cond

Using cond in functional pipelines (5 min. read)

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

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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.

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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

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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?

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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!

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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'));