Common JSON patterns in Haskell, Rust, and TypeScript

A lot of web development is transforming JSON one way or another. In TypeScript/JavaScript, this is straightforward, since JSON is built into the language. But can we also achieve good ergonomics in Haskell and Rust? Dear reader, I am glad you asked! 🙌

The comparisons we will see are not meant to show if one approach is better than another. Instead, it is intended to be a reference to become familiar with common patterns across multiple languages. Throughout this post, we will utilize several tools and libraries.

The core of working with JSON in Haskell and Rust is covered by:

Aeson: a Haskell JSON serialization/deserialization library¹.
Serde: a Rust JSON serialization/deserialization library.

The ergonomics is then improved in Haskell by grabbing one of the following options²:

Lens: a heavy-weight library to transform and work with records (and much more!)³.
Record Dot Syntax: an upcoming language extension in Haskell, which recently got accepted by the GHC steering Committee⁴.

We’ll go through typical use-cases seen in TypeScript/JavaScript codebases, and see how we can achieve the same in Haskell and Rust.

Table of Contents:

Preparation: Setting up our data
Comparison
Changelog

Preparation: Setting up our data

First, we will set up our data structures and a few examples, which we will use throughout this post. Haskell and Rust require a bit more ceremony because we will use packages/crates. For TypeScript we use ts-node to run TypeScript in a REPL.

TypeScript

Let us first set up our reference Object in TypeScript. Save the following in house.ts (or check out typescript-json):

interface Address {
  country: string;
  address: string;
}

interface Person {
  id: number;
  firstname: string;
  lastname: string;
}

interface Household {
  id: number;
  people: Person[];
  address?: Address;
  alternativeAddress?: Address;
  owner: Person;
}

const addr: Address = { country: "Ocean", address: "Under the sea" };
const mom: Person = { id: 1, firstname: "Ariel", lastname: "Swanson" };
const dad: Person = { id: 2, firstname: "Triton", lastname: "Swanson" };
const son: Person = { id: 3, firstname: "Eric", lastname: "Swanson" };
const house: Household = {
  id: 1,
  people: [mom, dad, son],
  address: addr,
  // We omit `alternativeAddress` which is optional.
  owner: mom,
};

Haskell

The included snippet serves to give you an idea of the data structures, types, and names that we will be working with.

You can find the setup for each specific solution in:

haskell-lens: Contains the Lens apporach.
haskell-record-dot: Contains the Record Dot Syntax apporach.

Check out src/House.hs for the data structures, and src/Main.hs for all the examples throughout this post.

data Address = Address
  { country :: String
  , address :: String
  } deriving (Show, Generic)
  deriving (ToJSON, FromJSON) via CustomJSON '[OmitNothingFields] Address

data Person = Person
  { id :: Int
  , firstname :: String
  , lastname :: String
  } deriving (Show, Generic)
  deriving (ToJSON, FromJSON) via CustomJSON '[OmitNothingFields] Person

data Household = Household
  { id :: Int
  , people :: [Person]
  , address :: Maybe Address
  , alternativeAddress :: Maybe Address
  , owner :: Person
  } deriving (Show, Generic)
  deriving (ToJSON, FromJSON) via CustomJSON '[OmitNothingFields] Household

house = Household
  { id = 1
  , people = [mom, dad, son]
  , address = Just addr
  , alternativeAddress = Nothing
  , owner = mom
  }
  where
    addr = Address { country = "Ocean", address = "Under the sea" }
    mom = Person { id = 1, firstname = "Ariel", lastname = "Swanson" }
    dad = Person { id = 2, firstname = "Triton", lastname = "Swanson" }
    son = Person { id = 3, firstname = "Eric", lastname = "Swanson" }

To allow overlapping record fields, we use DuplicateRecordFields along with OverloadedLabels (only in the Lens version), and a bunch of other extensions for deriving things via generics.

We control the details of the JSON serialization / deserialization using the derive-aeson package + the DerivingVia language extension.

Rust

The full setup can be found in rust-serde. Check out src/house.rs for the data structures, and src/main.rs for all the examples throughout this post.

