src/Rel8/Query/Filter.hs · rel8

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rel8/src/Rel8/Query/Filter.hs

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1module Rel8.Query.Filter

2 ( filter

3 , where_

4 )

5where

7-- base

8import Prelude hiding ( filter )

10-- opaleye

11import qualified Opaleye.Operators as Opaleye

13-- profunctors

14import Data.Profunctor ( lmap )

16-- rel8

17import Rel8.Expr ( Expr )

18import Rel8.Expr.Opaleye ( toColumn, toPrimExpr )

19import Rel8.Query ( Query )

20import Rel8.Query.Opaleye ( fromOpaleye )

23filter :: (a -> Expr Bool) -> a -> Query a

24filter f a = a <$ where_ (f a)

27-- | Drop any rows that don't match a predicate. @where_ expr@ is equivalent

28-- to the SQL @WHERE expr@.

30-- >>> :{

31-- select c $ do

32-- x <- values [ lit x | x <- [ 1..5 :: Int32 ] ]

33-- where_ $ x >. lit 2

34-- return x

35-- :}

36-- [3,4,5]

37where_ :: Expr Bool -> Query ()

38where_ condition =

39 fromOpaleye $ lmap (\_ -> toColumn $ toPrimExpr condition) Opaleye.restrict