src/Rel8/Order.hs · rel8

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rel8/src/Rel8/Order.hs

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1{-# language DerivingStrategies #-}

2{-# language GeneralizedNewtypeDeriving #-}

3{-# language StandaloneKindSignatures #-}

5module Rel8.Order

6 ( Order(..)

7 )

8where

10-- base

11import Data.Functor.Contravariant ( Contravariant )

12import Data.Kind ( Type )

13import Prelude

15-- contravariant

16import Data.Functor.Contravariant.Divisible ( Decidable, Divisible )

18-- opaleye

19import qualified Opaleye.Internal.Order as Opaleye

22-- | An ordering expression for @a@. Primitive orderings are defined with 'asc'

23-- and 'desc', and you can combine @Order@ via its various instances.

25-- A common pattern is to use '<>' to combine multiple orderings in sequence,

26-- and '>$<' (from 'Contravariant') to select individual columns. For example,

27-- to sort a @Query@ on two columns, we could do:

29-- >>> import Data.Functor.Contravariant ((>$<))

30-- >>> :{

31-- select c $ orderBy (mconcat [fst >$< asc, snd >$< desc]) $ do

32-- x <- values [ lit x | x <- [1..3 :: Int32 ] ]

33-- y <- values [ lit x | x <- [1..3 :: Int32 ] ]

34-- return (x, y)

35-- :}

36-- [(1,3),(1,2),(1,1),(2,3),(2,2),(2,1),(3,3),(3,2),(3,1)]

37type Order :: Type -> Type

38newtype Order a = Order (Opaleye.Order a)

39 deriving newtype (Contravariant, Divisible, Decidable, Semigroup, Monoid)