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POHow do I get out of a type? JSON Parsing with Data.Aeson
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copied!<p>Haskell beginner here. I want to pass a "type" parameter with JSON, and have it parse into a native Haskell type using read. The following works: </p> <pre><code>data CreatureType = Bot | Player deriving (Generic, Typeable, Show, Read) data Creature = Creature CreatureType deriving (Typeable, Show) instance FromJSON Creature where parseJSON (A.Object v) = Creature <$> read <$> (v .: "type") parseJSON _ = mzero </code></pre> <p>I want to change it to work with <code>readMay</code>, so that it doesn't crash if read fails. I keep getting stuck. parseJSON needs to return <code>mzero</code> if <code>readMay</code> returns <code>Nothing</code>. </p> <p>Main Question: How can I get this function to use <code>readMay</code> to return <code>mzero</code> on a failure with read?</p> <p>Here's what I've tried: </p> <pre><code>instance FromJSON Creature where parseJSON (A.Object v) = do let t = (v .: "type") :: Parser String mt = readMay <$> t :: Parser (Maybe CreatureType) -- ?? -- guard -- I can't use this because it wants a Bool, and I only have Parser Bool -- Creature <$> read <$> mt parseJSON _ = mzero </code></pre> <p><code>mt</code> is <code>Parser (Maybe CreatureType)</code>. How can I change what I return if it is <code>Nothing</code>? I can't use pattern matching, because <code>Data.Aeson</code> doesn't seem to export a value constructor for Parser. I can't seem to find any methods that will give me the value inside a parser, so, is there a generic haskell method that does it? I've looked at <code>Control.Applicative</code> and can't find anything. Here are the docs for <code>Data.Aeson</code>. <a href="http://hackage.haskell.org/packages/archive/aeson/0.5.0.0/doc/html/Data-Aeson.html" rel="nofollow">http://hackage.haskell.org/packages/archive/aeson/0.5.0.0/doc/html/Data-Aeson.html</a></p>

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