src/Rel8/Query/Null.hs · rel8

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1module Rel8.Query.Null

2 ( catNullable

3 )

4where

6-- base

7import Prelude

9-- rel8

10import Rel8.Expr ( Expr )

11import Rel8.Expr.Null ( isNonNull, unsafeUnnullify )

12import Rel8.Query ( Query )

13import Rel8.Query.Filter ( where_ )

16-- | Filter a 'Query' that might return @null@ to a 'Query' without any

17-- @null@s.

19-- Corresponds to 'Data.Maybe.catMaybes'.

20--

21-- >>> select c $ pure (nullExpr :: Expr (Maybe Bool))

22-- [Nothing]

23--

24-- >>> select c $ catNullable (nullExpr :: Expr (Maybe Bool))

25-- []

26--

27-- >>> select c $ catNullable (lit (Just True))

28-- [True]

29--

30-- Notice how in the last example a @Bool@ is returned (rather than @Maybe

31-- Bool@):

32--

33-- >>> :t catNullable (lit (Just True))

34-- catMaybe (lit (Just True)) :: Query (Expr Bool)

35catNullable :: Expr (Maybe a) -> Query (Expr a)

36catNullable a = do

37 where_ $ isNonNull a

38 pure $ unsafeUnnullify a