Algebraic: modules and specifications

A set of specifications of signatures on algebraic modules and an implementation of a standard library built to match.

Truly modular functional programming in Javascript

Algebraic is a set of Javascript objects which are known as algebraic modules or modules when it's clear that I'm not referring to CommonJS, AMD, or ES6 import/export modules. A module contains a set of related behaviors operating over a set of types which the module pertains to.

For instance, there is a module Array which contains operations over the standard JS array type.

> var A = require("algebraic");
> A.Array.of(1)
[ 1 ]
> A.Array.plus(A.Array.of(1), A.Array.of(2))
[ 1, 2 ]

Seasoned OO-ers might find this awkward and backward—we should be doing something like [1].concat([2]). Algebraic intentionally eschews this design with its modular design. (If the noisy imports bother you then consider ES6 destructuring)

import { Array } from "algebraic";
const { plus, of } = Array;

const x = plus(of(1), of(2));

The fastest way to describe modular design in OO-terms is to say that a module is a class with only static methods. They, however, can also be much, much more.

Status

Principles

Behavior before state

A driving principle of Algebraic's design is behavior before state. What this means is that libraries and programs should be first designed against a specification of behavior and oblivious to the state required to pull the behavior off. The advantage of this adage is increased modularity driven by a clearer line between interface and implementation.

Unfortunately, standard OO practice conflates behavior and state heavily---indeed some take the definition of an object to be the conflation of behavior and state.

In most cases module objects themselves are completely state free. Indeed, in any case where they are not the observation of that state ought to be severely restricted and the intended behavior favored. Instead all of the state of Algebraic arises from the arguments passed to and received from module behaviors.

"Seamless" immutability by default

Immutability vastly reduces the state space of a program improving the ability for the author to correctly reason about it's behavior. It's also assumed in most descriptions of algebraic structures. For these reasons, Algebraic eschews behaviors which mutate values opting instead to return new copies as often as possible. This also provides standard sharing tricks like pre-emptive O(1) equality checks (which Algebraic uses by default) useful for comparison-driven programs like React-style UIs.

Note: Algebraic-style immutability is dangerous when mixed with mutable JS code. Algebraic does not attempt to freeze or seal values and also maximizes sharing, so mutation of an object involved in Algebraic behaviors can have long range effects.

Inspirations

Algebraic takes its design inspiration heavily from two other languages and their associated communities: ML and Haskell. Modular design of programs is extraordinarily well done in ML languages like OCaml and SML where the static type system helps ensure that your interfaces are well-defined and properly used. Algebraic specifications of operations is extraordinarily well done in Haskell where laziness, purity, and the Haskell "typeclass" mechanism drove the use of algebraic laws over immutable values out of both necessity and splendor.

Motivation

• Why not OO? Other FP-in-JS libraries (Fantasy Land, Ramda, Sanctuary, etc) provide FP-like functionality by attaching methods to objects so that a value x is, e.g., a Monoid when x.plus is a partially-applied monoid operation. algebraic eschews this by placing all functionality in an object external to x called a module.
• Immutable by default. (Obvious cache invalidation)
• Names, operations, and laws from algebra. Programming is an exercise in naming things. Algebra provides a large set of useful, consistent names along with well-defined expectations for what they mean.

FAQ

• Why ES6 and not something like Typescript? Three reasons.
  • This is a packaging of some code originally written for a real project using ES6, so using Typescript was not an option initially.
  • I'm not confident that I like the design decisions of Typescript's type system sufficiently to want to endorse it. An untyped system is arguably worse, but it gives me the freedom to write exactly the code intended.
  • I am writing some functions which could be well-typed but certainly would not be in most standard type systems. Lacking a real type checker I can introduce semi-formal notation for thee ideas and work with them.

Modules

Special

• Naive: Decidable

Simply kinded

• String: Decidable, Hashable, Decidable Monoid, Least, Total Order
• Boolean
• Number
• Integer
• Natural

Higher kinded

• Array: Decidable Monoid, Covariant, Foldable, Traversable, Monad, Reflects Decidability

• Set.OfDecidable

• Set.OfHashable

• Set.OfTotalOrder

• Dict

• Trie

• Validation

• Either

• Contract

Specifications

Identification

• Decidable {X}

  • definitions
    • X : Type
    • eq : (X, X) -> Bool
  • notation
    • When unambiguous, x = y is written to mean eq(x, y).
  • laws
    • reflexivity: for all (a : X) a = a
    • symmetry: for all (a b : X) a = b implies b = a.
    • transitivity: for all (a b c : X) a = b and b = c implies a = c.

