#10891: `reify` doesn't find associated type families -------------------------------------+------------------------------------- Reporter: antalsz | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.0.1 Component: Template Haskell | Version: 7.10.2 Resolution: | Keywords: type | families; newcomer Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect result | Unknown/Multiple at runtime | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Description changed by osa1: Old description:
Template Haskell's `reify` doesn't find the the associated type families of a type class. Consider the following two classes:
{{{#!hs 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`:
{{{#!hs -- 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:
{{{!#hs -- 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
{{{#!hs instance C' [a] where type F [a] = a f' = length }}}
we get
{{{#!hs -- 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.)
New description: Template Haskell's `reify` doesn't find the the associated type families of a type class. Consider the following two classes: {{{#!hs 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`: {{{#!hs -- 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: {{{#!hs -- 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 {{{#!hs instance C' [a] where type F [a] = a f' = length }}} we get {{{#!hs -- 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.) -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10891#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler