[GHC] #14782: typeclass polymorphism defeats bang patterns

[GHC]

#14782: typeclass polymorphism defeats bang patterns
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           Reporter:  dmwit          |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I think the following program should crash, but it doesn't:

 {{{
 {-# LANGUAGE BangPatterns #-}
 main = let n :: Num a => a; !n = undefined in return ()
 }}}

 Interestingly, my attempt to translate this out of BangPatterns into
 standard Haskell according to the translation on the wiki page led to a
 compile-time error:

 {{{
 main = let n :: Num a => a; x at n = undefined in x `seq` return ()
 }}}

 Produces the error:

 {{{
 test.hs:1:12: error:
     • Overloaded signature conflicts with monomorphism restriction
         n :: forall a. Num a => a
     • In the expression:
         let
           n :: Num a => a
           x at n = undefined
         in x `seq` return ()
       In an equation for ‘main’:
           main
             = let
                 n :: Num a => a
                 x at n = undefined
               in x `seq` return ()
   |
 1 | main = let n :: Num a => a; x at n = undefined in x `seq` return ()
   |            ^^^^^^^^^^^^^^^
 }}}

 Even giving `x` its own explicitly polymorphic type signature -- what I
 thought was the standard way to prevent the monomorphism restriction from
 being a problem -- doesn't help. I'm uncertain what to make of that, but
 it certainly seems related: earlier versions of the compiler both do not
 give that error and do produce a crashing program, as I expected.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14782>
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