You want mixfix? You got it!

fix.js

function _(x) {
  if (x instanceof _) return new _(x._);
  if (!(this instanceof _)) return new _(x);
  this._ = x;
}

_.mixfix = function mixfix(pattern, f) {
  const [name, ...names] = pattern.split('_');

  let startIndex = name === '' ? 1 : 0;
  let endIndex = pattern[pattern.length-1] === '_' ? -1 : void 0;

  _[pattern.slice(startIndex, endIndex)] = f;

  if (name === '') {
    names.length > 1
      ? Object.defineProperty(_.prototype, names[0], {
        enumerable: true,
        get() {
          return x => go(names.slice(1), new _(f (this._) (x)));
        }
      })
    : Object.defineProperty(_.prototype, names[0], {
      enumerable: true,
      get() {
        return new _(f(this._));
      }
    });
  } else {
    _[name] = x => go(names, new _(f(x)));
  }
  return _;

  function go([name, ...names], parent) {
    if (name !== '') Object.defineProperty(parent, name, {
      get() {
        return x => go(names, new _(this._(x)));
      }
    });
    return parent;
  }
};

_._extend = function extend(lib) {
  Object.keys(lib).forEach(k => {
    if(lib[k].length < 1 || k[k.length-1] === '_') {
      _[k] = lib[k];
    } else if (lib[k].length === 1) {
      _.mixfix(`_${k}`, lib[k]);
    } else {
      _.mixfix(`_${k}_`, lib[k]);
    }
  });
  return _;
};

_.mixfix('if_then_else', t => c => a => t ? c() : a());

_.if_then_else (true) (() => 'and...') (() => 'no, really. what\'s your point?'); // "and..."

_.if(true)
  .then(() => 'you\'re crazy!')
  .else(() => 'like a fox')._;

_.mixfix('_and_', x => y => x && y);

_.and (true) (false); // false

_(true).and(true)._ // true