be reified? e.g. if I have something like:

class C a where
  type T a :: *
  type T a = a

Syntactically it looks like TySynD but semantically it's different.

That's true that it's different semantically. But Template Haskell is all about syntax, not semantics. But I'd make it a TySynInstD instead of a TySynD (even though the latter is more syntactically accurate). It seems a bit overwrought to make a new constructor for Dec for this case.

Another question: Normally we don't fill InstanceD contents, probably because we can't anyway -- because we need source code for this, right? But we can still fill information about associated type instances. Should we do this or are there any problems with that? (maybe it's not possible etc.)

(By InstanceD I mean the one generated for ClassI)

assigned to @osa1

That's a good point. If it's easy to do, I think adding the associated type stuff would be nice. But don't go too far out of way to do it. Thanks for jumping in!

added 1 deleted label

Trac metadata

Trac field	Value
Differential revisions	→ D1254

mentioned in commit 5c115236

This commit was reverted in 39a262e5.

removed 1 deleted label

mentioned in commit b4d43b4e