rescript-promise

This is a proposal for replacing the original Js.Promise binding that is shipped within the ReScript compiler. It will be upstreamed as Js.Promise2 soon. This binding was made to allow our users to try out the implementation in their codebases first.

See the PROPOSAL.md for the rationale and design decisions.

Feature Overview:

• t-first bindings
• Fully compatible with the builtin Js.Promise.t type
• make for creating a new promise with a (resolve, reject) => {} callback
• resolve for creating a resolved promise
• reject for creating a rejected promise
• catch for catching any JS or ReScript errors (all represented as an exn value)
• then for chaining functions that return another promise
• thenResolve for chaining functions that transform the value inside a promise
• all and race for running promises concurrently
• finally for arbitrary tasks after a promise has rejected / resolved
• Globally accessible Promise module that doesn't collide with Js.Promise

Non-Goals of rescript-promise:

• No rejection tracking or other complex type hackery
• No special utilities (we will add docs on how to implement common utils on your own)

Caveats:

• There are 2 edge-cases where returning a Promise.t<Promise.t<'a>> value within then / thenResolve is not runtime safe (but also quite rare in practise). Refer to the Common Mistakes section for details.
• These edge-cases shouldn't happen in day to day use, also, for those with general concerns about runtime safetiness, it is recommended to use a catch call in the end of each promise chain to prevent runtime crashes anyways (just like in JS).

Requirements

[email protected] and above.

Installation

npm install @ryyppy/rescript-promise --save

Add @ryyppy/rescript-promise as a dependency in your bsconfig.json:

{
  "bs-dependencies": ["@ryyppy/rescript-promise"]
}

This will expose a global Promise module (don't worry, it will not mess with your existing Js.Promise code).

Examples

• examples/FetchExample.res: Using the fetch api to login / query some data with a full promise chain scenario

Usage

Creating a Promise:

let p1 = Promise.make((resolve, _reject) => {
  
  // We use uncurried functions for resolve / reject
  // for cleaner JS output without unintended curry calls
  resolve(. "hello world")
})

let p2 = Promise.resolve("some value")

// You can only reject `exn` values for streamlined catch handling
exception MyOwnError(string)
let p3 = Promise.reject(MyOwnError("some rejection"))

Access and transform a promise value:

open Promise
Promise.resolve("hello world")
->then(msg => {
  // then callbacks require the result to be resolved explicitly
  resolve("Message: " ++ msg)
})
->then(msg => {
  Js.log(msg);

  // Even if there is no result, we need to use resolve() to return a promise
  resolve()
})
->ignore // Requires ignoring due to unhandled return value

Chain promises:

open Promise

type user = {"name": string}
type comment = string

// mock function
let queryComments = (username: string): Js.Promise.t<array<comment>> => {
  switch username {
  | "patrick" => ["comment 1", "comment 2"]
  | _ => []
  }->resolve
}

// mock function
let queryUser = (_: string): Js.Promise.t<user> => {
  resolve({"name": "patrick"})
}

let queryUser = queryUser("u1")
->then(user => {
  // We use `then` to automatically
  // unnest our queryComments promise
  queryComments(user["name"])
})
->then(comments => {
  // comments is now an array<comment>
  Belt.Array.forEach(comments, comment => Js.log(comment))

  // Output:
  // comment 1
  // comment 2

  resolve()
})
->ignore

You can also use thenResolve to chain a promise, and transform its nested value:

open Promise

let createNumPromise = (n) => resolve(n)

createNumPromise(5)
->thenResolve(num => {
  num + 1
})
->thenResolve(num => {
  Js.log(num)
})
->ignore

Catch promise errors:

Important: catch needs to return the same return value as its previous then call (e.g. if you pass a promise of type Promise.t<int>, you need to return an int in your catch callback). This usually implies that you'll need to use a result value to express successful / unsuccessful operations:

exception MyError(string)

open Promise

Promise.reject(MyError("test"))
->then(str => {
  Js.log("this should not be reached: " ++ str)

  // Here we use the builtin `result` constructor `Ok`
  Ok("successful")->resolve
})
->catch(e => {
  let err = switch e {
  | MyError(str) => "found MyError: " ++ str
  | _ => "Some unknown error"
  }

  // Here we are using the same type (`t<result>`) as in the previous `then` call
  Error(err)->resolve
})
->then(result => {
  let msg = switch result {
  | Ok(str) => "Successful: " ++ str
  | Error(msg) => "Error: " ++ msg
  }
  Js.log(msg)
  resolve()
})
->ignore

