Modernizing Haskell 98

The introduction of Applicative, along with Foldable and Traversable, have had a big impact on Haskell.

The Foldable and Traversable type­classes lift the Prelude fold and map functions to a much higher level of abstraction. Moreover, Foldable and Traversable also bring a clean separation between processes that preserve or discard the shape of the structure being processed:

• Traversable describes a process that preserves the shape of the data structure being traversed over

• Foldable discards (or transforms) the shape of the structure being folded over

Since Traversable is a specialization of Foldable, we can say that shape preservation is a special case of shape transformation. This line between shape preservation and transformation is clearly visible from the fact that functions that discard their result (for example, mapM_, forM_, sequence_, and so on) are in Foldable, while their shape-preserving counterparts are in Traversable.

Due to the relatively late introduction...