Their Variations

Once you get how Foldable and Traversable works, the following variations should be pretty easy to grasp

Variants that include the a value's index

The same as their base type classes, but an additional Int argument represent the a's index in f is included in the map/fold/traverse functions.

• FunctorWithIndex
• FoldableWithIndex
• TraversableWithIndex

Variants where the f cannot be empty

The same as their base type classes, but the f type must always have at least 1 a value. As a result, the Applicative requirement in the type class' functions can be downgraded to just Apply:

• Foldable1
• Traversable1

Variants where the f can have 2 types

The same as their base type classes, but as though it was f (Tuple a b) rather than f a:

• Bifoldable
• Bitraversable

Note: as of 0.15.7, a type class instance for BiFoldable and BiTraversable can be derived by the compiler.