[GHC] #1974: length "foo" doesn't work with -XOverloadedStrings

[GHC]

#1974: length "foo" doesn't work with -XOverloadedStrings
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 Reporter:  ganesh           |          Owner:         
     Type:  feature request  |         Status:  new    
 Priority:  low              |      Milestone:         
Component:  Compiler         |        Version:  6.8.1  
 Severity:  minor            |     Resolution:         
 Keywords:                   |     Difficulty:  Unknown
 Testcase:                   |   Architecture:  Unknown
       Os:  Unknown          |  
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Comment (by simonpj):

 Fair enough.  Would you care to specify the new defaulting rule?  The
 starting points are:
  0. http://www.haskell.org/ghc/docs/latest/html/users_guide/interactive-
 evaluation.html#extended-default-rules
  0. http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-
 extensions.html#overloaded-strings

 In particular, (1) requires the constraints to be of form (C a), where 'a'
 is a type variable; whereas you want it to be of form (C ty), where ty is
 a type. You probably want the type to have at most one free variable, to
 avoid complicated interactions between defaults.

 But what if the constraints are `(IsString [a], Eq a)`.  Then to try a
 candidate default type, say `[Char]` we presumably match `[a]` against
 `[Char]`, and everything works out.  But if the `(Eq a)` was first, we
 might try matching `a` against `[Char]`, which doesn't work.  So perhaps
 each candidate type must be matched against each constraint (and if the
 match succeeds, try to solve the constraints under that substitution)?

 If you write the spec, I'm pretty sure it'll be easy enough to implement.

 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1974#comment:2>
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