[GHC] #11052: Standalone derived Typeable instance for promoted lists is not found
#11052: Standalone derived Typeable instance for promoted lists is not found -------------------------------------+------------------------------------- Reporter: liamoc | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- On GHC 7.10.2, if I try and compile the following short program: {{{#!hs {-# LANGUAGE DataKinds, PolyKinds, DeriveDataTypeable, StandaloneDeriving #-} module Test where import Data.Typeable deriving instance Typeable '[] deriving instance Typeable '(:) nilTyCon = typeRepTyCon (typeRep (Proxy :: Proxy '[])) consTyCon = typeRepTyCon (typeRep (Proxy :: Proxy '(:) )) }}} Then GHC reports the following errors: {{{ Test.hs:9:27: No instance for (Typeable '[]) arising from a use of ‘typeRep’ In the first argument of ‘typeRepTyCon’, namely ‘(typeRep (Proxy :: Proxy '[]))’ In the expression: typeRepTyCon (typeRep (Proxy :: Proxy '[])) In an equation for ‘nilTyCon’: nilTyCon = typeRepTyCon (typeRep (Proxy :: Proxy '[])) Test.hs:10:27: No instance for (Typeable (':)) (maybe you haven't applied enough arguments to a function?) arising from a use of ‘typeRep’ In the first argument of ‘typeRepTyCon’, namely ‘(typeRep (Proxy :: Proxy (:)))’ In the expression: typeRepTyCon (typeRep (Proxy :: Proxy (:))) In an equation for ‘consTyCon’: consTyCon = typeRepTyCon (typeRep (Proxy :: Proxy (:))) }}} It seems a bug to me that the very instances I asked to be derived (and apparently were without complaint from GHC) are now not found by the typechecker in the very next line. I think that if the instance cannot be derived, GHC should say so, or if it can, then GHC should find it. I can't really see what's going on with the dump flags, because the typechecker fails before the desugarer runs. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11052 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#11052: Standalone derived Typeable instance for promoted lists is not found -------------------------------------+------------------------------------- Reporter: liamoc | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.10.2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by liamoc): * component: Compiler => Compiler (Type checker) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11052#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#11052: Standalone derived Typeable instance for promoted lists is not found -------------------------------------+------------------------------------- Reporter: liamoc | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 7.10.2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by rwbarton): The error is correct, though the error message is bad (especially the second one). With `-fprint-explicit-kinds` the error is better: {{{ Test2.hs:10:28: No instance for (Typeable [k1] ('[] k1)) arising from a use of ‘typeRep’ In the first argument of ‘typeRepTyCon’, namely ‘(typeRep (Proxy :: Proxy []))’ In the expression: typeRepTyCon (typeRep (Proxy :: Proxy [])) In an equation for ‘nilTyCon’: nilTyCon = typeRepTyCon (typeRep (Proxy :: Proxy [])) Test2.hs:11:28: No instance for (Typeable (k0 -> [k0] -> [k0]) ((':) k0)) (maybe you haven't applied enough arguments to a function?) arising from a use of ‘typeRep’ In the first argument of ‘typeRepTyCon’, namely ‘(typeRep (Proxy :: Proxy (:)))’ In the expression: typeRepTyCon (typeRep (Proxy :: Proxy (:))) In an equation for ‘consTyCon’: consTyCon = typeRepTyCon (typeRep (Proxy :: Proxy (:))) }}} The type `'[]` can have any kind `[k1]`, and these must have different Typeable instances. So, the (implicit, and not even mentioned in the original error message) kind variable is really ambiguous. You could choose a particular kind with a signature: {{{ nilStarTyCon = typeRepTyCon (typeRep (Proxy :: Proxy ('[] :: [*]))) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11052#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#11052: Standalone derived Typeable instance for promoted lists is not found -------------------------------------+------------------------------------- Reporter: liamoc | Owner: Type: bug | Status: new Priority: high | Milestone: 8.0.1 Component: Compiler (Type | Version: 7.10.2 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by goldfire): * priority: normal => high * milestone: => 8.0.1 Comment: (Written concurrently with comment:2) This is actually expected behavior, but perhaps shouldn't be. The problem is that, at your call sites of `typeRep`, GHC doesn't know what ''kinds'' you want to specialize `'[]` and `'(:)` to. We can't have type representations of poly-kinded things (for various good reasons that would take a bit of academic research to sort out), so compilation fails. But we sure can improve the error message. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11052#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#11052: Standalone derived Typeable instance for promoted lists is not found -------------------------------------+------------------------------------- Reporter: liamoc | Owner: Type: bug | Status: closed Priority: high | Milestone: 8.0.1 Component: Compiler (Type | Version: 7.10.2 checker) | Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by bgamari): * status: new => closed * resolution: => fixed Comment: It appears that the error message has been improved. With `master` as of today the test above produces the following error, {{{ $ inplace/bin/ghc-stage2 hi.hs [1 of 1] Compiling Test ( hi.hs, hi.o ) hi.hs:9:27: error: No instance for (Typeable '[]) arising from a use of ‘typeRep’ GHC can't yet do polykinded Typeable ('[] :: [k1]) In the first argument of ‘typeRepTyCon’, namely ‘(typeRep (Proxy :: Proxy '[]))’ In the expression: typeRepTyCon (typeRep (Proxy :: Proxy '[])) In an equation for ‘nilTyCon’: nilTyCon = typeRepTyCon (typeRep (Proxy :: Proxy '[])) hi.hs:10:27: error: No instance for (Typeable (':)) arising from a use of ‘typeRep’ GHC can't yet do polykinded Typeable ((':) :: k0 -> [k0] -> [k0]) In the first argument of ‘typeRepTyCon’, namely ‘(typeRep (Proxy :: Proxy (:)))’ In the expression: typeRepTyCon (typeRep (Proxy :: Proxy (:))) In an equation for ‘consTyCon’: consTyCon = typeRepTyCon (typeRep (Proxy :: Proxy (:))) }}} Given that representation of polykinded types is an open research question it seems that the bug covered by this issue (the error message) has been resolved. Closing. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11052#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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