#10387: toRational should error out on NaN and Infinity values for Double/Floats -------------------------------------+------------------------------------- Reporter: lerkok | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- The current implementation of `toRational` for `Float`/`Double` types accept `NaN` and `Infinity` values and produce results. For `Infinity` the result nicely maps back to `Infinity`: {{{#!hs Prelude> fromRational (toRational (1/0)) :: Double Infinity Prelude> fromRational (toRational (-1/0)) :: Double -Infinity Prelude> }}} But the same isn't true for NaNs: {{{#!hs Prelude> fromRational (toRational (0/0)) :: Double -Infinity }}} Also, these results are probably architecture specific. In any case, I think `toRational` should simply error out if it receives `NaN` or `Infinity`, as there are really no legitimate values we can map them to, even though we might be able to complete the round-trip correctly in case if infinities. The following just doesn't make sense: {{{#!hs Prelude> toRational (1/0) 179769313486231590772930519078902473361797697894230657273430081157732675805500963132708477322407536021120113879871393357658789768814416622492847430639474124377767893424865485276302219601246094119453082952085005768838150682342462881473913110540827237163350510684586298239947245938479716304835356329624224137216 % 1 }}} So, I propose the `toRational` instance for Float/Double's should simply recognize `NaN` and `Infinite` values and throw an error. [Note that this is also how the HW implementations for the corresponding conversions work, raising a floating-point-exception (in the unmasked case). See `CVTPS2PI`/`CVTPS2DQ` etc., for instance, in x86.] -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10387 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler