[GHC] #8659: GHCi told me to tell you that it crashed

10 Jan 2014

      #8659: GHCi told me to tell you that it crashed
------------------------------+-------------------------------------
       Reporter:  ishkabible  |             Owner:
           Type:  bug         |            Status:  new
       Priority:  normal      |         Milestone:
      Component:  GHCi        |           Version:  7.4.2
       Keywords:              |  Operating System:  Unknown/Multiple
   Architecture:  x86         |   Type of failure:  None/Unknown
     Difficulty:  Unknown     |         Test Case:
     Blocked By:              |          Blocking:
Related Tickets:              |
------------------------------+-------------------------------------
 I'm using an x86 build of GHCi version 7.4.2 on Windows 7 professional.

 I attempted to interpret the following code and GHC crashed and told me to
 report the issue.

 {{{
 module ConstructorTypes where

 import Data.List (intercalate)
 import VarGenMonad

 data ConstructorSpec = Spec {
     parrentType :: ConstructorSpec,
     specName :: String,
     specArgs :: [Type]
 } deriving(Eq, Ord)

 data ConstructorTypeSpec = ConTypeSpec { conTypeName :: String, conSpecs
 :: [ConstructorSpec], numParams :: Int }

 data Pattern = Pat ConstructorSpec [Pattern]
              | Id String
     deriving(Eq, Ord)

 --this holds
 data Type = Poly Type
           | DeBruijnTypeVar Int
           | ConType ConstructorTypeSpec
           | Type :-> Type
     deriving(Eq, Ord, show)

 --define Eq and Ord to break infinte chain of type/spec/type/spec/...
 instance Eq ConstructorTypeSpec where
     t1 == t2 = conTypeName t1 == conTypeName t2

 instance Ord ConstructorTypeSpec where
     t1 <= t2 = conTypeName t1 <= conTypeName t2

 --create own show types to give a nice syntax and avoid infinite loops
 instance Show ConstructorSpec where
     show (Spec ty name args) = name ++ (concatMap ((' ':) . show) args) ++
 " :: " ++ conTypeName ty

 instance Show ConstructorTypeSpec where
     show (ConTypeSpec name specs vars) = name ++ "=" ++ (concatMap
 (("\n\t" ++) . show) specs)

 genUniqueVars lst n = take n $ runVarGen lst gen
     where gen = do
               x <- drawVar
               return $ x : gen

 {--
 toString env (Poly t) = do
     a <- drawVar            --should advance to next varible
     b <- toString (a:env) t --any calls to drawVar in toString should
 advance it as well
     return $ "(forall " ++ a ++ ")" ++ b
 toString env (DeBruijn x) = do
     return $ env !! x       --no changes are made here however
 toString env (ConType spec) = do
     names <- mapM_ (toString env) (typeVars spec) --each call to drawVar
 in each call toString should advance the internal state
     return $ conTypeName spec ++ intercalate " " names
 toString env (x :-> y) = do
     t1 <- toString env x
     t2 <- toString env y
     return $ "(" ++ t1 ++ ") -> " ++ t2
 --}
 }}}

 "VarGenMonad" is the following code

 {{{
 module VarGenMonad (VarGen, runVarGen, runVarGenWith, drawVar) where

 import Control.Monad

 --effectivlly a state monad of type State a [String]
 newtype VarGen a = VarGen { getFunc :: [String] -> (a, [String]) }

 --just gets the resulting value (this is what users will interface to)
 runVarGen vl vg = runVarGenWith showInt
     where showInt 0 = ""
           showInt x = show x

 --another function the user can interacte with
 runVarGenWith f vl vg = fst $ runState vg (makeVarListWith f vl)

 --gets the result from a var gen monad after telling it what varibles to
 use
 runState :: VarGen a -> [String] -> (a, [String])
 runState vg vl = (getFunc vg) vl

 --creates an infinte list of unique varibles from a finite one
 makeVarListWith :: Integral a => (a -> String) -> [String] -> [String]
 makeVarListWith f lst = gen lst lst 0
     where gen t []     c = gen t t (c + 1)
           gen t (x:xs) c = (x ++ f c) : gen t xs c

 --implements the monad operations in same fashion as a state monad
 instance Monad VarGen where
     return x = VarGen $ \s -> (x, s)
     cur >>= transform = VarGen func
         where func s = runState (transform v) nextList
                   where (v, nextList) = runState cur s

 drawVar :: VarGen String
 drawVar = VarGen $ \(x:xs) -> (x, xs)
 }}}

 this worked before I added "show" (note that it is lowercase cuz I messed
 up) to the deriving cluase of my 'Type' type. After fixing it to make it
 an upper case S GHC promptly told me what else I had messed up in my code
 (that is, it worked correctly).

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8659
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