What does it actually mean Left and Right in sanctuary

Question

> S.reduce(S.flip(S.K),[],S.Left([1,2]))
[]
> S.reduce(S.flip(S.K),[],S.Right([1,2]))
[ 1, 2 ]

I was trying to understand sanctuary and its working can anybody explain the above outcomes in detail In my understanding S.reduce takes a mapping function and uses the input array which should be of type either and reduces it But why its empty array in first case and same array in second case

Answer 1

Let's start with a more familiar example: using S.reduce on an array.

> S.reduce (acc => s => acc + s) ('initial:') (['foo', 'bar', 'baz'])
'initial:foobarbaz'

Now, let's specialize the type of S.reduce to explain the behaviour above.

S.reduce :: Foldable f => (b -> a -> b) -> b -> f a -> b

-- replace ‘Foldable f => f’ with ‘Array’ --

S.reduce :: (b -> a -> b) -> b -> Array a -> b

Next, let's specialize the type of S.reduce to see how it will operate on Either values.

S.reduce :: Foldable f => (b -> a -> b) -> b -> f a -> b

-- replace ‘Foldable f => f’ with ‘Either x’ --

S.reduce :: (b -> a -> b) -> b -> Either x a -> b

What can we do when given S.Left ('foo') as the Either x a? We have an x ('foo') but no a. Because we don't have an a we cannot make use of the b -> a -> b function. Thus, the only b we can possibly return is the initial value.

> S.reduce (acc => s => acc + s) ('initial:') (S.Left ('foo'))
'initial:'

What can we do when given S.Right ('bar') as the Either x a? We have an a, which we could feed to the b -> a -> b function along with the initial value to produce another value of type b.

> S.reduce (acc => s => acc + s) ('initial:') (S.Right ('bar'))
'initial:bar'

If S.reduce were to return 'initial:' or 'initial:bar:bar:bar' in the case above it would still be conforming to the type signature, but the fantasy-land/reduce implementation for Sanctuary's Either type applies the function exactly once when given a Right.