[GHC] #16184: GHC said to report a bug

15 Jan 2019

      #16184: GHC said to report a bug
-------------------------------------+-------------------------------------
           Reporter:  TheKing01      |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
  (Type checker)                     |
           Keywords:  panic, skolem  |  Operating System:  Linux
       Architecture:  x86_64         |   Type of failure:  Compile-time
  (amd64)                            |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 I tried to run
 {{{#!hs
 {-# LANGUAGE Rank2Types, PartialTypeSignatures #-}
 import Control.Monad
 newtype Search b = Search {optimum :: forall a. Ord a => (b -> a) -> b}

 instance Monad Search where
   return a = Search $ const a
   Search ma >>= f = Search $ \p ->
     optimum (f (ma (\a -> p (optimum (f a) p)))) p

 instance Functor Search where
   fmap = liftM

 instance Applicative Search where
   pure = return
   (<*>) = ap

 pair a b = Search $ \p -> if p a >= p b then a else b
 toList s = igo [] where
     igo ls = let x = optimum s (\x' -> x' `notElem` ls) in if x `elem` ls
 then ls else igo (x:ls)

 cantor :: Search (Integer -> Bool)
 cantor = let
     igo :: _
     igo p n = q n (optimum cantor $ q n) where
         q n a = p undefined
     in Search igo

 main = return ()
 }}}

 When run, it said

 {{{
 source_file.hs:24:5:
     No instance for (Ord a)
     When checking that ‘igo’ has the specified type
       igo :: forall t a t1. (t -> a) -> t1 -> a
     Probable cause: the inferred type is ambiguous
     In the expression:
       let
         igo :: _
         igo p n
           = q n (optimum cantor $ q n)
           where
               q n a = ...
       in Search igo
     In an equation for ‘cantor’:
         cantor
           = let
               igo :: _
               igo p n
                 = q n (optimum cantor $ q n)
                 where
                     ...
             in Search igo

 source_file.hs:26:15:
     Couldn't match type ‘t’ with ‘Integer -> Bool’
       ‘t’ is untouchable
         inside the constraints (Ord a1)
         bound by a type expected by the context:
                    Ord a1 => ((Integer -> Bool) -> a1) -> Integer -> Bool
         at source_file.hs:26:8-17ghc: panic! (the 'impossible' happened)
   (GHC version 7.10.3 for x86_64-unknown-linux):
         No skolem info: t_aU3[sk]

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 Since it said to report a bug, I am.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16184
GHC http://www.haskell.org/ghc/
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