[GHC] #6022: GHC infers over-general types

[GHC]

#6022: GHC infers over-general types
---------------------------------+------------------------------------------
    Reporter:  simonpj           |       Owner:                  
        Type:  bug               |      Status:  new             
    Priority:  high              |   Milestone:  7.6.2           
   Component:  Compiler          |     Version:  7.4.1           
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown    
  Difficulty:  Unknown           |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:                    |  
---------------------------------+------------------------------------------

Comment(by carter):

 While this is likely wrong code, it is perfectly reasonable to have vector
 function num instances! (its not overly general, its just not
 pedagogically helpful for new people starting in haskell)

 eg   given Num a ,  the instance  Num (Vector a -> a), has a perfectly
 natural (and useful) definition. and thus a Num instance for [a]-> a is
 reasonable too!

 however, maybe an alternative approach would be add a suppressable warning
 for when a type class constraint is used which has no known instances?

 here are example "similar ways" to write the same function, and their
 respective types, in ghci 7.6.1


 Prelude> let g x = (+) x  $ head
 Prelude> :t g
 g :: Num ([a] -> a) => ([a] -> a) -> [a] -> a

 Prelude> let h x y = x + head y
 Prelude> :t h
 h :: Num a => a -> [a] -> a

 Prelude> let q x = (x +) . head
 Prelude> :t q
 q :: Num c => c -> [c] -> c

 so it seems that the core of the issue in the case of the bad example in
 this ticket  is a syntactical one of making the intended composition.
 Likewise, I think this isn't a bug, the type checker is giving a type that
 the user can look at and see is "wrong" for their code!

 i think rejecting code with that Num instance constraint would be bad.
 Unless someone has a counter point, I think this issue should be closed.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6022#comment:2>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler