haskell aeson

JSON strings can trivially be decoded to Haskell values, provided that the types Now that we know what our target values are, we need a Haskell representation for them. See what's changed in recent (and upcoming) releases: (You can create and contribute changes using either git or Mercurial.). them? tree (AST), using, When decoding a value, the process is reversed: the bytes are So, this example could be rewritten like this: Working with Value is easy as well, just keep in mind that. Tuples of various lengths are arrays. How could you do it? The ways to work with aeson ranges from simple (but inflexible) to complex (but flexible). That’s all. drop 1” means that the decoder would, -- *expect* fields to correspond to their lowercase counterparts – e.g. If nothing happens, download the GitHub extension for Visual Studio and try again. above operates in an Applicative Parser context. A common situation is that some field can be optional, and if it’s not present you want to provide a default value. takes a JSON value (of the aeson type Value) and returns a Haskell value of Parsing an array, in comparison, is much less messy: To understand what’s going on, look at the type of mapM: (In GHC 7.10 and newer it’s more general, but nothing changes if you consider this less general version.). compiler generate the implementation for you. “expected an array of tuples” instead of “expected an array”). decode and encode ), (And there’s also a type called Result that is for results of parsers as well but doesn’t use CPS. Its main purpose is simple injection of fields into JSONs produced by aesonlibrary. The result is Nothing if the key is not present, or empty if It’s supposed to be the most comprehensive Aeson tutorial on the web; if there’s anything missing from it, send me an email! (This trick depends on GHC knowing the internals of your data types and willing to share them with Aeson.). The Pair type is simply defined In order to use the encode and decode function from the Data.Aeson package we need to make Person an instance of ToJSON and FromJSON. aeson comes (If you need to convert JSON to/from Text, there’s a possibility you’re doing something wrong.) Well, we can just use fail to signal an error (similarly to how we’d use Left to signal an error if Parser actually was Either String a): Parser, just like Either, is a monad (and also Applicative and Alternative), so we can use all the usual things on it; if you ever used Parsec, this shouldn’t be too unfamiliar for you. For convenience, the encode and also need to launch GHCi with ghci -XOverloadedStrings. simply returns Nothing if the field wasn’t found or was a null. the Generic module, and you can do the following: Be aware that the encoding may not always be what you'd naively Table of contents Basic operations The reason I wrote this version of the parser first is that otherwise you wouldn’t understand how Aeson works, and that would’ve bitten you later. A JSON value represented as a Haskell value. Consider the following common data type in web service developing: use '(. Haskell: Reusing FromJSON instances with lenses, lens-aeson, and nested JSON. Since this library follows the standard, calling they're used to log you in. There's plenty Sometimes you want to work with JSON data directly, without first For more information, see our Privacy Statement. as (Text, Value), and .= is a helper to encode the right-hand side to its Since we derive Generic for Person, we can create empty instances for these classes. An example: Another interesting case is extended records – that is, when one datatype is a subset of another: So, RussianName is just a Name with patronymic added to it. to complex (but flexible). decode' :: FromJSON a => ByteString -> Maybe aSource. So how do you define FromJSON and ToJSON instances for types that don't have Documentation in Data.Aeson.TH is pretty good, so I’ll just give an example: You can also generate instances for several types at once: (This was already kinda discussed in one of the past sections, but I want to provide the whole solution so that I can link people to it.). Data.Map, Data.Text and Data.Time.Clock in order to try out the examples above. github issue tracker. from an object without affecting its validity and we know a default It has got 6 constructors: So, if you want to construct JSON directly, you can do it by constructing a Value and then converting it to JSON with encode: We had to use fromList (which has to be imported from GHC.Exts) 2 times – once to convert a list of pairs to Object, another time to convert a list to Array. -- Do all the same for "b" (in a slightly terser way, to save space): -- Vector is the type Aeson uses to represent JSON arrays. withBool :: String -> (Bool -> Parser a) -> Value -> Parser aSource. For example the previous example can be simplified to just: fromJSON :: FromJSON a => Value -> Result aSource. For compatibility reasons, the default DeriveGeneric ToJSON implementation This is what parseJSON This function parses immediately, but defers conversion. some specific type first. NominalDiffTime is represented as the number of seconds. Nowadays I'm running a functional programming consultancy at to do it automatically: To use this, simply add a deriving Generic clause to your datatype and So beware that this We can use HM.toList to turn it. aeson-0.6.1.0: Fast JSON parsing and encoding. This library was originally written by Bryan O'Sullivan. There's plenty The result is empty if the key is not present or the value cannot Please report bugs via the For compatibility reasons, the default DeriveGeneric ToJSON implementation (Moreover, when you parse JSON, you won’t be able to parse e.g. Note the use of the OverloadedStrings language extension which enables

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