Design principles

There are some principles that guide the design of Folktale 2. This document describes these principles.


Arrow functions over currying

Curried functions can be composed in several ways. And this is a good thing, because it means it’s easier to create new functionality by combining existing ones. However, in an untyped language, you have no way of enforcing a correct composition of functions, so currying in JS tends to lead to silent or late errors. Coupled with JavaScript’s heavy use of variadic functions, and the VM’s unhelpful error reporting, these have the potential for being very frustrating, especially for people new to FP.

ES2015 lessens a bit the burden of being more explicit with your composition, since arrow functions have less visual overhead, and can enforce some compositional constraints syntactically. They also side-step the problems with JavaScript’s variadic functions to an extent.

Free-method versions of functions

JavaScript’s syntax is limited, but some of the proposed additions to it lead to programs that are easier to read. One of them is the This-Binding syntax, which allows using functions in infix position:

const _groupBy = require('folktale/core/iterator/group-by');

function groupBy(f) {
  return _groupBy(this, f);

[1, 2, 3, 4, 5]::groupBy(isOdd)
// => _groupBy([1, 2, 3, 4, 5], isOdd)

// Some combinators may be provided specifically to use as infix
// => list.sort(on(compare, first))

Reuse the language’s native methods

Where possible, as long as laws aren’t broken, reuse the language’s native methods. This makes it easier to combine Folktale with existing projects and libraries, as they’d have similar expectations.

Monolithic package

There are plenty of advantages of micro-libraries. They are more modular, since they have to work with less expectations; they are easier to replace; we can make a better use of semantic versioning; and they are easier to understand.

But there are plenty of drawbacks to them as well:

  • They’re harder to use when you need to combine more than one module. For example, if you want an Either module to be able to convert its values to a Maybe structure, you need to provide the Either module with the Maybe structure to use. This, while modular, adds some friction for people to use these features, which goes against our aim to make functional programming easier to newcomers.

  • They’re harder to maintain. Keeping modules, tooling, and issues in sync between different modules takes much more effort, and it’s easy to miss things. When all of the modules are related anyway, you’ll have to keep them in sync all the time.

  • They’re less consistent. Because each module is independent, exposes its own API, and evolves at its own pace, it’s easy to get inconsistent behaviour across them. This was the case with how methods worked in the old versions of Maybe, Either, and Validation.

Interactive (REPL) development

One of the long-term goals with Folktale is to have really good support for interactive development in the REPL. The first step for that is to be able to view documentation directly from the REPL, which is being done by annotating objects with the Meta:Magical library.

As newer features get added to Meta:Magical, like searching functions by approximate type signature, Folktale and its users will benefit from them without having to do anything.

Logically grouped, minimal modules

Each module should be entirely self-contained, and only provide the absolute minimum necessary for that particular functionality. Most of the time this will mean one function = one module, but not always. See the folktale/adt/union module, and the Siren’s Platform Design document for examples.

There are two reasons behind this:

  1. Extracting a minimal application from a large framework/environment (tree-shaking) requires full type inference in JavaScript, because modules are first class, and you need to know which properties of which objects are accessed where. For historical data on this, see the report on Foundations of Object-Oriented Languages and Aegesen at al’s work on type inference for Self.

  2. Because all modules are annotated, inline, for the Meta:Magical library, even modules comprised of simple, one line functions end up being more than 40 lines of code when you consider documentation.

Modules are grouped in an hierarchy of categories

This is mostly to make using the libraries easier given the previous point. Modules that are higher in the hierarchy should re-export all features below it. This allows people to require a bag of things, like require('folktale/result'), or a very specific functionality, when they are concerned about the resulting size of the application (if they are deploying to a Browser, for example), like require('folktale/result/from-nullable').