[commit: base] master: Add RatioZeroDenominator to ArithException, and use it instead of error (3a3ab80)

[Simon Marlow]

Repository : ssh://darcs.haskell.org//srv/darcs/packages/base

On branch  : master

http://hackage.haskell.org/trac/ghc/changeset/3a3ab8080104b6f6dacf03e4413c592532c9f22e

>---------------------------------------------------------------

commit 3a3ab8080104b6f6dacf03e4413c592532c9f22e
Author: Simon Marlow <marlowsd at gmail.com>
Date:   Tue Jul 3 11:16:40 2012 +0100

    Add RatioZeroDenominator to ArithException, and use it instead of error
    
    As per discussion on the libraries list.

>---------------------------------------------------------------

 GHC/Err.lhs       |    5 +++++
 GHC/Exception.lhs |    2 ++
 GHC/Real.lhs      |    6 +++---
 3 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/GHC/Err.lhs b/GHC/Err.lhs
index 5352a06..ed1dcb9 100644
--- a/GHC/Err.lhs
+++ b/GHC/Err.lhs
@@ -27,6 +27,7 @@ module GHC.Err
        (
          absentErr                 -- :: a
        , divZeroError              -- :: a
+       , ratioZeroDenominatorError -- :: a
        , overflowError             -- :: a
 
        , error                     -- :: String -> a
@@ -82,6 +83,10 @@ in the libraries before the Exception type has been defined yet.
 divZeroError :: a
 divZeroError = throw DivideByZero
 
+{-# NOINLINE ratioZeroDenominatorError #-}
+ratioZeroDenominatorError :: a
+ratioZeroDenominatorError = throw RatioZeroDenominator
+
 {-# NOINLINE overflowError #-}
 overflowError :: a
 overflowError = throw Overflow
diff --git a/GHC/Exception.lhs b/GHC/Exception.lhs
index 74f8fea..76a9eb4 100644
--- a/GHC/Exception.lhs
+++ b/GHC/Exception.lhs
@@ -182,6 +182,7 @@ data ArithException
   | LossOfPrecision
   | DivideByZero
   | Denormal
+  | RatioZeroDenominator
   deriving (Eq, Ord, Typeable)
 
 instance Exception ArithException
@@ -192,5 +193,6 @@ instance Show ArithException where
   showsPrec _ LossOfPrecision = showString "loss of precision"
   showsPrec _ DivideByZero    = showString "divide by zero"
   showsPrec _ Denormal        = showString "denormal"
+  showsPrec _ RatioZeroDenominator = showString "Ratio has zero denominator"
 
 \end{code}
diff --git a/GHC/Real.lhs b/GHC/Real.lhs
index d4e407e..a011989 100644
--- a/GHC/Real.lhs
+++ b/GHC/Real.lhs
@@ -92,7 +92,7 @@ their greatest common divisor.
 \begin{code}
 reduce ::  (Integral a) => a -> a -> Ratio a
 {-# SPECIALISE reduce :: Integer -> Integer -> Rational #-}
-reduce _ 0              =  error "Ratio.%: zero denominator"
+reduce _ 0              =  ratioZeroDenominatorError
 reduce x y              =  (x `quot` d) :% (y `quot` d)
                            where d = gcd x y
 \end{code}
@@ -412,7 +412,7 @@ instance  (Integral a)  => Num (Ratio a)  where
 instance  (Integral a)  => Fractional (Ratio a)  where
     {-# SPECIALIZE instance Fractional Rational #-}
     (x:%y) / (x':%y')   =  (x*y') % (y*x')
-    recip (0:%_)        = error "Ratio.%: zero denominator"
+    recip (0:%_)        = ratioZeroDenominatorError
     recip (x:%y)
         | x < 0         = negate y :% negate x
         | otherwise     = y :% x
@@ -628,7 +628,7 @@ x ^^ n          =  if n >= 0 then x^n else recip (x^(negate n))
     | e > 0     = (n ^ e) :% (d ^ e)
     | e == 0    = 1 :% 1
     | n > 0     = (d ^ (negate e)) :% (n ^ (negate e))
-    | n == 0    = error "Ratio.%: zero denominator"
+    | n == 0    = ratioZeroDenominatorError
     | otherwise = let nn = d ^ (negate e)
                       dd = (negate n) ^ (negate e)
                   in if even e then (nn :% dd) else (negate nn :% dd)