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    <p>This seems to be a known issue with Generics-based parsing. <a href="http://hackage.haskell.org/package/aeson" rel="noreferrer">Aeson</a> has had the same issue and the maintainers have decided to deprecate that functionality in favor of a Template Haskell-based strategy: <a href="https://github.com/bos/aeson/issues/118" rel="noreferrer">https://github.com/bos/aeson/issues/118</a></p> <p>With Aeson, your code would look very similar:</p> <pre><code>import Data.Aeson import Data.Aeson.TH data Record = Record { myMandatoryField :: Integer, myOptionalField :: Maybe Integer } deriving (Eq, Show) $(deriveJSON id ''Record) </code></pre> <p>This way, the <code>Maybe</code> field encodes and decodes as expected:</p> <pre><code>$ ghci λ :l Main.hs Ok, modules loaded: Main. λ encode $ Record 5 Nothing "{\"myOptionalField\":null,\"myMandatoryField\":5}" λ decode it :: Maybe Record Just (Record {myMandatoryField = 5, myOptionalField = Nothing}) </code></pre> <p><strong>UPDATE</strong>: As mentioned in the comments, null-field omission is available with Template Haskell in Aeson HEAD, but that's not yet on Hackage. You can get that behavior today with hand-written <code>FromJSON</code>/<code>ToJSON</code> instances:</p> <pre><code>instance FromJSON Record where parseJSON = withObject "record" $ \o -&gt; Record &lt;$&gt; o .: "myMandatoryField" &lt;*&gt; o .:? "myOptionalField" instance ToJSON Record where toJSON (Record manf optf) = object $ catMaybes [ ("myMandatoryField" .=) &lt;$&gt; pure manf , ("myOptionalField" .=) &lt;$&gt; optf ] </code></pre>
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    1. POHow do I use Text.JSON.Generic with optional JSON fields?
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    1. USMike Craig
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    1. COThank you for this answer. However, it still doesn't give exactly the behavior I'd like, which is for the field not to appear at all (as opposed to being generated with a value of `null`).
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    2. COThis feature, I believe, is in Aeson HEAD but hasn't been released to Hackage yet.
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