isNothing

The isNothing function returns True iff its argument is Nothing.

Examples

Basic usage:

>>> isNothing (Just 3)
False

>>> isNothing (Just ())
False

>>> isNothing Nothing
True

Only the outer constructor is taken into consideration:

>>> isNothing (Just Nothing)
False

Yields True iff the argument is Nothing.

IS NULL comparison. For IS NOT NULL, you can negate this with not_, as in not_ (isNothing (person ^. PersonAge)) Warning: Persistent and Esqueleto have different behavior for != Nothing: TODO: table In SQL, = NULL and != NULL return NULL instead of true or false. For this reason, you very likely do not want to use !=. Nothing in Esqueleto. You may find these hlint rules helpful to enforce this:

- error: {lhs: v Database.Esqueleto.==. Database.Esqueleto.nothing, rhs: Database.Esqueleto.isNothing v, name: Use Esqueleto's isNothing}
- error: {lhs: v Database.Esqueleto.==. Database.Esqueleto.val Nothing, rhs: Database.Esqueleto.isNothing v, name: Use Esqueleto's isNothing}
- error: {lhs: v Database.Esqueleto.!=. Database.Esqueleto.nothing, rhs: not_ (Database.Esqueleto.isNothing v), name: Use Esqueleto's not isNothing}
- error: {lhs: v Database.Esqueleto.!=. Database.Esqueleto.val Nothing, rhs: not_ (Database.Esqueleto.isNothing v), name: Use Esqueleto's not isNothing}

Check if the symbolic value is nothing.

>>> isNothing (sNothing :: SMaybe Integer)
True

>>> isNothing (sJust (literal "nope"))
False

Operator corresponding SQL IS NULL , and extended against record types.

Convert between Nothing and True (see isNothing). (not . isJust)

Operator corresponding SQL IS NULL , and extended against record types.

Test that a Maybe is Nothing.

Analogous to isNothing, but for MaybeT

An alias for isNothing that avoids clashing with the function from Data.Maybe isNothing.

Returns a QGenExpr that evaluates to true when the first argument is null

Type class for types which can be safely cast to NothingAction, for instance with toNothingAction.