1.1.1 • Public • Published


fast-check bindings for ReScript



npm i --save-dev rescript-fast-check


yarn add -D rescript-fast-check

Then add rescript-fast-check to bsconfig.json:

  "bs-dev-dependencies": [


Ported from the fast-check example, using bs-mocha

open FastCheck
open Arbitrary
open Property.Sync

// Code under test
let contains = (text, pattern) => text->Js.String2.indexOf(pattern) >= 0

// Properties
describe("properties", () => {
  // string text always contains itself
  it("should always contain itself", () =>
    assert_(property1(string(), text => contains(text, text)))
  // string a + b + c always contains b, whatever the values of a, b and c
  it("should always contain its substrings", () =>
      property3(string(), string(), string(), (a, b, c) =>
        contains(a ++ b ++ c, b)

For further examples please browse the tests. The tests are included in the NPM package for this purpose.


These bindings aim to be zero cost as much as possible. The library is almost exclusively external bindings, with a handful of methods for convenience and helping to map edge cases to the ReScript type system. The above example compiles to JavaScript that is semantically identical to the fast-check example it was ported from; this can be seen by compiling the example test file.


Everything is under the FastCheck namespace. Rather than use a flat namespace, as the fast-check API does, ReScript affords us the ability to use some logical module groupings while still compiling to the flat namespace.

The rescript-fast-check API groups are roughly mapped to the fast-check documentation, although not everything lines up as well as it used to after some of the fast-check documentation was restructured.


This module covers built-in arbitraries and custom arbitraries. Top level functions are simple arbitraries that generate values with no (or very little) input, sub-modules are used to group more complicated arbitraries together.

  • Combinators contains most of the APIs grouped under combinators in the fast-check documentation.
  • Objects groups the objects and letrec APIs, which are technically also combinators but a bit more advanced to work within the ReScript type system.
  • Derive includes most of the custom arbitrary APIs, the ones that derive arbitraries from arbitraries.
  • Scheduler is a dedicated module for fc.scheduler(), for information on this see the detect race conditions documentation.
  • Context groups methods for using fc.context(), for more information on this see the log within a predicate documentation.


To maintain type safety, this library does not bind with varadic arguments. Property test bindings are available up to 5 arguments; if you need more a PR would be welcome.

Fast-check is very strict about the return types of property testing methods. If a predicate does not return undefined, a truthy check is used to validate the response. ReScript does not have a convenient mapping for returning bool | unit without jumping through hoops so two modules are used for each of the sync / async styles.

Property testing is therefore split into 4 sub-modules with an identical list of methods covering the property based runners:

  • Sync for synchronous boolean checks
  • SyncUnit for synchronous exception-throwing checks
  • Async for promise-based checks that resolve to a boolean
  • AsyncUnit for promise-based checks that reject in case of failure

Because I was a JSVerify fan I have added a helper method to combine assert and property1 into a single function call, assertProperty1, a feature JSVerify offers but fast-check does not. This is implemented in each of the 4 property testing sub-modules, also extending to 5 property arguments. The example test file shows the difference.

As a further inconvenience, assert is a keyword in ReScript so the binding was renamed to assert_. This means a zero cost binding to a single property assertion is assert_(property1(arb, f)). To improve the developer experience, two options are provided:

  1. Te recommended approach is the helper method assertProperty1, but it is not zero cost.
  2. The Property.FcAssert sub-module contains sync and async, which are zero-cost bindings. The example then becomes FcAssert.sync(property1(arb, f)).


There are a lot of places that could use assistance! Contributions are very welcome. Here are some suggestions:

  • Move the usage section of this file to a proper documentation area and include more examples.
  • Add doc comments to every binding to aid autocomplete results. Do this by copying descriptions from the fast-check documentation or API reference, whichever provides the better description.
  • Doc comments would also mean an automated documentation tool can be added.
  • Better tests to validate that the values produced by the API calls match the binding types. The existing tests only validate that bindings are able to call APIs without a runtime error.
  • More complete fast-check API bindings. Most of the missing parts are due to difficulty in figuring out a type-safe binding. Brand new APIs may also lack bindings if I haven't noticed they were added.
  • As an example, FastCheck.Global is intended to map to fast-check's global configuration options but it is unimplemented.
  • Consider converting from ReasonML syntax to ReScript. However until the rescript-vscode extension is complete I'm likely to prefer ReasonML due to the better development experience. It makes no difference which syntax this repository uses when installed as a dependency.
  • If there is enough feedback that the binding module structure differences to fast-check documentation are confusing, a big refactor to line them all up again would be necessary.

Implementation notes

The insistence on zero-cost bindings results in a lot of duplication in the Property module. Functors are used to avoid duplication between bool and unit property checks, but the design of external types requires duplication between definitions and fc.assertProperty. The only methods where duplication can be avoided are fc.assert and fc.check, which doesn't seem worth the extra hassle (but they are de-duplicated in the Intf_Property module types).

License and Credits

All code is licensed as BSD. See LICENSE.

The original inspiration for this project came from bs-jsverify after jsverify was abandoned.

Some development has been done during work hours by employees at Tiny.

Package Sidebar


npm i rescript-fast-check

Weekly Downloads






Unpacked Size

64.3 kB

Total Files


Last publish


  • thespyder