[GHC] #12801: Misleading error message when deriving Functor
#12801: Misleading error message when deriving Functor -------------------------------------+------------------------------------- Reporter: erikd | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Poor/confusing Unknown/Multiple | error message Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Compiling this trivial demo program: {{{ data Container = Container [Wibble Int] deriving (Eq, Show) data Wibble a = Wibble a | Wobble deriving (Eq, Functor, Show) }}} results in the error message: {{{ a.hs:6:13: error: • No instance for (Eq (Wibble Int)) arising from the first field of ‘Container’ (type ‘[Wibble Int]’) Possible fix: use a standalone 'deriving instance' declaration, so you can specify the instance context yourself • When deriving the instance for (Eq Container) a.hs:6:17: error: • No instance for (Show (Wibble Int)) arising from the first field of ‘Container’ (type ‘[Wibble Int]’) Possible fix: use a standalone 'deriving instance' declaration, so you can specify the instance context yourself • When deriving the instance for (Show Container) a.hs:12:17: error: • Can't make a derived instance of ‘Functor Wibble’: You need DeriveFunctor to derive an instance for this class • In the data declaration for ‘Wibble’ }}} The "No instance for (Eq|Show)" are rather misleading, because the *actual* error is that the `{-# LANGUAGE DeriveFunctor #-}` pragma is missing. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12801 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#12801: Misleading error message when deriving Functor -------------------------------------+------------------------------------- Reporter: erikd | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): I think this is a duplicate of #10684, as the failure to derive `Functor` for `Wibble` prevents `Eq` and `Show` from being derived for `Wibble`, causing a cascade of error messages for `Container`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12801#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#12801: Misleading error message when deriving Functor -------------------------------------+------------------------------------- Reporter: erikd | Owner: Type: bug | Status: closed Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 8.0.1 Resolution: duplicate | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by erikd): * status: new => closed * resolution: => duplicate Comment: Yes, this is a dupe of #10684. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12801#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#12801: Misleading error message when deriving Functor
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Reporter: erikd | Owner: (none)
Type: bug | Status: closed
Priority: normal | Milestone: 8.2.1
Component: Compiler | Version: 8.0.1
Resolution: duplicate | Keywords:
Operating System: Unknown/Multiple | Architecture:
Type of failure: Poor/confusing | Unknown/Multiple
error message | Test Case:
Blocked By: | Blocking:
Related Tickets: | Differential Rev(s):
Wiki Page: |
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Comment (by Ryan Scott
#12801: Misleading error message when deriving Functor -------------------------------------+------------------------------------- Reporter: erikd | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: 8.4.1 Component: Compiler | Version: 8.0.1 Resolution: duplicate | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: Poor/confusing | Test Case: error message | deriving/should_fail/T12801 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * testcase: => deriving/should_fail/T12801 * milestone: 8.2.1 => 8.4.1 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12801#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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