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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.)