cond

Using cond in functional pipelines (5 min. read)

Sometimes we have too many conditions, making switch statements a great choice.

const findAnimal = (animal) => {
  switch (animal) {
    case 'lion':
      return 'Africa and India';
      
    case 'tiger':
      return 'China, Russia, India, Vietnam, and many more';
      
    case 'hyena':
      return 'African Savannah'
      
    case 'grizzly bear':
      return 'North America';
      
    default:
      return 'Not sure, try Googling it!'
  }
};
console.log(findAnimal('cow'));

We could mimic this with pipe and when, but coding for a default case is tough.

import { always, equals, ifElse, pipe, when } from 'ramda';
const findAnimal = pipe(
  when(equals('lion'), always('Africa and India')),
  when(equals('tiger'), always('China, Russia, India, Vietnam, and many more')),
  when(equals('hyena'), always('African Savannah')),
  when(equals('grizzly bear'), always('North America')),
);
console.log(findAnimal('cow'));

Or we can use cond, which is built into languages like Lisp. It takes an array of if/then statements, which are arrays themselves.

The first function is the predicate, and the second function is what to run if the predicate returns true.

Here’s an example

import { always, cond, equals } from 'ramda';
const findAnimal = cond([
  [equals('lion'), always('Africa and India')],
  [equals('tiger'), always('China, Russia, India, Vietnam, and many more')],
  [equals('hyena'), always('African Savannah')],
  [equals('grizzly bear'), always('North America')]
]);
console.log(findAnimal('lion'));

It runs through each array. If the array’s first function returns true, the array’s second function is called and the logic’s cut off.

But how do we support the default case?

default:
      return 'Not sure, try Googling it!';

Have a function that always returns true at the very end. If nothing else runs, it will succeed and be called!

import { always, cond, equals } from 'ramda';
const findAnimal = cond([
  [equals('lion'), always('Africa and India')],
  [equals('tiger'), always('China, Russia, India, Vietnam, and many more')],
  [equals('hyena'), always('African Savannah')],
  [equals('grizzly bear'), always('North America')],
  [always(true), always('Not sure, try Googling it!')]
]);
console.log(findAnimal('cow'));