* package:hledger-web

Assign a field by multiplication (*=).

Examples

multiplyAge :: MonadIO m => ReaderT SqlBackend m ()
multiplyAge = updateWhere [UserName ==. "SPJ" ] [UserAge *=. 2]

The above query when applied on dataset-1, will produce this:

+-----+-----+--------+
|id   |name |age     |
+-----+-----+--------+
|1    |SPJ  |40 -> 80|
+-----+-----+--------+
|2    |Simon|41      |
+-----+-----+--------+

Sequence actions, discarding the value of the first argument.

Examples

If used in conjunction with the Applicative instance for Maybe, you can chain Maybe computations, with a possible "early return" in case of Nothing.

>>> Just 2 *> Just 3
Just 3

>>> Nothing *> Just 3
Nothing

Of course a more interesting use case would be to have effectful computations instead of just returning pure values.

>>> import Data.Char

>>> import Text.ParserCombinators.ReadP

>>> let p = string "my name is " *> munch1 isAlpha <* eof

>>> readP_to_S p "my name is Simon"
[("Simon","")]

Sequence actions, discarding the value of the second argument.

Sequential application. A few functors support an implementation of <*> that is more efficient than the default one.

Example

Used in combination with (<$>), (<*>) can be used to build a record.

>>> data MyState = MyState {arg1 :: Foo, arg2 :: Bar, arg3 :: Baz}

>>> produceFoo :: Applicative f => f Foo

>>> produceBar :: Applicative f => f Bar

>>> produceBaz :: Applicative f => f Baz

>>> mkState :: Applicative f => f MyState

>>> mkState = MyState <$> produceFoo <*> produceBar <*> produceBaz