Complete support for GHC 9.0 & 9.2 as well as aeson 2.0 #35
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Arguably, emptyVariant is more like Void than (), given that it is
actually not possible to obtain its value via fromVariant and if it
were, a program would explode due to undefined.
In JSON there is not really a way to have a void value, a JSON value is
always *something*, so it makes no sense to have a Variant '[] parser
that succeeds in any condition. More importantly, the FromJSON instance
for Variant (t ': ts) actually *requires* the instance for Variant '[]
to fail! If parseJSON is called for Variant '[] from that instance, it
means that the value didn't match any of the types the Variant accepts.
This causes the bug that, if parseJSON :: Parser () succeeds for a JSON
value, we can obtain an arbitrary Variant that would fail to evaluate,
as it'd be constructed via extendVariant emptyVariant. For aeson < 1.6
this is a pretty obscure bug, since FromJSON () only accepts the empty
array:
>>> decode "[]" :: Maybe (Variant '[Int])
Just *** Exception: Prelude.undefined
Since aeson 2.0, FromJSON () [accepts] all JSON values, meaning that any
failed parse to Variant results in a program crash!
Tagged Variants also exhibit this problem, of course. The bug is fixed
by making the FromJSON (Variant '[]) instance fail always via parseFail.
[accepts]: haskell/aeson@677daf0
This adds a bit more ifdef zoo to be compatible with the breaking change in aeson. Tested manually against aeson 2.0.3.0 and 1.5.6.0. Might want to update the CI as well, but I don’t have the time to touch CircleCI yaml. Co-authored-by: Lukas Epple <lukas.epple@possehl-analytics.com>
Support for * is disabled in GHC 9.2 by default and causes a lot of annoying warnings in 9.0. Tested against 8.10.7, 9.0.2 and 9.2.4. The base constraint seems to be correct, as Data.Kind was added in base 4.9. Co-authored-by: Lukas Epple <lukas.epple@possehl-analytics.com>
This seems to be required now to compile the Variant code.
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This PR builds on #32, adding support for aeson 2.0 as well as GHC >= 9.0, but fixes some mistakes, gets the
Variantcode to work again and adds support for GHC 9.2.Typeover*, also gets rid of the warnings for 9.0FlexibleContextswhere necessaryFromJSONfor (tagged)Variants. This bug hinges onparseJSON x :: Parser ()succeeding which is always the case foraeson >= 2.0, making this bug quite prevalent. See the commit message for further details.aeson == 1.5.*aeson == 2.0.*aeson-1.5.6.0❔