• Hashable {X}

  • definitions
    • include Decidable {X}
    • definitions
      • hash : X -> Int
  • laws
    • identity compatibility: for all (a b : X) if a = b then hash(a) = hash(b).

Order Theory

• Preorder {X}

  • definitions
    • X : Type
    • leq : (X, X) -> Bool
    • indistinguishable : (X, X) -> Bool
      • λ(a, b) . leq(a, b) && leq(b, a)
    • incomparable : (X, X) -> Bool
      • λ(a, b) . not(leq(a, b) || leq(b, a))
  • notation
    • Where unambiguous, x <= y is written to mean leq(x, y)
  • laws
    • reflexivity: for all (a : X) a <= a
    • transitivity: for all (a b c : X) a <= b and b <= c implies a <= c

• Partial Order {X} or Poset {X}

  • include Preorder {X}
  • implies unique Decidable {X = X}
    • define eq(a, b) = indistinguishable(a, b)
  • laws
    • antisymmetry: indistinguishable is an equivalence relation

• Weak Order {X}

  • include Poset {X}
  • laws
    • incomparable is transitive

• Total Order {X}

  • include Poset {X}
  • implies unique Weak Order {X = X}
    • proof: totality implies that incomparable is the empty relation
  • laws
    • totality: for all (a b : X) either a <= b or b <= a

Boundings

• Least {X}

  • definitions
    • X : Type
    • bottom : () -> X

• Greatest {X}

  • definitions
    • X : Type
    • top : () -> X

• Bounded {X}

  • include Least {X} and Greatest {X}

Lattice Theory

• Join {X} / Meet {X}

  • include Decidable {X}
  • definitions (one of, called op)
    • join : (X, X) -> X
    • meet : (X, X) -> X
  • notation
    • Where unambiguous, join is written |
    • Where unambiguous, meet is written &
  • laws
    • associativity of op
    • commutativity of op
    • idempotence of op: for all (a : X) op(a, a) = a
  • implies unique
    • Semigroup {X} with
      • define plus(a, b) = op(a, b)
    • Poset {X} with
      • define leq(a, b) = eq(a | b, b) or
      • define leq(a, b) = eq(a, a & b) or

• Join^★ {X} / Meet^★ {X}

  • include Join {X} / Meet {X} as op
  • definitions
    • joinN : [X] -> X or
    • meetN : [X] -> X
  • laws
    • op is a Monoid morphism from arrays to X
    • operation compatibility: for all (a b : X) op(a, b) = opN([a, b])
  • implies unique Least {X} / Greatest {X}
    • define bottom() = joinN([]) or
    • define top() = meetN([])
  • implies unique Monoid {X}
    • define zero() = opN([]])

• Lattice

  • include Decidable {X}
  • definitions
    • join : (X, X) -> X and
    • meet : (X, X) -> X
  • notation
    • Where unambiguous, join is written |
    • Where unambiguous, meet is written &
  • laws
    • join implies unique Join {X}
    • meet implies unique Meet {X}
    • absorption: for all (a b : X) if q = a | (a & b) and p = a & (a | b) then eq(q, p) and q = a and p = a
    • Poset {X} via Join {X} is compatible with Poset {X} via Meet {X}
  • implies unique Poset {X}
    • defined via either Join {X} or Meet {X}
  • projects
    • Semigroup {X} via Join {X}
    • Semigroup {X} via Meet {X}

• Lattice^★

  • include Decidable {X}
  • definitions
    • join : (X, X) -> X and
    • meet : (X, X) -> X
    • joinN : (X, X) -> X and
    • meetN : (X, X) -> X
  • notation
    • Where unambiguous, join is written |
    • Where unambiguous, meet is written &
  • implies Bounded {X}
    • via implication of Least {X} and Greatest {X} from Join^★ {X} and Meet^★ {X} respectively
  • laws
    • joinN implies unique Join^★ {X}
    • meetN implies unique Meet^★ {X}
    • absorption: for all (a b : X) if q = a | (a & b) and p = a & (a | b) then eq(q, p) and q = a and p = a
    • Poset {X} via Join {X} is compatible with Poset {X} via Meet {X}
  • projects
    • Monoid {X} via Join^★ {X}
    • Monoid {X} via Meet^★ {X}

• Distributive {X}

  • include Lattice {X}
  • laws
    • distributivity
      • for all (a b c : X) let p = a & (b | c)) and q = (a & b) | (a & c) then eq(p, q) or
      • dually, for all (a b c : X) let p = a | (b & c) and q = (a | b) & (a | c) then p = q

First-order Algebraic Theories

• Magma {X}

  • definitions
    • X : Type
    • plus : (X, X) -> X
  • notation
    • Where unambiguous, plus(x, y) is written x + y

