kontrakt

A functional contract library for Javascript.

Even though examples on this page will use ES6 features for brevity, this library does not require it and is backwards compatible with earlier versions.

Installation

npm install kontrakt --save

Basic use

Call kontrakt(opts)(fn) to get a wrapper function that will inject constraints into any given function.

var kontrakt = require('kontrakt'),
    fn = function(x) { return x + 2 }

var fn2 = kontrakt({
    pre: function(x) {
      return x < 2
    },
    post: function(x, y) {
      return y > 0
    }
})(fn)

fn2(0) // works
fn2(-3) // throws a kontrakt.CalleeViolation
fn2(4)  // throws a kontrakt.CallerViolation

Constraint functions should be pure functions that return boolean values. A falsy return value from a constraint function is interpreted as a contract violation, and a suitable error will be thrown. Errors thrown from constraints are either instances of CalleeViolation or CallerViolation, indicating respectively that either the calling code or called function is to blame.

Both pre- and post-constraints are optional. A pre-constraint receives all arguments given in the function call as parameters. A post-constraint will receive the function return value as the first argument, followed by original function call parameters.

Combinators

A set of combinators is included in kontrakt/combinators module. These include the most basic combinators:

The and() combinator subjects all argument constraint return values to the boolean AND operation and returns the result. or() combinator works in similar fashion, using a boolean OR operator instead. Both combinators accept 1-N constraint functions as arguments.

The not() combinator inverts the return value from any one constraint function.

const kontrakt = require('kontrakt'),
    { and, or } = require('kontrakt/combinators'),
    gt = (x) => (y) => x > y,
    lt = (x) => (y) => x > y,
    fn = (x) => x + 2

const fn2 = kontrakt({
    pre: and(
      gt(2),
      lt(5)
    ),
    post: or(
      lt(-2),
      gt(10)
    )
})(fn)