#[derive(Serialize, Deserialize, Debug, Clone)]
pub struct Address {
    pub country: String,
    pub address: String,
}

#[derive(Serialize, Deserialize, Debug, Clone)]
pub struct Person {
    pub id: u32,
    pub firstname: String,
    pub lastname: String,
}

#[derive(Serialize, Deserialize, Debug, Clone)]
#[serde(rename_all = "camelCase")]
pub struct Household {
    pub id: u32,
    pub people: Vec<Person>,
    #[serde(skip_serializing_if = "Option::is_none")]
    pub address: Option<Address>,
    #[serde(skip_serializing_if = "Option::is_none")]
    pub alternative_address: Option<Address>,
    pub owner: Person,
}

pub fn house() -> Household {
    let addr = Address { country: "Ocean".to_string(), address: "Under the sea".to_string() };
    let mom = Person { id: 1, firstname: "Ariel".to_string(), lastname: "Swanson".to_string() };
    let dad = Person { id: 2, firstname: "Triton".to_string(), lastname: "Swanson".to_string() };
    let son = Person { id: 3, firstname: "Eric".to_string(), lastname: "Swanson".to_string() };
    Household {
        id: 1,
        people: vec![mom.clone(), dad, son],
        address: Some(addr),
        alternative_address: None,
        owner: mom
    }
}

Comparison

If you wish to follow along, you can fire up a REPL for each approach. For the TypeScript and Rust versions, where we utilize mutability, we will clone the objects each time, to keep them consistent across examples and in our REPL.

💡

In TypeScript this would more commonly be done using the spread operator, ..., or using something like _.cloneDeep(value).

TypeScript

$ cd typescript-json
$ npm i
$ npm run repl
> import data from './house'
> let newData

Haskell

$ cd haskell-lens
$ stack build
$ stack ghci
*Main Data>

Unfortunately, GHC plugins don’t play nicely with ghci. We will instead build the project to play around with the examples in src/Main.hs.

$ cd haskell-record-dot
$ stack build
$ # Open src/Main.hs in your editor
$ stack run

Rust

Since Rust doesn’t have a REPL, we will instead build the project, so we play around with the examples in src/main.rs.

$ cd rust-serde
$ cargo build
$ # Open src/main.rs in your editor
$ cargo run

Get a field

The first one is simple: we will get a value from our object.

First, our TypeScript version:

> data.house.owner
{ id: 1, firstname: 'Ariel', lastname: 'Swanson' }

Let’s see how we achieve this in Haskell with Lenses:

*Main Data> house ^. #owner
Person {id = 1, firstname = "Ariel", lastname = "Swanson"}

There’s probably already two unfamiliar pieces of syntax here.

The first, ^., comes from Lens and is the view function that we use as an accessor to the object/record. The second, the # prefix of #owner, comes from the OverloadedLabels extension and allows us to have multiple record fields of the same name in scope.

Let’s see how we achieve this in Haskell with Record Dot Syntax:

house.owner
--> Person {id = 1, firstname = "Ariel", lastname = "Swanson"}

Finally, let’s check out Rust:

house.owner
--> Person { id: 1, firstname: "Ariel", lastname: "Swanson" }

Get a nested field

We slowly increase the difficulty by accessing a nested field.

TypeScript:

> data.house.owner.firstname
'Ariel'

Haskell with Lenses:

*Main Data> house ^. #owner . #firstname
"Ariel"

Haskell with Record Dot Syntax:

house.owner.firstname
--> "Ariel"

Rust:

house.owner.firstname
--> "Ariel"

Get an optional field

How do we handle optional fields?

TypeScript:

// A field that exists.
> data.house.address.address
'Under the sea'

// A field that does *NOT* exist (throws an exception.)
> data.house.alternativeAddress.address
TypeError: Cannot read property 'address' of undefined
    at ....

// A field that does *NOT* exist, using optional-chaining.
> data.house.alternativeAddress?.address
undefined

Optional chaining (?) is a significant step toward writing safer and cleaner code in JS/TS.

Haskell with Lenses:

-- Return the value in a Maybe.
*Main Data> house ^. #address
Just (Address {country = "Ocean", address = "Under the sea"})

-- A field on an object that exists.
*Main Data> house ^. #address . #_Just . #address
"Under the sea"

-- A field on an object that does *NOT* exist (falls back to an empty value.)
*Main Data> house ^. #alternativeAddress . #_Just . #address
""

#_Just from Lens gives us convenient access to fields wrapped in Maybes, with a fallback value.

Haskell with Record Dot Syntax:

-- Return the value in a Maybe.
house.address
--> Just (Address {country = "Ocean", address = "Under the sea"})

-- A field on an object that exists.
maybe "" (.address) house.address
--> "Under the sea"

-- A field on an object that does *NOT* exist (falls back to an empty value.)
maybe "" (.address) house.alternativeAddress
--> ""

We end up writing more regular code to dive into the Maybe value by using maybe⁵ to proceed or fallback to a default value.