• Semigroup {X}

  • include Magma {X} and Decidable {X = X}
  • laws
    • associativity of plus: for all (a b c : X) eq(plus(a, plus(b, c)), plus(plus(a, b), c))

• Monoid {X}

  • include Semigroup {X}
  • definitions
    • zero : () -> X
    • plusN : [X] -> X
      • λ(xs) . xs.reduce(plus, zero())
  • notation
    • Where unambiguous, zero() is written 0
  • laws
    • zero is a zero: for all (a : X) a + 0 = a and 0 + a = a

• Decidable Monoid {X}

  • include Monoid {X}
  • definitions
    • isZero : X -> Bool
  • laws
    • zero isZero: isZero(zero())

• Group {X}

  • include Monoid {X}
  • definitions
    • negate : X -> X
    • minus : (X, X) -> X
      • λ(a, b) . plus(a, negate(b))
  • notation
    • Where unambiguous, negate(x) is written -x
    • Where unambiguous, minus(x, y) is written x - y
  • laws
    • negate undoes plus: for all (a : X) x - x = 0 and -x + x = 0

The following 3 are very close to one another such that Ring {X} subsumes both Semiring {X} and Rng {X} but those two are incomparable.

• Semiring {X}

  • include Decidable {X}
  • definitions
    • zero : () -> X
    • one : () -> X
    • plus : (X, X) -> X
    • times : (X, X) -> X
  • projects
    • zero and plus form the zero-plus Monoid {X}
    • one and times form the one-times Monoid {X}
  • notation
    • Where unambiguous, zero() is written 0
    • Where unambiguous, one() is written 1
    • Where unambiguous, plus(x, y) is written x + y
    • Where unambiguous, times(x, y) is written x * y
  • laws
    • the zero-plus Monoid {X} is commutative
    • times-plus distributivity: for all (a b c : X)
      • left distributivity: a * (b + c) = (a * b) + (a * c)
      • right distributivity: (a + b) * c = (a * c) + (b * c)
    • zero annihilation: for all (a : X) 0 * a = 0 and a * 0 = 0

• Rng

  • include Decidable {X}
  • definitions
    • zero : () -> X
    • negate : X -> X
    • plus : (X, X) -> X
    • times : (X, X) -> X
    • minus : (X, X) -> X
      • λ(a, b) . plus(a, negate(b))
  • notation
    • Where unambiguous, zero() is written 0
    • Where unambiguous, plus(x, y) is written x + y
    • Where unambiguous, times(x, y) is written x * y
    • Where unambiguous, negate(x) is written -x
    • Where unambiguous, minus(x, y) is written as x - y
  • projects
    • zero, plus, and negate form the zero-plus Group {X}
    • times forms the times Semigroup {X}
  • laws
    • the zero-plus Group {X} is commutative
    • times-plus distributivity: for all (a b c : X)
      • left distributivity: a * (b + c) = (a * b) + (a * c)
      • right distributivity: (a + b) * c = (a * c) + (b * c)

• Ring

  • include (all compatible)
    • Decidable {X}
    • Semiring {X}
    • Rng {X}
  • definitions
    • zero : () -> X
    • one : () -> X
    • negate : X -> X
    • plus : (X, X) -> X
    • times : (X, X) -> X
    • minus : (X, X) -> X
      • λ(a, b) . plus(a, negate(b))
  • notation
    • Where unambiguous, zero() is written 0
    • Where unambiguous, one() is written 1
    • Where unambiguous, plus(x, y) is written x + y
    • Where unambiguous, times(x, y) is written x * y
    • Where unambiguous, negate(x) is written -x
    • Where unambiguous, minus(x, y) is written as x - y
  • projects
    • zero, plus, and negate form the zero-plus Group {X}
    • one-times form the one-times Monoid {X}
  • laws
    • the zero-plus Group {X} is commutative
    • times-plus distributivity: for all (a b c : X)
      • left distributivity: a * (b + c) = (a * b) + (a * c)
      • right distributivity: (a + b) * c = (a * c) + (b * c)

Higher-order Algebraic Theories

Three notational shorthands are used in this section: id = x => x, f . g = x => f(g(x)), and . = f => g => x => f(g(x)).

