[GHC] #1488: Code that won't ever be executed should not give rise to constraints

[GHC]

#1488: Code that won't ever be executed should not give rise to constraints
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  Reporter:  iampure at gmail.com  |          Owner:         
      Type:  feature request    |         Status:  new    
  Priority:  normal             |      Milestone:         
 Component:  Compiler           |        Version:  6.6.1  
  Severity:  normal             |       Keywords:         
Difficulty:  Moderate (1 day)   |             Os:  Unknown
  Testcase:                     |   Architecture:  Unknown
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In the code

 a x = let b = show x in 5,
 ghci derives:
 a :: (Show a, Num t) => a -> t, which is correct, but I think the rules
 should be slightly altered, since in reality giving something to a which
 is not an instance of Show wouldn't be a problem, since it's not used in
 the first place.

 One important application of this is testing a general data structure for
 one specific element type with assertions in the the general code and
 having a switch to turn generic and non-generic mode on and off.
 Unfortunately, this approach doesn't work today, since all kinds of
 constraints that are essentially part of the debugging process get added.
 Since deciding whether a certain variable is used is very easy in this
 setting(for example CPP macros that expand into "if debugging", where
 debugging will have a constant value. (I don't expect you to solve the
 Halting Problem).

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1488>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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