Rust:

// Return the value in an Option.
house.address
--> Some(Address { country: "Ocean", address: "Under the sea" })

// A field on an object that exists.
house.address.and_then(|a| Some(a.address)).unwrap_or("".to_string())
--> "Under the sea"

// A field on an object that does *NOT* exist (falls back to an empty value.)
house.alternative_address.and_then(|a| Some(a.address)).unwrap_or("".to_string())
--> ""

We utilize and_then a bit like maybe, passing a function to act on our value if it’s Some, and then creating a default case with unwrap_or.

Set a field

We’ll start with updating a non-nested field.

TypeScript:

> newData = JSON.parse(JSON.stringify(data)) // Clone our data object.
> const newAriel = { id: 4, firstname: 'New Ariel', lastname: 'Swandóttir' }
> newData.house.owner = newAriel
{ id: 4, firstname: 'New Ariel', lastname: 'Swandóttir' }

Haskell with Lenses:

*Main Data> let newAriel = Person { id = 4, firstname = "New Ariel", lastname = "Swanson" }
*Main Data> house & #owner .~ newAriel
Household { {- Full Household object... -} }

We add two new pieces of syntax here. The & is a reverse application operator, but for all intents and purposes think of it as the ^. for setters. Finally, .~ is what allows us to actually set our value.

Haskell with Record Dot Syntax:

let newAriel = Person { id = 4, firstname = "New Ariel", lastname = "Swanson" }
house{ owner = newAriel}
--> Household { {- Full Household object... -} }

Pretty neat. Note that the lack of spacing in house{ is intentional.

Rust:

let mut new_house = house.clone();
let new_ariel = Person { id: 4, firstname: "New Ariel".to_string(), lastname: "Swanson".to_string() };
new_house.owner = new_ariel;
--> Household { /* Full Household object... */ }

Alternatively we could use Rust’s Struct Update syntax, .., which works much like the spread syntax (...) in JavaScript. It would look something like Household { owner: new_ariel, ..house }.

Set a nested field

Now it gets a bit more tricky.

TypeScript:

> newData = JSON.parse(JSON.stringify(data)) // Clone our data object.
> newData.house.owner.firstname = 'New Ariel'
'New Ariel'

Haskell with Lenses:

*Main Data> house & #owner . #firstname .~ "New Ariel"
Household { {- Full Household object... -} }

Note that we mix & and . to dig deeper into the object/record, much like accessing a nested field.

Haskell with Record Dot Syntax:

house{ owner.firstname = "New Ariel"}
--> Household { {- Full Household object... -} }

Note that the lack of spacing in house{ is actually important, at least in the current state of RecordDotSyntax.

Rust:

let mut new_house = house.clone();
new_house.owner.firstname = "New Ariel".to_string();
--> Household { /* Full Household object... */ }

Set each item in a list

Let’s work a bit on the people list in our household. We’ll make those first names a bit more fresh.

TypeScript:

> newData = JSON.parse(JSON.stringify(data)) // Clone our data object.
> newData.house.people.forEach(person => { person.firstname = `Fly ${person.firstname}` })
> newData.house.people
[
  { id: 1, firstname: 'Fly Ariel', lastname: 'Swanson' },
  { id: 2, firstname: 'Fly Triton', lastname: 'Swanson' },
  { id: 3, firstname: 'Fly Eric', lastname: 'Swanson' }
]

Haskell with Lenses:

-- You can usually also use `traverse` instead of `mapped` here.
*Main Data> house & #people . mapped . #firstname %~ ("Fly " <>)
Household { {- Full Household object... -} }

mapped allows us to map a function over all the values in #people.

Haskell with Record Dot Syntax:

house{ people = map (\p -> p{firstname = "Fly " ++ p.firstname}) house.people}
--> Household { {- Full Household object... -} }

Using map feels very natural, and is quite close to the regular code you would write in Haskell.

Rust:

let mut new_house = house.clone();
new_house.people.iter_mut().for_each(|p| p.firstname = format!("Fly {}", p.firstname));
--> Household { /* Full Household object... */ }

Encode / Serialize

Encoding JSON from our data is quite simple. In TypeScript/JavaScript it’s built-in, and in Haskell and Rust, we simply reach for Aeson and Serde. Each of the libraries gives us control over the details in various ways, such as omitting Nothing/None values.

