Moritz:

Looks like you're raising the same concern as in here: https://ghc.haskell.org/trac/ghc/ticket/13124

Long story short: You cannot encode -0.0 directly. You have to represent it as an expression, multiplying `-1` with `fromRational 0`.

Similar concerns apply to NaN and Infinity as well, though those cannot be written as literals to start with, so it isn't a big issue there.

-Levent.

On Sun, Sep 17, 2017 at 7:02 AM, Moritz Angermann <moritz.angermann@gmail.com> wrote:

Hi *,

I just ran into the following case:

> fromRat (-0 :: Rational) :: Double
0.0

> -0 :: Rational
0 % 1

How do I encode negative zero as a rational? The question come
up as `CmmFloat` is defined as `CmmFloat Rational Width` in
compiler/cmm/CmmExpr.hs. And my code generator thus turned
(Int, Rational) into a floating point expression, and when trying
to turn it into a `double` value, I ended up getting bad negative
zeros.

This makes me believe I can’t actually encode `-0` as a Rational,
and such `CmmFloat` will never contain `-0`?

Cheers,
Moritz
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