User-facing changes to the Folktale project are summarised here, ordered from the most recent releases to the oldest ones.


How is this file structured?

Each version entry is written as a heading in the format [<version number>] - YYYY-MM-DD and contains the sections:

  • New features — Functionality that has been added from the previous version to the referred one;
  • Bug fixes — Incorrect behaviour that has been corrected from the previous version to the referred one;
  • Optimisations — Performance and memory improvements from the previous version to the referred one;
  • Documentation — Improvements made to the documentation;
  • Miscellaneous — Any other change worth mentioning that doesn’t fall in the previous ones;
  • DEPRECATED FEATURES — Features that have been deprecated in the referred version, and should be avoided for new codebases;
  • BREAKING CHANGES — Backwards-incompatible changes that have been introduced by the version, along with the changes necessary to existing codebases. Upgrading from previous versions is not safe;

[2.1.0-rc1] - 2017-12-31

New features

  • nullableToResult can now take a fallback value (PR #166 by @diasbruno)

Bug fixes

  • Derived .toString methods can now handle objects without a .toString method (though not yet circular objects). (PR #169)


  • Many typos were fixed (PR #151 by @gvillalta99; PR #165 by @MichaelQQ, PR #145 by @stabbylambda, PR #143 and PR #144 by @floriansimon1)
  • Migration documentation on Task has been improved, and examples fixed (PR #160)
  • Added notes on shims for older platforms (PR #161)
  • Fixed some Validation examples (PR #154 by @scotttrinh)


  • Renamed Future.recover to Future.orElse. Future.recover was deprecated. (PR #146)


  • We’ve moved from Make to Furipota, so the build system is cross-platform now (PR #148)

[2.0.0] - 2017-07-15

New features

  • filter for both Maybe and Result (PR #135 by @floriansimon1).
  • waitAll now supports empty arrays (PR #134 by @jreut)
  • allows using direct style with Tasks, like in async/await for Promises (PR #128 by @framp)

[2.0.0-rc1] - 2017-06-22

New features

  • Maybe now implements Semigroup and Monoid (PR #125 by @diasbruno).
  • Result now implements Semigroup.


  • folktale/data/future is now folktale/concurrency/future.
  • folktale/data/task is now folktale/concurrency/task.
  • folktale/data/conversions is now folktale/conversions.
  • folktale/core/fantasy-land is now folktale/fantasy-land.
  • folktale/data/maybe is now folktale/maybe.
  • folktale/data/result is now folktale/result.
  • folktale/data/validation is now folktale/validation.
  • The folktale/core/adt module is now folktale/adt/union module, which specifies that it creates union types. The data function was also renamed to union, and the ADT namespace was renamed to Union — although this one was used mostly internally.


    const { data, derivations } = require('folktale/core/adt');
    const List = data('List', {
      Empty(){ },
      Cons(value, rest) {
        return { value, rest };


    const { union, derivations } = require('folktale/adt/union');
    const List = union('List', {
      Empty(){ },
      Cons(value, rest) {
        return { value, rest };

[2.0.0-beta1] - 2017-05-03


  • Documentation for Data.Future was added.
  • Fixed some of the Data.Task documentation.

New features

  • nodebackToTask (and Task.fromNodeback) allows converting callback-based functions in Node-style to Tasks automatically (PR #116 by @rpearce)
  • futureToPromise and promiseToFuture allow converting between promises and futures. The conversion isn’t strictly loss-less due to the difference in semantics (PR #119, details in #98)
  • promisedToTask (and Task.fromPromised) allows converting functions that return promises to functions that return tasks (PR #120, details in #97)


  • (f43441b) Future#mapRejection has been renamed to Future#mapRejected for consistency. To fix just replace any calls of mapRejection on Futures to mapRejected:

    // Before
    Future.rejected(1).mapRejection(x => x + 1);
    // Now
    Future.rejected(1).mapRejected(x => x + 1);

[2.0.0-alpha4] - 2017-04-08


  • Guides for installing and contributing to Folktale were added.
  • An annex describing the type annotation used in Folktale was added.
  • Documentation for Data.Task was added.
  • Other documentation improvements were made.
  • Top-level async/await is now supported in Documentation examples.
  • Experimental features now have a section noting such in their pages.

