modify

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.

modify f is an action that updates the state to the result of applying f to the current state.

• modify f = get >>= (put .
  f)

modify f is an action that updates the state to the result of applying f to the current state.

• modify f = get >>= (put .
  f)

modify f is an action that updates the state to the result of applying f to the current state.

• modify f = get >>= (put .
  f)

Apply a destructive operation to a vector. The operation may be performed in place if it is safe to do so and will modify a copy of the vector otherwise (see New for details).

Examples

>>> import qualified Data.Vector as V

>>> import qualified Data.Vector.Mutable as MV

>>> V.modify (\v -> MV.write v 0 'x') $ V.replicate 4 'a'
"xaaa"

Apply a destructive operation to a vector. The operation may be performed in place if it is safe to do so and will modify a copy of the vector otherwise (see New for details).

Examples

>>> import qualified Data.Vector.Strict as V

>>> import qualified Data.Vector.Strict.Mutable as MV

>>> V.modify (\v -> MV.write v 0 'x') $ V.replicate 4 'a'
"xaaa"

Modify the element at the given position.

Modify the element at the given position.

Apply a destructive operation to a vector. The operation may be performed in place if it is safe to do so and will modify a copy of the vector otherwise (see New for details).

Examples

>>> import qualified Data.Vector.Primitive as VP

>>> import qualified Data.Vector.Primitive.Mutable as MVP

>>> VP.modify (\v -> MVP.write v 0 'x') $ VP.replicate 4 'a'
"xaaa"

Modify the element at the given position.

Apply a destructive operation to a vector. The operation may be performed in place if it is safe to do so and will modify a copy of the vector otherwise (see New for details).

Examples

>>> import qualified Data.Vector.Storable as VS

>>> import qualified Data.Vector.Storable.Mutable as MVS

>>> VS.modify (\v -> MVS.write v 0 'x') $ VS.replicate 4 'a'
"xaaa"

Modify the element at the given position.

Apply a destructive operation to a vector. The operation may be performed in place if it is safe to do so and will modify a copy of the vector otherwise (see New for details).

Examples

>>> import qualified Data.Vector.Unboxed as VU

>>> import qualified Data.Vector.Unboxed.Mutable as MVU

>>> VU.modify (\v -> MVU.write v 0 'x') $ VU.replicate 4 'a'
"xaaa"

Modify the element at the given position.

A slight variation on modifyMVar_ that allows a value to be returned (b) in addition to the modified value of the MVar.

Like modifyMVar, but the IO action in the second argument is executed with asynchronous exceptions masked.

Like modifyMVar_, but the IO action in the second argument is executed with asynchronous exceptions masked.

An exception-safe wrapper for modifying the contents of an MVar. Like withMVar, modifyMVar will replace the original contents of the MVar if an exception is raised during the operation. This function is only atomic if there are no other producers for this MVar. In other words, it cannot guarantee that, by the time modifyMVar_ gets the chance to write to the MVar, the value of the MVar has not been altered by a write operation from another thread.

Mutate the contents of an IORef, combining readIORef and writeIORef. This is not an atomic update, consider using atomicModifyIORef when operating in a multithreaded environment. Be warned that modifyIORef does not apply the function strictly. This means if the program calls modifyIORef many times, but seldom uses the value, thunks will pile up in memory resulting in a space leak. This is a common mistake made when using an IORef as a counter. For example, the following will likely produce a stack overflow:

ref <- newIORef 0
replicateM_ 1000000 $ modifyIORef ref (+1)
readIORef ref >>= print

To avoid this problem, use modifyIORef' instead.

Strict version of modifyIORef. This is not an atomic update, consider using atomicModifyIORef' when operating in a multithreaded environment.

Mutate the contents of an STRef.

>>> :{
runST (do
ref <- newSTRef ""
modifySTRef ref (const "world")
modifySTRef ref (++ "!")
modifySTRef ref ("Hello, " ++)
readSTRef ref )
:}
"Hello, world!"

Be warned that modifySTRef does not apply the function strictly. This means if the program calls modifySTRef many times, but seldom uses the value, thunks will pile up in memory resulting in a space leak. This is a common mistake made when using an STRef as a counter. For example, the following will leak memory and may produce a stack overflow:

>>> import GHC.Internal.Control.Monad (replicateM_)

>>> :{
print (runST (do
ref <- newSTRef 0
replicateM_ 1000 $ modifySTRef ref (+1)
readSTRef ref ))
:}
1000

To avoid this problem, use modifySTRef' instead.

Strict version of modifySTRef