Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC

Commits:

• f8d9d016
  by Andrew Lelechenko at 2025-07-22T21:13:28-04:00

  Fix issues with toRational for types capable to represent infinite and not-a-number values
  
  This commit fixes all of the following pitfalls:
  
  > toRational (read "Infinity" :: Double)
  179769313486231590772930519078902473361797697894230657273430081157732675805500963132708477322407536021120113879871393357658789768814416622492847430639474124377767893424865485276302219601246094119453082952085005768838150682342462881473913110540827237163350510684586298239947245938479716304835356329624224137216 % 1
  > toRational (read "NaN" :: Double)
  269653970229347386159395778618353710042696546841345985910145121736599013708251444699062715983611304031680170819807090036488184653221624933739271145959211186566651840137298227914453329401869141179179624428127508653257226023513694322210869665811240855745025766026879447359920868907719574457253034494436336205824 % 1
  
  > realToFrac (read "NaN" :: Double) -- With -O0
  Infinity
  > realToFrac (read "NaN" :: Double) -- With -O1
  NaN
  
  > realToFrac (read "NaN" :: Double) :: CDouble
  Infinity
  > realToFrac (read "NaN" :: CDouble) :: Double
  Infinity
  
  Implements https://github.com/haskell/core-libraries-committee/issues/338
  

4 changed files:

• libraries/base/changelog.md
• libraries/ghc-internal/src/GHC/Internal/Float.hs
• libraries/ghc-internal/src/GHC/Internal/Real.hs
• testsuite/tests/numeric/should_run/T9810.stdout

Changes:

 ## 4.23.0.0 *TBA*

   * Add `Data.List.NonEmpty.mapMaybe`. ([CLC proposal #337](https://github.com/haskell/core-libraries-committee/issues/337))

+  * Fix issues with toRational for types capable to represent infinite and not-a-number values ([CLC proposal #338](https://github.com/haskell/core-libraries-committee/issues/338))

 ## 4.22.0.0 *TBA*

   * Shipped with GHC 9.14.1

 -- | @since base-2.01

 --

--- Beware that 'toRational' generates garbage for non-finite arguments:

---

--- >>> toRational (1/0 :: Float)

--- 340282366920938463463374607431768211456 % 1

--- >>> toRational (0/0 :: Float)

--- 510423550381407695195061911147652317184 % 1

---

 instance  Real Float  where

+    toRational x

+      | isInfinite x = if x > 0 then infinity else -infinity

+      | isNaN x = notANumber

     toRational (F# x#)  =

         case decodeFloat_Int# x# of

           (# m#, e# #)

 -- | @since base-2.01

 --

--- Beware that 'toRational' generates garbage for non-finite arguments:

---

--- >>> toRational (1/0)

--- 179769313 (and 300 more digits...) % 1

--- >>> toRational (0/0)

--- 269653970 (and 300 more digits...) % 1

---

 instance  Real Double  where

+    toRational x

+      | isInfinite x = if x > 0 then infinity else -infinity

+      | isNaN x = notANumber

     toRational (D# x#)  =

         case integerDecodeDouble# x# of

           (# m, e# #)

 -- | General coercion to 'Fractional' types.

 --

--- WARNING: This function goes through the 'Rational' type, which does not have values for 'NaN' for example.

--- This means it does not round-trip.

---

--- For 'Double' it also behaves differently with or without -O0:

---

--- > Prelude> realToFrac nan -- With -O0

--- > -Infinity

--- > Prelude> realToFrac nan

--- > NaN

 realToFrac :: (Real a, Fractional b) => a -> b

 {-# NOINLINE [1] realToFrac #-}

 -- See Note [Allow time for type-specialisation rules to fire]

 ## Double ##

 Infinity

 -Infinity

-Infinity

+NaN

 Infinity

 -Infinity

 Infinity

 ## Float ##

 Infinity

 -Infinity

-Infinity

+NaN

 Infinity

 -Infinity

 Infinity