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GHC 9.10.3 · lts/ghc-9.10.x · c74966e · 2026-09-27

Modulenewtype-0.2.2.0Haskell2010

Control.Newtype

SPDX-License-Identifier : BSD-3-Clause

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

Example1 expression
ala Sum foldMap [1,2,3,4] -- foldMaps the list ala the Sum newtype10
Example1 expression
ala Product foldMap [1,2,3,4] -- foldMaps the list ala the Product newtype24
Example1 expression
ala Endo foldMap [(+1), (+2), (subtract 1), (*2)] 3 -- foldMaps the list ala the Endo newtype8

NB: 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.

  • 1 class
  • 7 values
  • Packagenewtype-0.2.2.0
  • Exports8
  • LanguageHaskell2010
  • LicenceBSD-3-Clause
  • SourceNewtype.hs
classclass Newtype n o | n -> o where
#

Given a newtype n, we will always have the same unwrapped type o, meaning we can represent this with a fundep n -> o.

Any instance of this class just needs to let pack equal to the newtype's constructor, and let unpack destruct the newtype with pattern matching.

Starting with newtype-0.2.2.0, default method implementations are provided using Data.Coerce for GHC 7.8 (i.e. base-4.7.0.0) and later, i.e.:

pack   = coerce
unpack = coerce

When omitting the method definitions with GHC 7.4 and 7.6 a compile error will be triggered.

Consequently, if your code relies on these default methods make sure to state

build-depends: newtype ^>= 0.2.2.0

In your .cabal package description.

Methods

Instances20Newtype, …
  • Newtype All BoolDefined in newtype-0.2.2.0 · Control.Newtype
  • Newtype Any BoolDefined in newtype-0.2.2.0 · Control.Newtype
  • Newtype (Identity a) aDefined in newtype-0.2.2.0 · Control.Newtype
  • Newtype (Down a) aDefined in newtype-0.2.2.0 · Control.Newtype

    NOTE: Type & instance only available with base ≥ 4.6.0

  • Newtype (Dual a) aDefined in newtype-0.2.2.0 · Control.Newtype
  • Newtype (Product a) aDefined in newtype-0.2.2.0 · Control.Newtype
  • Newtype (Sum a) aDefined in newtype-0.2.2.0 · Control.Newtype
  • Newtype (First a) (Maybe a)Defined in newtype-0.2.2.0 · Control.Newtype
  • Newtype (Last a) (Maybe a)Defined in newtype-0.2.2.0 · Control.Newtype
  • Newtype (ZipList a) [a]Defined in newtype-0.2.2.0 · Control.Newtype
  • Newtype (Endo a) (a -> a)Defined in newtype-0.2.2.0 · Control.Newtype
  • Newtype (Fixed a) IntegerDefined in newtype-0.2.2.0 · Control.Newtype
  • Newtype (WrappedMonad m a) (m a)Defined in newtype-0.2.2.0 · Control.Newtype
  • ArrowApply a => Newtype (ArrowMonad a b) (a () b)Defined in newtype-0.2.2.0 · Control.Newtype
  • Newtype (Const a x) aDefined in newtype-0.2.2.0 · Control.Newtype
  • Newtype (Ap f a) (f a)Defined in newtype-0.2.2.0 · Control.Newtype

    NOTE: Type & instance only available with base ≥ 4.12.0

  • Newtype (Alt f a) (f a)Defined in newtype-0.2.2.0 · Control.Newtype

    NOTE: Type & instance only available with base ≥ 4.8.0

  • Newtype (WrappedArrow a b c) (a b c)Defined in newtype-0.2.2.0 · Control.Newtype
  • Newtype (Kleisli m a b) (a -> m b)Defined in newtype-0.2.2.0 · Control.Newtype
  • Newtype (Compose f g a) (f (g a))Defined in newtype-0.2.2.0 · Control.Newtype
valueop :: Newtype n o => (o -> n) -> n -> o
#

This function serves two purposes:

  1. Giving you the unpack of a newtype without you needing to remember the name.

  2. Showing that the first parameter is completely ignored on the value level, meaning the only reason you pass in the constructor is to provide type information. Typeclasses sure are neat.

valueala
  1. :: (Newtype n o, Newtype n' o')
  2. => o -> n
  3. -> (o -> n) -> b -> n'
  4. -> b
  5. -> o'
#

The workhorse of the package. Given a pack and a "higher order function", it handles the packing and unpacking, and just sends you back a regular old function, with the type varying based on the hof (higher-order function) you passed.

The reason for the signature of the hof is due to ala not caring about structure. To illustrate why this is important, another function in this package is under. It is not extremely useful; under2 might be more useful (with e.g., mappend), but then we already digging the trench of "What about under3? under4?". The solution utilized here is to just hand off the "packer" to the hof. That way your structure can be imposed in the hof, whatever you may want it to be (e.g., List, Traversable).

valueala'
  1. :: (Newtype n o, Newtype n' o')
  2. => o -> n
  3. -> (a -> n) -> b -> n'
  4. -> a -> o
  5. -> b
  6. -> o'
#

This is the original function seen in Conor McBride's work. The way it differs from the ala function in this package, is that it provides an extra hook into the "packer" passed to the hof. However, this normally ends up being id, so ala wraps this function and passes id as the final parameter by default. If you want the convenience of being able to hook right into the hof, you may use this function.

valueunder :: (Newtype n o, Newtype n' o') => (o -> n) -> (n -> n') -> o -> o'
#

A very simple operation involving running the function "under" the newtype. Suffers from the problems mentioned in the ala function's documentation.

valueover :: (Newtype n o, Newtype n' o') => (o -> n) -> (o -> o') -> n -> n'
#

The opposite of under. I.e., take a function which works on the underlying types, and switch it to a function that works on the newtypes.