[GHC] #451: GHC poor type-checker error message

[GHC]

#451: GHC poor type-checker error message
--------------------------------------+-------------------------------------
  Reporter:  isaacdupree              |          Owner:  simonpj
      Type:  bug                      |         Status:  new    
  Priority:  normal                   |      Milestone:         
 Component:  Compiler (Type checker)  |        Version:  6.4    
  Severity:  normal                   |     Resolution:  None   
  Keywords:                           |             Os:  Unknown
Difficulty:  Unknown                  |   Architecture:  Unknown
--------------------------------------+-------------------------------------
Changes (by simonpj):

  * architecture:  => Unknown
  * difficulty:  => Unknown
  * status:  assigned => new
  * owner:  nobody => simonpj
  * os:  => Unknown

Old description:

> {{{
> Here is a very tricky GHC (6.4) error message I found.
>  I have simplified the context from where I found it,
> but the error is basically the same (still rather less
> confusing than the real thing, where Value ::
> [TString], TString :: [(String,Textdomain)], and error
> messages talked about, e.g., [[([Char],Textdomain)]]
> instead of [[Char]]). I had to run the original through
> Hugs to find my mistake there. Here is the code:
>
> > import Data.List(intersperse)
> > type Value = String
> > -- unifyEnd :: [key] -> [Value] -> [Value]
> > -- this example assumes (length ks <= length vs)
> > unifyEnd ks vs =
> >       let (fvs,evs) = splitAt (length ks - 1) vs
> >       in fvs ++ concat (intersperse "," evs)
>
> Here is the GHC-6.4 error message:
> BadErrorMessage.lhs:10:41:
>     Couldn't match `[Char]' against `Char'
>       Expected type: [[Char]]
>       Inferred type: [Char]
>     In the second argument of `intersperse', namely `evs'
>     In the first argument of `concat', namely
> `(intersperse "," evs)'
>
> The error message when the type signature is
> uncommented at least might lead to less pursuing of the
> wrong things, claiming the literal `","' is in error
> instead, but does not get to the location of the error.
>
> The Hugs error, while not perfect, has got the location
> correct: it shows me the part I erred in:
> ERROR "BadErrorMessage.lhs":8 - Type error in application
> *** Expression     : fvs ++ concat (intersperse "," evs)
> *** Term           : fvs
> *** Type           : [[Char]]
> *** Does not match : [Char]
>
> I had forgotten to put [ ] around concat (...), i.e.
>         fvs ++ [concat (intersperse "," evs)]
> is the corrected fragment of the definition.
>
> GHC did not appear to realize that the two arguments to
> 'intersperse' were currently consistent with each
> other, given intersperse's type signature of a -> [a]
> -> [a], but would not be if the type of the one claimed
> to be in error were changed to the "expected" type. (If
> it could say that they were both the wrong type, that
> would be another choice it had that makes sense, but
> that would be two human errors, perhaps less likely
> than one.)
>
> }}}

New description:

 {{{
 Here is a very tricky GHC (6.4) error message I found.
  I have simplified the context from where I found it,
 but the error is basically the same (still rather less
 confusing than the real thing, where Value ::
 [TString], TString :: [(String,Textdomain)], and error
 messages talked about, e.g., [[([Char],Textdomain)]]
 instead of [[Char]]). I had to run the original through
 Hugs to find my mistake there. Here is the code:

 > import Data.List(intersperse)
 > type Value = String
 > -- unifyEnd :: [key] -> [Value] -> [Value]
 > -- this example assumes (length ks <= length vs)
 > unifyEnd ks vs =
 >       let (fvs,evs) = splitAt (length ks - 1) vs
 >       in fvs ++ concat (intersperse "," evs)

 Here is the GHC-6.4 error message:
 BadErrorMessage.lhs:10:41:
     Couldn't match `[Char]' against `Char'
       Expected type: [[Char]]
       Inferred type: [Char]
     In the second argument of `intersperse', namely `evs'
     In the first argument of `concat', namely
 `(intersperse "," evs)'

 The error message when the type signature is
 uncommented at least might lead to less pursuing of the
 wrong things, claiming the literal `","' is in error
 instead, but does not get to the location of the error.

 The Hugs error, while not perfect, has got the location
 correct: it shows me the part I erred in:
 ERROR "BadErrorMessage.lhs":8 - Type error in application
 *** Expression     : fvs ++ concat (intersperse "," evs)
 *** Term           : fvs
 *** Type           : [[Char]]
 *** Does not match : [Char]

 I had forgotten to put [ ] around concat (...), i.e.
         fvs ++ [concat (intersperse "," evs)]
 is the corrected fragment of the definition.

 GHC did not appear to realize that the two arguments to
 'intersperse' were currently consistent with each
 other, given intersperse's type signature of a -> [a]
 -> [a], but would not be if the type of the one claimed
 to be in error were changed to the "expected" type. (If
 it could say that they were both the wrong type, that
 would be another choice it had that makes sense, but
 that would be two human errors, perhaps less likely
 than one.)

 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/451>
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