• Parametric {F}

  • definitions
    • F : Type -> Type

• Reflects decidability {F}

  • include Parametric {F}
  • implications
    • Decidable {X} implies Decidable {F X}

• Covariant {F}

  • include Parametric {F}
  • definitions
    • map : ∀ a b . (a -> b) -> (F a -> F b)
  • notation
    • Where unambiguous map(f) is written [f]
  • laws (category morphism)
    • for all (X : Type), Decidable {F X}, (fa : F X), fa = [id](fa)
    • for all (X Y Z : Type), Decidable {F X}, (fx : F X), (g : X -> Y), (f : Y -> Z) [f]([g](fx)) = [f . g](fx)

• Contravariant {F}

  • include Parametric {F}
  • definitions
    • comap : ∀ a b . (b -> a) -> (F a -> F b)
  • laws (category morphism)

• Apply {F}

  • include Covariant {F}
  • notation
    • When unambiguous, f * x is written to mean ap(f, x).
  • definitions
    • ap : ∀ a b . F (a -> b) -> (F a -> F b)
    • smash : ∀ a b . (F a, F b) -> F {fst: a, snd: b}
      • smashWith((fst, snd) => {fst, snd})
    • smashWith : ∀ a b c . ((a, b) -> c) -> (F a, F b) -> F c
      • λ(f) . λ(fa, fb) . map(x => f(x.fst, x.snd))(smash(fa, fb))
  • laws
    • composition reassociates: for all (X Y Z : Type), Decidable {F Z}, (f : F (Y -> Z)), (g : F (X -> Y)), (a : F x), we let ([.](f) * g) * a = f * (g * a).
  • notes
    • can be used to traverse non-empty structures

• Applicative {F}

  • include Apply {F}
  • defintions
    • of : ∀ a . a -> F a
    • traversals of "container-like" structures
      • traverseArray : ((a_i, i) -> F b_i) -> ([a_i] -> F [b_i])
      • traverseDict : ((a_i, k_i) -> F b_i) -> (Obj_i (k_i : a_i) -> F (Obj_i (k_i : b_i)))
  • notation
    • Where unambiguous, of(x) is written {x}
  • laws
    • identity reflection: {id} * v = v
    • application reflection: {f} * {x} = {f(x)}
    • interchange: u * {y} = {f => f(y)} * u

• Bind {F}

  • include Apply {F}
  • definitions
    • bind : ∀ a b . (a -> F b) -> (F a -> F b)
    • seq : ∀ a b . (F a, F b) -> F b
      • λ(fa, fb) . bind(_ => fb)(fa)
    • kseq : ∀ a b . (a -> F b, b -> F c) -> (a F c)
      • λ(afb, bfc) . λa . bind(bfc, afb(a))
    • collapse : ∀ a . F (F a) -> F a
      • bind(id)
  • notation
    • Where unambiguous bind(k, m) is written m >>= k
    • Where unambiguous seq(m1, m2) is written m1 >> m2
    • Where unambiguous kseq(f1, f2) is written f1 >=> f2
  • projects
    • Apply {F} via
      • define ap(mf, mx) = mf >>= (f => [f](mx))
  • laws
    • associativity: f >=> (g >=> h) = (f >=> g) >=> h

• Monad {F}

  • include (compatibly)
    • Bind {F}
    • Applicative {F}
  • laws
    • left identity: of >=> f = f
    • right identity: f >=> of = f

• Foldable {F}

  • include Covariant {F}
  • definitions
    • foldMap : ∀ a x . (Monoid {x}, a -> x) -> (F a -> x)
    • toArray : ∀ x . F x -> [x]
      • defined using the universal Monoid {[X]} available for all X
    • forEach : ∀ x . (F x, x -> ()) -> ()
      • defined by passing through arrays
    • reduceLeft
      • defined by passing through arrays
    • reduceRight
      • defined by passing through arrays
  • laws
    • "seek and find": for all a : Type, all functions F a -> a?, and all values x : F a, if f(x) exists then f(x) is in the array toArray(x)

• Traversable {F}

  • include Foldable {F}
  • definitions
    • traverse : ∀ a b k . (Applicative {k}, a -> k b) -> (F a -> t (F b))
  • laws
    • See Haskell documentation

Container-like Theories

• Cons {F}
  • include Foldable {F}
  • definitions
    • cons : ∀ a . (a, F a) -> F a
  • laws
    • "place and find": for all a : Type with Decidable {a}, for all c : F a and x : a, we have contains(x, toArray(cons(x, c))

References

• tel/ocaml-cats: An attempt to bring statically typed algebraic interfaces to OCaml.

• fantasyland/fantasy-land: A specification of objects which provide suitable algebraic operations. It's another, somewhat popular point in the design space of algebraic FP in Javascript.

• Ramda.js: An implementation of some algebraic and some higher-order functional techniques via object-oriented Javascript.

• plaid/sanctuary: An implementation of more advanced algebraic data types and associated functionality written For compatibility with Ramda.

License

Copyright 2015, Reify Health (<joseph@reifyhealth.com>)

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.