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 )

23-- | @filter f x@ will be a zero-row query when @f x@ is @False@, and will

24-- return @x@ unchanged when @f x@ is @True@. This is similar to

25-- 'Control.Monad.guard', but as the predicate is separate from the argument,

26-- it is easy to use in a pipeline of 'Query' transformations.

28-- >>> select c $ values [ lit x | x <- [ 1..5 :: Int32 ] ] >>= filter (>. 3)

29-- [4,5]

30filter :: (a -> Expr Bool) -> a -> Query a

31filter f a = a <$ where_ (f a)

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

35-- to the SQL @WHERE expr@.

37-- >>> :{

38-- select c $ do

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

40-- where_ $ x >. lit 2

41-- return x

42-- :}

43-- [3,4,5]

44where_ :: Expr Bool -> Query ()

45where_ condition =

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