Catch promise errors caused by a thrown JS exception:

open Promise

let causeErr = () => {
  Js.Exn.raiseError("Some JS error")->resolve
}

Promise.resolve()
->then(_ => {
  causeErr()
})
->catch(e => {
  switch e {
  | JsError(obj) =>
    switch Js.Exn.message(obj) {
    | Some(msg) => Js.log("Some JS error msg: " ++ msg)
    | None => Js.log("Must be some non-error value")
    }
  | _ => Js.log("Some unknown error")
  }
  resolve()
  // Outputs: Some JS error msg: Some JS error
})
->ignore

Catch promise errors that can be caused by ReScript OR JS Errors (mixed error types):

Every value passed to catch are unified into an exn value, no matter if those errors were thrown in JS, or in ReScript. This is similar to how we handle mixed JS / ReScript errors in synchronous try / catch blocks.

exception TestError(string)

let causeJsErr = () => {
  Js.Exn.raiseError("Some JS error")
}

let causeReScriptErr = () => {
  raise(TestError("Some ReScript error"))
}

// imaginary randomizer function
@bs.val external generateRandomInt: unit => int = "generateRandomInt"

open Promise

resolve()
->then(_ => {
  // We simulate a promise that either throws
  // a ReScript error, or JS error
  if generateRandomInt() > 5 {
    causeReScriptErr()
  } else {
    causeJsErr()
  }->resolve
})
->catch(e => {
  switch e {
  | TestError(msg) => Js.log("ReScript Error caught:" ++ msg)
  | JsError(obj) =>
    switch Js.Exn.message(obj) {
    | Some(msg) => Js.log("Some JS error msg: " ++ msg)
    | None => Js.log("Must be some non-error value")
    }
  | _ => Js.log("Some unknown error")
  }
  resolve()
})
->ignore

Using a promise from JS (interop):

open Promise

@val external someAsyncApi: unit => Js.Promise.t<string> = "someAsyncApi"

someAsyncApi()->Promise.then((str) => Js.log(str)->resolve)->ignore

Running multiple Promises concurrently:

open Promise

let place = ref(0)

let delayedMsg = (ms, msg) => {
  Promise.make((resolve, _) => {
    Js.Global.setTimeout(() => {
      place := place.contents + 1
      resolve(.(place.contents, msg))
    }, ms)->ignore
  })
}

let p1 = delayedMsg(1000, "is Anna")
let p2 = delayedMsg(500, "myName")
let p3 = delayedMsg(100, "Hi")

all([p1, p2, p3])->then(arr => {
  // arr = [ [ 3, 'is Anna' ], [ 2, 'myName' ], [ 1, 'Hi' ] ]

  Belt.Array.forEach(arr, ((place, name)) => {
    Js.log(`Place ${Belt.Int.toString(place)} => ${name}`)
  })
  // forEach output:
  // Place 3 => is Anna
  // Place 2 => myName
  // Place 1 => Hi

  resolve()
})
->ignore

Race Promises:

open Promise

let racer = (ms, name) => {
  Promise.make((resolve, _) => {
    Js.Global.setTimeout(() => {
      resolve(. name)
    }, ms)->ignore
  })
}

let promises = [racer(1000, "Turtle"), racer(500, "Hare"), racer(100, "Eagle")]

race(promises)
->then(winner => {
  Js.log("Congrats: " ++ winner)->resolve
  // Congrats: Eagle
})
->ignore

Common Mistakes

Don't return a Promise.t<Promise.t<'a>> within a then callback:

open Promise

resolve(1)
  ->then((value: int) => {
    let someOtherPromise = resolve(value + 2)

    // BAD: this will cause a Promise.t<Promise.t<'a>>
    resolve(someOtherPromise)
  })
  ->then((p: Promise.t<int>) => {
    // p is marked as a Promise, but it's actually an int
    // so this code will fail
    p->then((n) => Js.log(n)->resolve)
  })
  ->catch((e) => {
    Js.log("luckily, our mistake will be caught here");
    Js.log(e)
    // p.then is not a function
    resolve()
  })
  ->ignore

Don't return a Promise.t<'a> within a thenResolve callback:

open Promise
resolve(1)
  ->thenResolve((value: int) => {
    // BAD: This will cause a Promise.t<Promise.t<'a>>
    resolve(value)
  })
  ->thenResolve((p: Promise.t<int>) => {
    // p is marked as a Promise, but it's actually an int
    // so this code will fail
    p->thenResolve((n) => Js.log(n))->ignore
  })
  ->catch((e) => {
    Js.log("luckily, our mistake will be caught here");
    // e: p.then is not a function
    resolve()
  })
  ->ignore

Development

# Building
npm run build

# Watching
npm run dev

Run Test

Runs all tests

node tests/PromiseTest.js

Run Examples

Examples are runnable on node, and require an active internet connection to be able to access external mockup apis.

node examples/FetchExample.js