ramda-fantasy/test/types.js

var interfaces = {
  semigroup:      ['concat'],
  monoid:         ['concat', 'empty'],
  functor:        ['map'],
  apply:          ['map', 'ap'],
  applicative:    ['map', 'ap', 'of'],
  chain:          ['map', 'ap', 'chain'],
  monad:          ['map', 'ap', 'chain', 'of'],
  extend:         ['extend']
};

function correctInterface(type) {
  return function(obj) {
    return interfaces[type].every(function(method) {
      return obj[method] && typeof obj[method] === 'function';
    });
  };
}

function identity(x) { return x; }

module.exports = {
  semigroup: {
    iface: correctInterface('semigroup'),
    associative: function(a, b, c) {
      return a.concat(b).concat(c).equals(a.concat(b.concat(c)));
    }
  },

  functor: {
    iface: correctInterface('functor'),
    id: function(obj) {
      return obj.equals(obj.map(identity));
    },
    compose: function(obj, f, g) {
      return obj.map(function(x) { return f(g(x)); }).equals(
        obj.map(g).map(f)
      );
    }
  },

  apply: {
    iface: correctInterface('apply'),
    compose: function(a, u, v) {
      return a.ap(u.ap(v)).equals(
        a.map(function(f) {
          return function(g) {
            return function(x) {
              return f(g(x));
            };
          };
        }).ap(u).ap(v)
      );
    }
  },

  applicative: {
    iface: correctInterface('applicative'),
    id: function(obj, obj2) {
      return obj.of(identity).ap(obj2).equals(obj2);
    },
    homomorphic: function(obj, f, x) {
      return obj.of(f).ap(obj.of(x)).equals(obj.of(f(x)));
    },
    interchange: function(obj1, obj2, x) {
      return obj2.ap(obj1.of(x)).equals(
        obj1.of(function(f) { return f(x); }).ap(obj2)
      );
    }
  },

  chain: {
    iface: correctInterface('chain'),
    associative: function(obj, f, g) {
      return obj.chain(f).chain(g).equals(
        obj.chain(function(x) { return f(x).chain(g); })
      );
    }
  },

  monad: {
    iface: correctInterface('monad')
  },

  extend: {
    iface: correctInterface('extend')
  }
};