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Use scientific to fix floating point rounding problems #58

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@mightybyte
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Here's a proof of concept. Do you have a preference about how to resolve the LANGUAGE Safe bit? I was unsure about that.

@andreasabel
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Testsuite fails, missing instances From/ToYaml Double:

[1 of 1] Compiling Main             ( tests/Tests.hs, /__w/HsYAML/HsYAML/dist-newstyle/build/x86_64-linux/ghc-9.2.1/HsYAML-0.2.1.0/t/tests/build/tests/tests-tmp/Main.o )

tests/Tests.hs:26:[32](https://github.com/haskell-hvr/HsYAML/pull/58/checks#step:19:32): error:
    • No instance for (ToYAML Double) arising from a use of ‘outputStr’
    • In the second argument of ‘(==)’, namely ‘outputStr d’
      In the expression: ".nan" == outputStr d

tests/Tests.hs:60:32: error:
    • No instance for (FromYAML Double)
        arising from a use of ‘roundTrip’
    • In the second argument of ‘($)’, namely
        ‘roundTrip approxEq (1 :: Double)’

@sjakobi sjakobi linked an issue May 11, 2022 that may be closed by this pull request
Decimal numbers have rounding error #57
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@mightybyte mightybyte marked this pull request as ready for review October 28, 2022 06:32
sjakobi
sjakobi reviewed Oct 28, 2022
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src/Data/YAML.hs
Comment on lines -476 to 477

instance FromYAML Double where
instance FromYAML Scientific where
parseYAML = withFloat "!!float" pure
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What's the rationale for removing FromYAML and ToYAML Double?

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I guess there's an argument for going either way. My thought was that it's mainly there for the vales actually supported as scalar primitives, and since this PR changes the scalar numeric primitive from Double to Scientific, that seemed to be the obvious route to go.

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Well, we also provide instances for types like Int8 and tuples, so I think we ought to keep the Double instances.

sjakobi
sjakobi reviewed Nov 2, 2022
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cabal.project

package HsYAML
flags: +exe
flags: +exe
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Could you undo this change?

src/Data/YAML.hs
Comment on lines -476 to 477

instance FromYAML Double where
instance FromYAML Scientific where
parseYAML = withFloat "!!float" pure
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Well, we also provide instances for types like Int8 and tuples, so I think we ought to keep the Double instances.

src/Data/YAML/Schema/Internal.hs
Comment on lines +455 to +456
encodeDouble :: Scientific -> T.Text
encodeDouble d = T.pack . show $ d
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This function should be named encodeFloat for consistency with encodeInt etc.

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Also, is there a more efficient way to perform this encoding?

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sjakobi commented Nov 2, 2022

Do you have a preference about how to resolve the LANGUAGE Safe bit?

I don't really have any experience with -XSafe. I'm fine with removing it for now and waiting until someone comes along who cares. :)

@andreasabel
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What is the error if LANGUAGE Safe is left there?

@ulysses4ever ulysses4ever mentioned this pull request May 13, 2023
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Decimal numbers have rounding error

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@mightybyte @andreasabel @sjakobi