newtype-generics

A typeclass and set of functions for working with newtypes. Fork of the code published by Darius Jahandarie here, with the addition of generics.

The ‘Newtype’ typeclass and related functions: op, ala, ala', under. Primarly pulled from Conor McBride’s Epigram work. Some examples:

-- foldMaps the list ala the Sum newtype. This results in 10.
ala Sum foldMap [1,2,3,4] 

-- foldMaps the list ala the Product newtype. This results in 24.
ala Product foldMap [1,2,3,4] 

-- foldMaps the list ala the Endo newtype. This results in 8.
ala Endo foldMap [(+1), (+2), (subtract 1), (*2)] 3 

NB: Data.Foldable.foldMap is a generalized mconcatMap which is a generalized concatMap.

This package includes Newtype instances for all the (non-GHC/foreign) newtypes in base (as seen in the examples). However, there are neat things you can do with this with /any/ newtype and you should definitely define your own ‘Newtype’ instances for the power of this library. For example, see ala Cont traverse, with the proper Newtype instance for Cont.

This could of course be eased with the addition of generics for version 0.3:

{-# LANGUAGE DeriveGeneric              #-}

import GHC.Generics
(...)
newtype Example = Example Int (deriving Generic)
instance Newtype Example