[GHC] #14302: ghc panic on simple program

[GHC]

#14302: ghc panic on simple program
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        Reporter:  aberent           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by aberent):

 This appears to be an error in the error handling. Once I fix the
 compilation errors reported by ghci, ghc is happy to compile it.

 FYI my fixed code is:

 {{{
 module Tiltmaze (solve) where

 import Data.Sequence as Sequence
 -- A maze is a grid of empty cells and walls
 -- true means wall
 type Maze = Seq (Seq Bool)

 data Direction = North | South | East | West deriving (Eq, Show)
 directions = [North, South, East, West]

 step :: Direction -> Int -> Int -> (Int, Int)
 step d x y
   | d == North = (x, y-1)
   | d == South = (x, y+1)
   | d == East = (x+1, y)
   | d == West = (x-1, y)

 solve :: Maze -> Int -> Int -> Int -> Int -> Bool
 solve m sx sy tx ty
   | sx == tx && sy == ty = True
   | otherwise =
     let solved m x y d
           | x' < 0 = False
           | y' < 0 = False
           | x' >= Sequence.length m  = False
           | y' >= Sequence.length (m `index` x') = False
           | not ((m `index` x') `index` y') = False
           | otherwise = solve m' x' y' tx ty
               where (x', y') = step d x y
                     m' = update x (update y False (m `index` x)) m
       in any (solved m sx sy) directions
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14302#comment:1>
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