The Semialign typeclass lets us line up two structures of the same type. We can use this to merge two structures, or add additional typeclasses to do filtering, Applicative effects, or tracking indices.

merge is the simplest function. It takes five arguments: functions to handle values present in the left / right / both structures and two structures to merge.

merge :: Semialign t
  => (a -> c) -> (b -> c) -> (a -> b -> c)
  -> t a -> t b
  -> t c

Every other function in this module is a variant with modified functionality based on its name:

• Prefix i means an "indexed" variant: each function argument takes an additional index (i) parameter.
• Suffix Maybe means "filter results": each function argument returns Maybe c, and Nothings are filtered out.
• Suffix A means "applicative": each function argument returns actions in some Applicative f, and the whole operation collects these actions to produce the a merged structure in f.