json-decode-pipeline
Build JSON decoders using the pipeline (|>)
operator.
Motivation
It's common to decode into a record that has a type alias. Here's an example
of this from the map3
docs:
type alias Job = { name : String, id : Int, completed : Bool }
point : Decoder Job
point =
map3 Job
(field "name" string)
(field "id" int)
(field "completed" bool)This works because a record type alias can be called as a normal function. In that case it accepts one argument for each field (in whatever order the fields are declared in the type alias) and then returns an appropriate record built with those arguments.
The mapN decoders are straightforward, but require manually changing N
whenever the field count changes. This library provides functions designed to
be used with the |> operator, with the goal of having decoders that are both
easy to read and easy to modify.
Examples
Here is a decoder built with this library.
import Json.Decode as Decode exposing (Decoder, decodeString, float, int, nullable, string)
import Json.Decode.Pipeline exposing (required, optional, hardcoded)
type alias User =
{ id : Int
, email : Maybe String
, name : String
, percentExcited : Float
}
userDecoder : Decoder User
userDecoder =
Decode.succeed User
|> required "id" int
|> required "email" (nullable string) -- `null` decodes to `Nothing`
|> optional "name" string "(fallback if name is `null` or not present)"
|> hardcoded 1.0In this example:
required "id" intis similar to(field "id" int)optionalis likerequired, but if the field is eithernullor not present, decoding does not fail; instead it succeeds with the provided fallback value.hardcodeddoes not look at the provided JSON, and instead always decodes to the same value.
You could use this decoder as follows:
decodeString
userDecoder
"""
{"id": 123, "email": "[email protected]", "name": "Sam Sample"}
"""The result would be:
{ id = 123
, email = Just "[email protected]"
, name = "Sam Sample"
, percentExcited = 1.0
}Alternatively, you could use it like so:
decodeString
userDecoder
"""
{"id": 123, "email": null, "percentExcited": "(hardcoded)"}
"""In this case, the result would be:
{ id = 123
, email = Nothing
, name = "(fallback if name is `null` or not present)"
, percentExcited = 1.0
}
React
Typescript
Django
Laravel
Facebook
Microsoft
Google
Alibaba
D3
Tencent