#14047: "Illegal instance for type synonym" while deriving Typeable1 for data type ---------------------------------+-------------------------------------- Reporter: alexbiehl | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: Operating System: Windows | Architecture: x86_64 (amd64) Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | ---------------------------------+-------------------------------------- Description changed by alexbiehl: Old description:

This occured to me in `network-ip` package today:

{{{#!hs {-# LANGUAGE StandaloneDeriving #-} module Repro where

import Data.Typeable (Typeable1)

data InetAddr a = InetAddr deriving instance Typeable1 InetAddr }}}

Results in GHC complaining with:

{{{#!hs repro.hs:7:19: error: * Illegal instance for a type synonym A class instance must be for a class * In the stand-alone deriving instance for `Typeable1 InetAddr' | 7 | deriving instance Typeable1 InetAddr | ^^^^^^^^^^^^^^^^^^ }}}

This is due to `Typeable1` defined as type synonym for `Typeable` (see here http://hackage.haskell.org/package/base-4.10.0.0/docs/Data- Typeable.html#t:Typeable1).

I can see it is deprecated but I would still expect it to work correctly still.

New description: This occured to me in `network-ip` package today: {{{#!hs {-# LANGUAGE StandaloneDeriving #-} module Repro where import Data.Typeable (Typeable1) data InetAddr a = InetAddr deriving instance Typeable1 InetAddr }}} Results in GHC complaining with: {{{#!hs repro.hs:7:19: error: * Illegal instance for a type synonym A class instance must be for a class * In the stand-alone deriving instance for `Typeable1 InetAddr' | 7 | deriving instance Typeable1 InetAddr | ^^^^^^^^^^^^^^^^^^ }}} This is due to `Typeable1` defined as type synonym for `Typeable` (see here http://hackage.haskell.org/package/base-4.10.0.0/docs/Data- Typeable.html#t:Typeable1). I see it is deprecated but I would still expect it to work correctly still. -- -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14047#comment:1> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

#14047: "Illegal instance for type synonym" while deriving Typeable1 for data type -------------------------------------+------------------------------------- Reporter: alexbiehl | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #13267 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * os: Windows => Unknown/Multiple * architecture: x86_64 (amd64) => Unknown/Multiple * related: => #13267 Comment: This is expected behavior, see #13267. As it turns out, allowing type synonyms to be used as instance heads is extremely tricky to get right, and GHC decided to disallow this in 8.2.1 due to incorrect results one could achieve with nonsense like: {{{ type ShowF a = Show (a -> Bool) instance ShowF Int where show _ = "Fun" }}} As for your particular program, the workaround is simple: just don't derive `Typeable`! Every type automatically has `Typeable` instances these days, so deriving `Typeable` is a no-op. As you've noted `Typeable1` //et al.// have long been deprecated in favor of `Typeable`, and given that they're far less useful than they were in the past, I propose just removing `Typeable1` and friends altogether. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14047#comment:3> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

#14047: "Illegal instance for type synonym" while deriving Typeable1 for data type -------------------------------------+------------------------------------- Reporter: alexbiehl | Owner: (none) Type: bug | Status: patch Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #13267 | Differential Rev(s): Phab:D3817 Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: new => patch * differential: => Phab:D3817 Comment: I've submitted Phab:D3817 to remove `Typeable{1..7}`. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14047#comment:5> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler

#14047: "Illegal instance for type synonym" while deriving Typeable1 for data type -------------------------------------+------------------------------------- Reporter: alexbiehl | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: 8.4.1 Component: Compiler | Version: 8.2.1 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #13267 | Differential Rev(s): Phab:D3817 Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * status: patch => closed * resolution: => fixed * milestone: => 8.4.1 Comment: This is now "fixed", since `Typeable1` no longer exists. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14047#comment:7> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler