RFC: Nomenclature for fold functions

[harendra-kumar]

Currently we have foldl and foldlM functions whose signatures are more general than the standard ones and inline with the foldl library. It means that they are slightly different from the standard fold functions. To keep full compatibility with the nomenclature of standard lists and Foldable we should perhaps use different names for the functions created for the purposes of foldl.

The names foldl, foldl', foldl1 are all taken. The name fold is used by Foldable typeclass. If we are not going to have a Foldable instance we can use fold, foldM for these general functions. Note that we also have scan that is a general version of scanl.

[harendra-kumar]

Instead of making it a breaking change we can also deprecate the old function names.

[harendra-kumar]

We should perhaps use foldl' for strict left fold to be inline with Foldable. Do we need a lazy left fold?

[harendra-kumar]

It might make sense to provide a fold and a foldMap function in line with the Foldable typeclass. So those names should probably not be reused.

How about foldx and foldxM where the x can be thought of as a mnemonic for extraction function required in the foldl library compatible fold functions?