TypeScript:

> JSON.stringify(data)
'{"mom": ... }'

Haskell with Lenses + Haskell with Record Dot Syntax:

-- You can usually also use `traverse` instead of `mapped` here.
*Main Data> import Data.Aeson (encode)
*Main Data Data.Aeson> encode house
"{\"id\":1, ... }}"

Rust:

let serialized = serde_json::to_string(&house).unwrap();

Decode / Deserialize

Decoding JSON into our data type is luckily also straightforward, although we will need to tell Haskell and Rust a bit more information than when encoding (as one would expect).

TypeScript:

> let houseJson = JSON.stringify(data)
> JSON.parse(houseJson)
{
  mom: { id: 1, firstname: 'Ariel', lastname: 'Swanson' },
  dad: { id: 2, firstname: 'Triton', lastname: 'Swanson' },
  son: { id: 3, firstname: 'Eric', lastname: 'Swanson' },
  house: {
    id: 1,
    people: [ [Object], [Object], [Object] ],
    address: { country: 'Ocean', address: 'Under the sea' },
    owner: { id: 1, firstname: 'Ariel', lastname: 'Swanson' }
  }
}

Haskell with Lenses + Haskell with Record Dot Syntax:

-- Setting up imports and language extensions.
*Main Data>:set -XTypeApplications
*Main Data> import Data.Aeson (decode, encode)
*Main Data Data.Aeson> let houseJson = encode house
-- Our decoding.
*Main Data Data.Aeson> decode @Household houseJson
Just (Household
  { id = 1
  , people =
      [ Person {id = 1, firstname = "Ariel", lastname = "Swanson"}
      , Person {id = 2, firstname = "Triton", lastname = "Swanson"}
      , Person {id = 3, firstname = "Eric", lastname = "Swanson"}
    ]
  , address = Just (Address {country = "Ocean", address = "Under the sea"})
  , alternativeAddress = Nothing, owner = Person {id = 1, firstname = "Ariel", lastname = "Swanson"}
  }
)

Since we are in the REPL, we manually enable the TypeApplications language extension. We then use this when decoding, in @Household, to let Haskell know what data type we are trying to convert this random string into.

Alternatively, we could have written (decode houseJson) :: Maybe Household. The Maybe is what the decoder wraps the value in, in case we fed it a malformed JSON string.

Rust:

let house_json = serde_json::to_string(&house).unwrap();
let deserialize: Household = serde_json::from_str(&house_json).unwrap();
--> Household {
    id: 1,
    people: [
        Person { id: 1, firstname: "Ariel", lastname: "Swanson", },
        Person { id: 2, firstname: "Triton", lastname: "Swanson", },
        Person { id: 3, firstname: "Eric", lastname: "Swanson", },
    ],
    address: Some(Address { country: "Ocean", address: "Under the sea", }),
    alternative_address: None,
    owner: Person { id: 1, firstname: "Ariel", lastname: "Swanson", },
}

Like with Haskell, we let Rust know what data type we are trying to convert our random string into. We do this by annotating the type of deserialize to with deserialize: Household. The unwrap here is for convenience, but in real code, you’re probably more likely to do serde_json::from_str(&house_json)? instead.

Have other common patterns you’d like to see? Feel like some of the approaches could be improved? Leave a comment, and I will try to expand this list to be more comprehensive!

Changelog

Thanks to all the feedback from the /r/rust and /r/haskell communities, the following changes have been made:

13th of April, 2020
- Added serialize and deserialize examples
- Included Aeson derive code in the Haskell snippet since the two Haskell data type examples ended up being almost identical
6th of April, 2020
- Made house & #people . mapped %~ (\p -> p & #firstname .~ "Fly " ++ p ^. #firstname) much more succint with house & #people . mapped . #firstname %~ ("Fly " <>).
- Added acceptable spacing between house{ and the rest of the RecordDotSyntax approaches (e.g. house{ owner.firstname = "New Ariel"}).
- Changed from map to forEach in TypeScript, since the return value was discarded.
- Switched the Rust approaches to use mutations instead of the unidiomatic immutable style it was written in.

Along with aeson, we will use the new deriving-aeson library to derive our instances.↩︎
There are of course more options, like Optics (usage example), but I won’t cover them all here.↩︎
We use generic-lens for Lens derivations instead of TemplateHaskell.↩︎
It will take a bit of time before it is merged and available in GHC, so we will use the record-dot-preprocessor plugin to get a sneak peak.↩︎
maybe from Data.Maybe has the type signature maybe :: b -> (a -> b) -> Maybe a -> b, taking in as argument (1) a default value (2) a function to run if the value is Just and (3) the Maybe value we want to operate on.↩︎