New features

  • .mapRejected() was added as a counterpart of .map() for failed Tasks (35fbd0d)
  • .orElse() was added as a convenient way of recovering from failed Tasks (35fbd0d)
  • waitAny and waitAll have been added as convenience over .or and .and methods in Task (ea4d1c0)
  • Equality derivation now uses a limited deep-equality (for arrays/objects) by default (373b518)

Bug fixes

  • Fixes fromJSON in the presence of getters, where it’d fail when trying to set values in the reified structure (f4026e0)
  • Serialises undefined as null so all keys are kept in the serialised structure (ffee127)


  • (da6c2e3) nullable → validation now takes a fallback value, so Failures are kept as semigroups. By default, if you don’t pass anything, the fallback value will be undefined, so this only breaks things if you were relying on nullableToValidation(null) giving you Failure(null). To fix it, simply pass an additional parameter:

    // Before
    // Now
    nullableToValidation(null, null);
    Validation.fromNullable(null, null);

    That said, you should keep your Failures as proper semigroups, so things like .apply and .concat will work correctly.

[2.0.0-alpha3] - 2017-03-11

New features

  • Adds a nullable → maybe conversion (9706ab7);
  • Adds a .fold() method to the Maybe structure. This works similarly to Result and Validation’s .fold() method, except the function for the Nothing case takes no arguments (6f59f61);
  • Adds the collect function (data/validation/collect), which makes aggregating errors from failures directly a bit simpler (#71, a677e96);

Bug fixes

  • Fixes the exported symbol for validation → either conversion (450cb70);
  • Fixes the Setoid implementation to work with the core/fantasy-land module (d761107);
  • Fixes a bunch of currying and argument order issues with the core/fantasy-land module (d5b0c74);


  • Annotated files are now only generated for testing and documentation, which makes browser bundles much smaller (e0186fa);


  • The old .get() methods are deprecated in favour of the new .unsafeGet() methods. There was no behavioural change, just a naming one. See #42. (278d5a7, 19910b4);


  • (8e1b27e) Modules that were previously named core.js now reflect the name of the functionality they implement. This affects you if you’ve been importing the core.js modules directly, rather than their folder. The following modules are affected by this change:

    • core/adt/core.jscore/adt/data.js (exports the data function);
    • data/either/core.jsdata/either/either.js (exports the either ADT);
    • data/maybe/core.jsdata/maybe/maybe.js (exports the maybe ADT);
    • data/validation/core.jsdata/validation/validation.js (exports the validation ADT).
  • (7a1ef33) Removes the data/either/fromNullable.js module, which was moved to the data/conversions module. This affects you if you have been importing that module directly. In that case you can import data/conversions/nullable-to-either.js instead:

    // Before
    const eitherFromNullable = require('data/either/fromNullable');
    // Now
    const eitherFromNullable = require('data/conversions/nullable-to-either');

    Note that no changes are necessary if you were importing the whole data/either module and taking the fromNullable method from there.

  • (5676d39) Either.try now takes a thunk, rather than being a curried-ish form of application:

    // Before
    Either.try((a) =>;   // ==> Left("cannot read property 'foo' of null")
    // Now
    Either.try(() =>;       // ==> Left("cannot read property 'foo' of null")
  • (PR #62) Renames EitherResult. No behaviour changes, but a bunch of terminology changes that break existing code.

    Where one used to write:

    const { Left, Right } = require('folktale/data/either');

    One would now write:

    const { Error, Ok } = require('folktale/data/result');

    The data structure’s name has been changed to Result. The Left case has been changed to Error, and the Right case has been changed to Ok. This affects all uses of .matchWith as well as constructing values.

  • (d5e780f) Remove the .cata() method from core/adt, but adds it to Maybe and Validation. This breaks any core/adt structure that was using .cata() instead of .matchWith(), but makes it easier for people to migrate their code with Maybe and Validation to Folktale 2 by just replacing require('data.maybe') with require('folktale/data/maybe').

