Re: [GHC] #2544: Improve Can't unify error messages from type functions

2011-01-12 Thread GHC 
#2544: Improve Can't unify error messages from type functions
+---
Reporter:  simonpj  |Owner:  
Type:  bug  |   Status:  new 
Priority:  low  |Milestone:  7.0.2   
   Component:  Compiler (Type checker)  |  Version:  6.8.3   
Keywords:   | Testcase:  
   Blockedby:   |   Difficulty:  Unknown 
  Os:  Unknown/Multiple | Blocking:  
Architecture:  Unknown/Multiple |  Failure:  None/Unknown
+---
Changes (by jwlato):

 * cc: jwl...@… (added)
  * failure:  = None/Unknown


Comment:

 With ghc-7.0.1, the error message is now:
 {{{
 bug.hs:11:12:
 Could not deduce (IxMap r ~ IxMap i1) from the context (Ix l, Ix r)
 NB: `IxMap' is a type function, and may not be injective
 Expected type: IxMap (l :|: r) [Int]
   Actual type: BiApp (IxMap l) (IxMap i1) [Int]
 In the expression: BiApp empty empty
 In an equation for `empty': empty = BiApp empty empty
 In the instance declaration for `Ix (l :|: r)'

 bug.hs:11:18:
 Could not deduce (IxMap i ~ IxMap l) from the context (Ix l, Ix r)
 NB: `IxMap' is a type function, and may not be injective
 Expected type: IxMap l [Int]
   Actual type: IxMap i [Int]
 In the first argument of `BiApp', namely `empty'
 In the expression: BiApp empty empty
 In an equation for `empty': empty = BiApp empty empty
 Failed, modules loaded: none.
 }}}

 Can this be closed? These error messages are much more useful (at least to
 my eye).

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/2544#comment:6
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Re: [GHC] #2544: Improve Can't unify error messages from type functions

2011-01-12 Thread GHC 
#2544: Improve Can't unify error messages from type functions
--+-
  Reporter:  simonpj  |  Owner: 
 
  Type:  bug  | Status:  closed 
 
  Priority:  low  |  Milestone:  7.0.2  
 
 Component:  Compiler (Type checker)  |Version:  6.8.3  
 
Resolution:  fixed|   Keywords: 
 
  Testcase:  indexed-types/should_fail/T2544  |  Blockedby: 
 
Difficulty:  Unknown  | Os:  
Unknown/Multiple
  Blocking:   |   Architecture:  
Unknown/Multiple
   Failure:  None/Unknown |  
--+-
Changes (by simonpj):

  * status:  new = closed
  * testcase:  = indexed-types/should_fail/T2544
  * resolution:  = fixed


Comment:

 It's even improved a bit more in HEAD
 {{{
 T2544.hs:15:12:
 Could not deduce (IxMap l ~ IxMap i0)
 from the context (Ix l, Ix r)
   bound by the instance declaration at T2544.hs:13:10-37
 NB: `IxMap' is a type function, and may not be injective
 Expected type: IxMap (l :|: r) [Int]
   Actual type: BiApp (IxMap i0) (IxMap r) [Int]
 In the return type of a call of `BiApp'
 In the expression: BiApp empty empty
 In an equation for `empty': empty = BiApp empty empty

 T2544.hs:15:24:
 Could not deduce (IxMap i1 ~ IxMap r)
 from the context (Ix l, Ix r)
   bound by the instance declaration at T2544.hs:13:10-37
 NB: `IxMap' is a type function, and may not be injective
 Expected type: IxMap r [Int]
   Actual type: IxMap i1 [Int]
 In the second argument of `BiApp', namely `empty'
 In the expression: BiApp empty empty
 In an equation for `empty': empty = BiApp empty empty
 }}}
 So let's close. I'll add a test for completeness.

 Thanks for bringing it up.

 Simon

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/2544#comment:7
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[GHC] #2544: Improve Can't unify error messages from type functions

2008-08-26 Thread GHC 
#2544: Improve Can't unify error messages from type functions
+---
Reporter:  simonpj  |   Owner: 
Type:  bug  |  Status:  new
Priority:  normal   |   Milestone:  6.12 branch
   Component:  Compiler (Type checker)  | Version:  6.8.3  
Severity:  normal   |Keywords: 
  Difficulty:  Unknown  |Testcase: 
Architecture:  Unknown  |  Os:  Unknown
+---
 Consider this example from a Haskell Cafe thread:
 {{{
 data (:|:) a b = Inl a | Inr b

 class Ix i where
type IxMap i :: * - *
empty  :: IxMap i [Int]

 data BiApp a b c = BiApp (a c) (b c)

 instance (Ix l, Ix r) = Ix (l :|: r) where
type IxMap (l :|: r) = BiApp (IxMap l) (IxMap r)
empty = BiApp empty empty
 }}}
 This elicits the following confusing error message:
 {{{
  Couldn't match expected type `IxMap l'
 against inferred type `IxMap i'
Expected type: IxMap (l :|: r) [Int]
Inferred type: BiApp (IxMap i) (IxMap i1) [Int]
  In the expression: BiApp empty empty
  In the definition of `empty': empty = BiApp empty empty
 }}}
 As Alexander Dunlap responds, the error message is correct, but I can't
 help feeling that we should try harder to give a better error message.
 Something like Since `IxMap` is a type function, knowing that `IxMap l` =
 `IxMap i` does not require that `l` = `i`.  Or something.

 This ticket is just to make sure we don't forget.  The thread is here
 http://www.haskell.org/pipermail/haskell-cafe/2008-August/046371.html

 Simon

-- 
Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/2544
GHC http://www.haskell.org/ghc/
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