Revisions to Getting associated type synonyms with template Haskell

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edited Apr 27, 2015 at 17:00

Antal Spector-Zabusky

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% ghci
GHCi, version 7.48.23: http://www.haskell.org/ghc/  :? for help
...
Prelude> -- I'll be inserting line breaks and whitespace for clarity
Prelude> -- in all GHCi output.
Prelude> :set -XTemplateHaskell 
Prelude> import Language.Haskell.TH
Prelude Language.Haskell.TH> class C a where f :: a -> Int
Prelude Language.Haskell.TH> putStrLn $(stringE . show =<< reify ''C)
ClassI (ClassD [] :InteractiveGhci1.C [PlainTV a_1627394862]a_1627398388] []
               [SigD :InteractiveGhci1.f
                     (ForallT [PlainTV a_1627394862]a_1627398388]
                              [ClassP :InteractiveGhci1.C [VarT a_1627394862]]a_1627398388]]
                              (AppT (AppT ArrowT (VarT a_1627394862a_1627398388))
                                    (ConT GHC.Types.Int)))])
       []

However, adding an associated type syonymsynonym to the class causes no change (up to renaming) in the output:

Prelude Language.Haskell.TH> :set -XTypeFamilies 
Prelude Language.Haskell.TH> class C' a where type F a :: * ; f' :: a -> Int
Prelude Language.Haskell.TH> putStrLn $(stringE . show =<< reify ''C')
ClassI (ClassD [] :InteractiveGhci3.C' [PlainTV a_1627398570]a_1627405973] []
               [SigD :InteractiveGhci3.f'
                     (ForallT [PlainTV a_1627398570]a_1627405973]
                              [ClassP :InteractiveGhci3.C' [VarT a_1627398570]]a_1627405973]]
                              (AppT (AppT ArrowT (VarT a_1627398570a_1627405973))
                                    (ConT GHC.Types.Int)))])
       []

Prelude Language.Haskell.TH> putStrLn $(stringE . show =<< reify ''F)
FamilyI (FamilyD TypeFam
                 :InteractiveGhci3.F
                 [PlainTV a_1627398570]a_1627405973]
                 (Just (ArrowK StarK StarK)StarT))
        []

Prelude Language.Haskell.TH> instance C' [a] where type F [a] = a ; f' = length
Prelude Language.Haskell.TH> f' "Haskell"
7
Prelude Language.Haskell.TH> 42 :: F [Integer]
42
Prelude Language.Haskell.TH> putStrLn $(stringE . show =<< reify ''C')
ClassI (ClassD [] :InteractiveGhci3.C' [PlainTV a_1627398570]a_1627405973] []
               [SigD :InteractiveGhci3.f'
                     (ForallT [PlainTV a_1627398570]a_1627405973]
                              [ClassP :InteractiveGhci3.C' [VarT a_1627398570]]a_1627405973]]
                              (AppT (AppT ArrowT (VarT a_1627398570a_1627405973))
                                    (ConT GHC.Types.Int)))])
       [InstanceD []
                  (AppT (ConT :InteractiveGhci3.C')
                        (AppT ListT (VarT a_1627398694a_1627406161)))
                  []]

I'm runningThis problem is present in GHC 7.8.3 with version 2.9.0.0 of the template-haskell package; it was also present in GHC 7.4.2, with version 2.7.0.0 of the template-haskell package. If reify or something simple works (I didn't check on a later version, that's good to know, but I'm stuck withGHC 7.46.2 for this project*, but I imagine it was present there too.) I'm interested in solutions for the foreseeable futureany version of GHC (including "this was only fixed in GHC version V").

% ghci
GHCi, version 7.4.2: http://www.haskell.org/ghc/  :? for help
...
Prelude> :set -XTemplateHaskell 
Prelude> import Language.Haskell.TH
Prelude Language.Haskell.TH> class C a where f :: a -> Int
Prelude Language.Haskell.TH> putStrLn $(stringE . show =<< reify ''C)
ClassI (ClassD [] :Interactive.C [PlainTV a_1627394862] []
               [SigD :Interactive.f
                     (ForallT [PlainTV a_1627394862]
                              [ClassP :Interactive.C [VarT a_1627394862]]
                              (AppT (AppT ArrowT (VarT a_1627394862))
                                    (ConT GHC.Types.Int)))])
       []

However, adding an associated type syonym to the class causes no change (up to renaming) in the output:

