#4335: fromRational broken for Ratio a
----------------------------------+-----------------------------------------
Reporter: daniel.is.fischer | Owner:
Type: bug | Status: new
Priority: normal | Component: libraries/base
Version: 6.12.3 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Incorrect result at runtime
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The `Fractional` instance for `Ratio a` in GHC.Real defines
{{{
fromRational (x:%y) = fromInteger x :% fromInteger y
}}}
For fixed-width Integral types, that produces invalid results:
{{{
Prelude Data.Ratio> fromRational (1 % 2^32) :: Ratio Int
1 % 0
Prelude Data.Ratio> fromRational (3 % (2^32+9)) :: Ratio Int
3 % 9
}}}
Via `toRational`, these can be ported back to `Rational`.
`Ratio a` is generally broken for fixed-width types:
{{{
Prelude Data.Ratio> 1 % (minBound :: Int)
(-1) % (-2147483648)
}}}
but the particular brokenness of `fromRational` can be alleviated by
reducing:
{{{
fromRational (x:%y) = fromInteger x % fromInteger y
{-# RULES
"fromRational/id" fromRational = id :: Rational -> Rational
#-}
}}}
I think it's better to throw a divide by zero error than to produce
invalid results here.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4335>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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