The newtype-generics package

[ Tags: bsd3, control, library ] [ Propose Tags ]

Per Conor McBride, the Newtype typeclass represents the packing and unpacking of a newtype, and allows you to operate under that newtype with functions such as ala. Generics support was added in version 0.4, making this package a full replacement for the original newtype package, and a better alternative to newtype-th.

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Versions 0.4,, 0.4.1, 0.4.2, 0.5,, 0.5.1, 0.5.2,
Change log
Dependencies base (>=4.6 && <4.11), transformers [details]
License BSD3
Author Darius Jahandarie, Conor McBride, João Cristóvão
Maintainer Simon Jakobi <>
Category Control
Source repo head: git clone
Uploaded Fri Feb 16 02:47:32 UTC 2018 by sjakobi
Distributions Debian:0.4.1, LTSHaskell:, NixOS:0.5.1, Stackage:, openSUSE:0.5.1
Downloads 9617 total (240 in the last 30 days)
Rating 2.0 (votes: 1) [estimated by rule of succession]
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Status Docs available [build log]
Last success reported on 2018-02-16 [all 1 reports]
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Readme for newtype-generics-

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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