  • (f0dd120) Several renamings to make the API consistent regarding its usage of English (see #21), so now all names in the API use US English spelling, although the documentation still uses British English spelling:

    • The Show derivation (core/adt/show.js) is now called DebugRepresentation (core/adt/derivations/debug-representation.js);
    • The Setoid derivation (core/adt/setoid.js) is now called Equality (core/adt/derivations/equality.js). The method for providing a custom comparison function, previously .withEquality(cmp) is now called .wthCustomComparison(cmp);
    • The Serialize derivation (core/adt/serialize.js) is now called Serialization (core/adt/derivations/serialization.js);
    • The derivations are now provided in the core/adt/derivations.js file, and consequently in a derivations property of the core/adt module, rather than directly there;
    • The partialise function (core/lambda/partialise.js) is now called partialize (core/lambda/partialize.js).

[2.0.0-alpha2] - 2016-12-05

New features

  • Adds the Show derivation for ADTs, which provides .toString() and .inspect() methods, as well as the internal ToStringTag symbol (PR #12 by @boris-marinov);
  • Adds the partialise function to the core/lambda module. Partialise allows one to pass “holes” into a function application in order to specify only part of the arguments. It’s similar to curry in that it supports partial application, but it works better with regular JavaScript functions since argument saturation is explicitly defined, rather than depending on the number of arguments provided;
  • Adds the Serialize derivation for ADTs, which provides .toJSON() and .fromJSON(value) methods (PR #15 by @boris-marinov);
  • Adds the data/validation module, with equivalent semantics to the older Data.Validation structure. The breaking changes described in the previous version for Either and Maybe apply to this structure as well (PR #22 by @boris-marinov);
  • Adds the data/conversions module, supporting conversions between nullable types, maybes, eithers, and validations. Some of these are also provided in each structure itself as convenience methods (PR #24 by @boris-marinov);
  • Adds the core/fantasy-land module, containing free functions for each Fantasy Land specification method. This allows codebases to interact seamlessly with code for older and newer Fantasy Land specifications (PR #37 by @boris-marinov, PR #43);
  • Adds the data/task module, with a new Task implementation (PR #50).

Bug fixes

  • Changes the .hasInstance(value) method in ADTs so it supports testing non-objects as well. Previously this method would fail when testing null and undefined values;


  • The .cata(pattern) method is now deprecated in favour of the new .matchWith(pattern) method. They have the same behaviour;
  • The .is{TAG} fields are deprecated in favour of the new .hasInstance(value) methods. The .hasInstance() versions allow safely testing any value, even non-objects, and also do an union instance checking, rather than a simple tag check;

[2.0.0-alpha1] - 2016-05-09

This is the initial release of the redesigned Folktale. Backwards compatibility with Folktale 1 was not provided.

New features

  • Adds core/adt module to allow constructing union types in an easier way (PR #11);
  • Adds the Setoid derivation for ADTs, which provides an .equals() method (PR #10 by @boris-marinov);
  • Adds the core/lambda module with compose, constant, identity, and curry. The new curry implementation only unrolls application for functions marked by Folktale as constructed by curry, avoiding some of the problems with composing curried and regular functions;
  • Adds the core/object module with fromPairs, mapEntries, mapValues, toPairs, and values;
  • Adds the data/maybe module, with equivalent semantics to the older Data.Maybe structure;
  • Adds the data/either module, with equivalent semantics to the older Data.Either structure.


  • Either and Maybe’s .cata() method now passes the whole instance instead of just the value:

    // Before
      Left: (value) => value + 1,
      Right: (value) => value - 1
    // ==> 0
    // Now
      Left: (leftInstance) => leftInstance.value + 1,
      Right: (rightInstance) => rightInstance.value - 1
    // ==> 0

    Using ES2015’s destructuring assignments:

      Left: ({ value }) => value + 1,
      Right: ({ value }) => value - 1
    // ==> 0