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Document exceptions to rules

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Taylor Fausak 2021-05-01 19:26:15 +00:00 committed by GitHub
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src/lib/Witch.hs
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@ -137,16 +137,28 @@ module Witch
-- - Conversions should be lossless. If you have @Cast a b@ then no two @a@
-- values should be converted to the same @b@ value.
--
-- - Some conversions necessarily lose information, like converting from a
-- list into a set.
--
-- - If you have both @Cast a b@ and @Cast b a@, then
-- @cast \@b \@a . cast \@a \@b@ should be the same as 'id'. In other
-- words, @a@ and @b@ are isomorphic.
--
-- - This often true, but not always. For example, converting a list into
-- a set will remove duplicates. And then converting back into a list
-- will put the elements in ascending order.
--
-- - If you have both @Cast a b@ and @Cast b c@, then you could also have
-- @Cast a c@ and it should be the same as @cast \@b \@c . cast \@a \@b@.
-- In other words, @Cast@ is transitive.
--
-- In general if @s@ is a @t@, then you should add a 'Witch.Cast.Cast'
-- instance for it. But if @s@ merely can be a @t@, then you could add a
-- - This is not always true. For example an @Int8@ may be represented as
-- a number in JSON, whereas an @Int64@ might be represented as a
-- string. That means @into \@JSON (into \@Int64 int8)@ would not be the
-- same as @into \@JSON int8@.
--
-- In general if @s@ /is/ a @t@, then you should add a 'Witch.Cast.Cast'
-- instance for it. But if @s@ merely /can be/ a @t@, then you could add a
-- 'Witch.TryCast.TryCast' instance for it. And if it is technically
-- possible to convert from @s@ to @t@ but there are a lot of caveats, you
-- probably should not write any instances at all.