evaluation fault in ghci.

[Malcolm Wallace]

The interactive loop of ghci displays an interesting evaluation fault
to do with derived equality.  In the attached source file, there is
a simple guard which tests some equalities, and basically the same
value is given on the left and right of the (==).  Yet, it evaluates
to False in interactive mode, whereas it (correctly) evaluates to True
when compiled.


$ ghci
   ___         ___ _
  / _ \ /\  /\/ __(_)
 / /_\// /_/ / /  | |      GHC Interactive, version 5.04.2, for Haskell 98.
/ /_\\/ __  / /___| |      http://www.haskell.org/ghc/
\____/\/ /_/\____/|_|      Type :? for help.

Loading package base ... linking ... done.
Loading package haskell98 ... linking ... done.
Prelude> :l Small
Compiling Main             ( Small.hs, interpreted )
Ok, modules loaded: Main.
*Main> main
"*** Exception: Four.bceFour: precondition fails:arrows not parallel
f = E
g = E


$ ghc -o Small Small.hs
$ ./Small
"it works"


I have verified that the fault exists at least as far back as 5.02.3.

Regards,
    Malcolm
-------------- next part --------------
module Main where

data FourArrow = A | B | C | D | E | ABE | AC | BD | CDE
		 deriving (Eq, Show)

data Category a
    = Category
	{ src     :: a -> a
        , tgt     :: a -> a
        , equal   :: a -> a -> Bool
        }

categoryFour :: Category FourArrow
categoryFour = Category
	       {src = dom,
		tgt = cod,
		equal = (==)
	       }
    where
    dom A   = ABE
    dom B   = ABE
    dom C   = AC
    dom D   = BD
    dom E   = ABE
    dom ABE = ABE
    dom AC  = AC
    dom BD  = BD
    dom CDE = CDE
    cod A   = AC
    cod B   = BD
    cod C   = CDE
    cod D   = CDE
    cod E   = CDE
    cod ABE = ABE
    cod AC  = AC
    cod BD  = BD
    cod CDE = CDE

----
bceFour :: FourArrow -> FourArrow -> String
bceFour f g
    | srcF == srcFour g && tgtF == tgtFour g
    = "it works"
    | otherwise = error ("Four.bceFour: precondition fails:"
			  ++ "arrows not parallel"
			  ++ "\nf = " ++ (show f)
			  ++ "\ng = " ++ (show g)
			  ++ "\n")
    where
        srcFour = src categoryFour
        tgtFour = tgt categoryFour
        srcF = srcFour f
        tgtF = tgtFour f

main = print (bceFour E E)