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

Modulenewtype-generics-0.6.2Haskell2010

Control.Newtype.Generics

The Newtype typeclass and related functions. Primarily pulled from Conor McBride's Epigram work. Some examples:

Example1 expression
ala Sum foldMap [1,2,3,4]10
Example1 expression
ala Endo foldMap [(+1), (+2), (subtract 1), (*2)] 38
Example1 expression
under2 Min (<>) 2 11
Example1 expression
over All not (All False)All {getAll = True)

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. You can easily define new instances for your newtypes with the help of GHC.Generics

{-# LANGUAGE DeriveGeneric #-}
import GHC.Generics

(...)
newtype Example = Example Int
  deriving (Generic)

instance Newtype Example

This avoids the use of Template Haskell (TH) to get new instances.

  • 1 class
  • 9 values
classclass Newtype n where
#

As long as the type n is an instance of Generic, you can create an instance with just instance Newtype n

Associated types

  • type family O n

Methods

Instances25Newtype, …
  • Newtype AllDefined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype AnyDefined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype (First a)Defined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype (Last a)Defined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype (Max a)Defined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype (Min a)Defined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype (WrappedMonoid m)Defined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype (Identity a)Defined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype (First a)Defined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype (Last a)Defined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype (Down a)Defined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype (Dual a)Defined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype (Endo a)Defined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype (Product a)Defined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype (Sum a)Defined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype (ZipList a)Defined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype (WrappedMonad m a)Defined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype (Fixed a)Defined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype (ArrowMonad a b)Defined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype (WrappedArrow a b c)Defined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype (Kleisli m a b)Defined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype (Const a x)Defined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype (Ap f a)Defined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype (Alt f a)Defined in newtype-generics-0.6.2 · Control.Newtype.Generics
  • Newtype (Compose f g a)Defined in newtype-generics-0.6.2 · Control.Newtype.Generics
valueop :: (Newtype n, o ~ O n) => (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.

Example1 expression
op Identity (Identity 3)3
valueala
  1. :: (Newtype n, Newtype n', o' ~ O n', o ~ O n)
  2. => o -> n
  3. -> (o -> n) -> b -> n'
  4. -> b
  5. -> o'
#

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

The reason for the signature of the hof is due to ala not caring about structure. To illustrate why this is important, consider this alternative implementation of under2:

under2 :: (Newtype n, Newtype n', o' ~ O n', o ~ O n)
       => (o -> n) -> (n -> n -> n') -> (o -> o -> o')
under2' pa f o0 o1 = ala pa (\p -> uncurry f . bimap p p) (o0, o1)

Being handed the "packer", the hof may apply it in any structure of its choosing – in this case a tuple.

Example1 expression
ala Sum foldMap [1,2,3,4]10
valueala'
  1. :: (Newtype n, Newtype n', o' ~ O n', o ~ O n)
  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.

Example1 expression
ala' Sum foldMap length ["hello", "world"]10
Example1 expression
ala' First foldMap (readMaybe @Int) ["x", "42", "1"]Just 42
valueunder
  1. :: (Newtype n, Newtype n', o' ~ O n', o ~ O n)
  2. => o -> n
  3. -> n -> n'
  4. -> o
  5. -> o'
#

A very simple operation involving running the function 'under' the newtype.

Example1 expression
under Product (stimes 3) 327
valueover
  1. :: (Newtype n, Newtype n', o' ~ O n', o ~ O n)
  2. => o -> n
  3. -> o -> o'
  4. -> n
  5. -> 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.

Example1 expression
over All not (All False)All {getAll = True}
valueunder2
  1. :: (Newtype n, Newtype n', o' ~ O n', o ~ O n)
  2. => o -> n
  3. -> n -> n -> n'
  4. -> o
  5. -> o
  6. -> o'
#

Lower a binary function to operate on the underlying values.

Example1 expression
under2 Any (<>) True FalseTrue