Dart Fold vs Reduce

Question

What's the difference between fold and reduce in Dart and when would I use one as opposed to the other? They seem to do the same thing according to the docs.

Reduces a collection to a single value by iteratively combining each element of the collection with an existing value using the provided function.

Answer 1

reduce can only be used on non-empty collections with functions that returns the same type as the types contained in the collection.

fold can be used in all cases.

For instance you cannot compute the sum of the length of all strings in a list with reduce. You have to use fold :

final list = ['a', 'bb', 'ccc'];
// compute the sum of all length
list.fold(0, (t, e) => t + e.length); // result is 6

By the way list.reduce(f) can be seen as a shortcut for list.skip(1).fold(list.first, f).

Answer 2

There are a few noticeable differences between them. Other than the ones mentioned above, it is worth highlighting that fold() is able to operate on collections that are empty without producing an error.

reduce() will throw an error saying Bad state: No element whereas fold() will return a non-null value back, using the initial value passed onto it as a fallback return value.

I've discussed about this at length here:

https://medium.com/@darsshanNair/demystifying-fold-in-dart-faacb3bd4efd