Prelude Language.Haskell.TH> :set -XTypeFamilies 
Prelude Language.Haskell.TH> class C' a where type F a :: * ; f' :: a -> Int
Prelude Language.Haskell.TH> putStrLn $(stringE . show =<< reify ''C')
ClassI (ClassD [] :Interactive.C' [PlainTV a_1627398570] []
               [SigD :Interactive.f'
                     (ForallT [PlainTV a_1627398570]
                              [ClassP :Interactive.C' [VarT a_1627398570]]
                              (AppT (AppT ArrowT (VarT a_1627398570))
                                    (ConT GHC.Types.Int)))])
       []

Prelude Language.Haskell.TH> putStrLn $(stringE . show =<< reify ''F)
FamilyI (FamilyD TypeFam
                 :Interactive.F
                 [PlainTV a_1627398570]
                 (Just (ArrowK StarK StarK)))
        []

Prelude Language.Haskell.TH> instance C' [a] where type F [a] = a ; f' = length
Prelude Language.Haskell.TH> f' "Haskell"
7
Prelude Language.Haskell.TH> 42 :: F [Integer]
42
Prelude Language.Haskell.TH> putStrLn $(stringE . show =<< reify ''C')
ClassI (ClassD [] :Interactive.C' [PlainTV a_1627398570] []
               [SigD :Interactive.f'
                     (ForallT [PlainTV a_1627398570]
                              [ClassP :Interactive.C' [VarT a_1627398570]]
                              (AppT (AppT ArrowT (VarT a_1627398570))
                                    (ConT GHC.Types.Int)))])
       [InstanceD []
                  (AppT (ConT :Interactive.C')
                        (AppT ListT (VarT a_1627398694)))
                  []]

I'm running GHC version 7.4.2, with version 2.7.0.0 of the template-haskell package. If reify or something simple works on a later version, that's good to know, but I'm stuck with 7.4.2 for this project for the foreseeable future.

% ghci
GHCi, version 7.8.3: http://www.haskell.org/ghc/  :? for help
...
Prelude> -- I'll be inserting line breaks and whitespace for clarity
Prelude> -- in all GHCi output.
Prelude> :set -XTemplateHaskell 
Prelude> import Language.Haskell.TH
Prelude Language.Haskell.TH> class C a where f :: a -> Int
Prelude Language.Haskell.TH> 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)))])
       []

However, adding an associated type synonym to the class causes no change (up to renaming) in the output:

Prelude Language.Haskell.TH> :set -XTypeFamilies 
Prelude Language.Haskell.TH> class C' a where type F a :: * ; f' :: a -> Int
Prelude Language.Haskell.TH> 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)))])
       []

Prelude Language.Haskell.TH> putStrLn $(stringE . show =<< reify ''F)
FamilyI (FamilyD TypeFam
                 Ghci3.F
                 [PlainTV a_1627405973]
                 (Just StarT))
        []

Prelude Language.Haskell.TH> instance C' [a] where type F [a] = a ; f' = length
Prelude Language.Haskell.TH> f' "Haskell"
7
Prelude Language.Haskell.TH> 42 :: F [Integer]
42
Prelude Language.Haskell.TH> 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 is present in GHC 7.8.3 with version 2.9.0.0 of the template-haskell package; it was also present in GHC 7.4.2 with version 2.7.0.0 of the template-haskell package. (I didn't check on GHC 7.6.*, but I imagine it was present there too.) I'm interested in solutions for any version of GHC (including "this was only fixed in GHC version V").

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edited Apr 22, 2015 at 17:27

Antal Spector-Zabusky

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Can Template HaskellHaskell find out the names and/or the declarations of the associated type synonyms declared in a type class? I expected reify would do what I want, but it doesn't seem to provide all the necessary information. It works for getting function type signatures:

Can Template Haskell find out the names and/or the declarations of the associated type synonyms declared in a type class? I expected reify would do what I want, but it doesn't seem to provide all the necessary information. It works for getting function type signatures:

Can Template Haskell find out the names and/or the declarations of the associated type synonyms declared in a type class? I expected reify would do what I want, but it doesn't seem to provide all the necessary information. It works for getting function type signatures:

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edited Apr 22, 2015 at 15:11

Hossein Narimani Rad

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Can Template HaskellHaskell find out the names and/or the declarations of the associated type synonyms declared in a type class? I expected reify would do what I want, but it doesn't seem to provide all the necessary information. It works for getting function type signatures:

Can Template Haskell find out the names and/or the declarations of the associated type synonyms declared in a type class? I expected reify would do what I want, but it doesn't seem to provide all the necessary information. It works for getting function type signatures:

Can Template Haskell find out the names and/or the declarations of the associated type synonyms declared in a type class? I expected reify would do what I want, but it doesn't seem to provide all the necessary information. It works for getting function type signatures:

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Antal Spector-Zabusky

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