[GHC] #11822: Pattern match checker exceeded (2000000) iterations

[GHC]

#11822: Pattern match checker exceeded (2000000) iterations
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        Reporter:  j.waldmann        |                Owner:  gkaracha
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1-rc3
      Resolution:                    |             Keywords:
                                     |  PatternMatchWarnings
Operating System:  Unknown/Multiple  |         Architecture:  x86_64
 Type of failure:  Compile-time      |  (amd64)
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dbaynard):

 I've just encountered this (on 8.6.3) while making some changes to stack.
 I'll try an option which doesn't need to do so much checking. But this
 appears much smaller than the previous examples here.

 {{{#!haskell
 {#- LANGUAGE OverloadedLists -#}
 {#- LANGUAGE PatternSynonyms -#}
 {#- LANGUAGE GeneralisedNewtypeDeriving -#}

 import Data.Seq (Seq, pattern (:<|))
 import Data.Set (Set)

 newtype SiblingDependencies = SiblingDependencies Int
   deriving (Eq, Ord, Enum, Integral, Real, Num)

 newtype Depth = Depth Int
   deriving (Eq, Ord, Enum, Integral, Real, Num)

 data TreeNode prefix
   = OnlyChild prefix
   | LeafLast prefix
   | LeafMid prefix
   | NodeLast prefix
   | NodeMid prefix
   | PrefixedLast prefix (Seq SiblingDependencies) (Set PackageName) Depth
   | PrefixedMid prefix (Seq SiblingDependencies) (Set PackageName) Depth

 mkTreeNode
   :: prefix
   -> Seq SiblingDependencies
   -> Set PackageName
   -> Depth
   -> TreeNode prefix
 mkTreeNode t     []      _   _   = OnlyChild t
 mkTreeNode t     [0]    []   _   = LeafLast t
 mkTreeNode t     [_]    []   _   = LeafMid t
 mkTreeNode t     [0]     _   0   = LeafLast t
 mkTreeNode t     [_]     _   0   = LeafMid t
 mkTreeNode t     [0]     _   _   = NodeLast t
 mkTreeNode t     [_]     _   _   = NodeMid t
 mkTreeNode t (0 :<| ns) ds depth = PrefixedLast t ns ds depth
 mkTreeNode t (_ :<| ns) ds depth = PrefixedMid t ns ds depth
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11822#comment:22>
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