modify package:relude

Monadic state transformer. Maps an old state to a new state inside a state monad. The old state is thrown away.

Main> :t modify ((+1) :: Int -> Int)
modify (...) :: (MonadState Int a) => a ()

This says that modify (+1) acts over any Monad that is a member of the MonadState class, with an Int state.

Strict version of modifyTVar.

Lifted version of modifyIORef.

>>> ref <- newIORef 42

>>> modifyIORef ref (\a -> a + 6)

>>> readIORef ref
48

• To avoid space-leaks, see modifyIORef' for stricter updates
• For atomic updates, see atomicModifyIORef

Lifted version of modifyIORef'.

>>> ref <- newIORef 42

>>> modifyIORef' ref (\a -> a + 3)

>>> readIORef ref
45

• For lazier updates, see modifyIORef
• For atomic updates, see atomicModifyIORef'

A variant of modify in which the computation is strict in the new state.

Lifted version of atomicModifyIORef.

>>> ref <- newIORef 42

>>> atomicModifyIORef ref (\a -> (a, a + 3))
45

>>> readIORef ref
42

• To avoid space-leaks, see atomicModifyIORef' for stricter updates
• If you are not interested in the return value, see atomicModifyIORef_

Lifted version of atomicModifyIORef'.

>>> ref <- newIORef 42

>>> atomicModifyIORef' ref (\a -> (a, a + 3))
45

>>> readIORef ref
42

• For lazier updates, see atomicModifyIORef
• If you are not interested in the return value, see atomicModifyIORef'_

Version of atomicModifyIORef' that discards return value. Useful when you want to update IORef but not interested in the returning result.

>>> ref <- newIORef 42

>>> atomicModifyIORef'_ ref (`div` 2)

>>> readIORef ref
21

Version of atomicModifyIORef that discards return value. Useful when you want to update IORef but not interested in the returning result.

>>> ref <- newIORef 42

>>> atomicModifyIORef_ ref (`div` 2)

>>> readIORef ref
21