Haskell aeson ParseJSON example using (.:)

Question

There is the following snippet of code in the Aeson package usage example:

data Coord = Coord { x :: Double, y :: Double }

instance FromJSON Coord where
  parseJSON (Object v) = Coord    <$>
                         v .: "x" <*>
                         v .: "y"

The type of parseJSON function is parseJSON :: Value -> Parser a. I have the following question about this code: what is the .: function? From the example I might say that its type is Object -> String -> Parser String, however I can't find anything about it on hoogle/hackage. Any help would be appreciated!

The index linked to at the top of documentation pages often helps in such cases. — duplode
– duplode, Commented Aug 1, 2015 at 14:13

duplode · Accepted Answer · 2015-08-01 14:21:08Z

2

It retrieves the value associated with the key. (.:) produces a parse failure (via empty from Alternative) if the key isn't there, so it is suitable for mandatory keys (as opposed to (.:?), which makes sense for optional ones).

answered Aug 1, 2015 at 14:21

duplode

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