aeson-picker

Tiny library to get fields from JSON format

Common use is the following:

JSON |-- FIELDS :: EXPECTED TYPE
Text |-- [Text] :: a

So operator (|--) gets JSON (represented as Text), “route” to field inside JSON (represented as [Text]), and tries to parse field from JSON with described route to expected type. If expected type can be deduced with type checker then it can be dropped down.

A little bit safer operator is (|-?). It returns not a but Maybe a.

Examples

First, add extension (to not pack String to Text every time):

ghci>:set -XOverloadedStrings

Then you can try something simple (empty list means that you try to parse Value from the top JSON-level):

ghci>"5" |-- [] :: Int
5
ghci>"5" |-- [] :: Float
5.0
ghci>"5" |-- [] :: String
"*** Exception: Data.Aeson.Picker: could not pick field with path: []

But what if field you are looking for somewhere inside JSON? That’s why are you here.

Let’s try to get something from inside JSON:

ghci>"{\"a\": 5}" |-- ["a"] :: Int
5

But be sure that the field is presented inside JSON:

ghci>"{\"a\": 5}" |-- ["b"] :: Int
*** Exception: Data.Aeson.Picker: could not pick field with path: ["b"]

We can go deeper (as deep as you want):

ghci>"{\"outer\": {\"inner\": [1,2,3]}}" |-- ["outer", "inner"] :: [Int]
[1,2,3]

But be sure that you JSON is really valid (by specification key in JSON should be String):

ghci>"{a: 5}" |-- ["a"] :: Int
*** Exception: Data.Aeson.Picker: input json is not valid

If you want more “safe” picker, you can use another operator:

ghci>"5" |-? [] :: Maybe Int
Just 5
ghci>"{\"a\": 5}" |-? ["a"] :: Maybe Int
Just 5
ghci>"{\"a\": 5}" |-? ["b"] :: Maybe Int
Nothing

In current logic even operator (|-?) will throw error if JSON is not valid:

ghci>"{a: 5}" |-? ["a"] :: Maybe Int
*** Exception: Data.Aeson.Picker: input json is not valid

You can open issue if you do not think that it is right logic.