Thanks a lot for the correction, I guess the right identity law would be violated, right? I was think about the type constructor stuff, next time should definitely think about the laws first. On Tuesday, June 17, 2014, Bob Ippolito wrote:

Your understanding is not correct. The monad laws would be violated If return did make a new mutable object. http://www.haskell.org/haskellwiki/Monad_laws

Side-effects typically have a `m ()` return type, so your "more Haskell" way is not idiomatic Haskell.

On Tue, Jun 17, 2014 at 7:53 PM, Baojun Wang javascript:_e(%7B%7D,'cvml','wangbj@gmail.com');> wrote:

...

To make my question more clearer, will test1/test2 have noticeable performance difference?

-- mutable1.hs

import qualified Data.Vector.Mutable as MV

import Control.Monad

import Control.Monad.Primitive

a1 :: (PrimMonad m) => MV.MVector (PrimState m) a -> m ()

a1 v = do

-- do something

return ()

a2 :: (PrimMonad m) => MV.MVector (PrimState m) a -> m ()

a2 v = do

-- do something else

return ()

b1 :: (PrimMonad m) => MV.MVector (PrimState m) a -> m (MV.MVector (PrimState m) a)

b2 :: (PrimMonad m) => MV.MVector (PrimState m) a -> m (MV.MVector (PrimState m) a)

b1 v = do

-- do something different

return v

b2 v = do

-- do something else different

return v

test1 :: IO ()

test1 = do

v1 <- MV.replicate 1000 0

a1 v1

a2 v1

return ()

test2 :: IO ()

test2 =

MV.replicate 1000 0 >>= b1 >>= b2 >> return () -- I'd prefer this way cause it's more haskell.

On Tue, Jun 17, 2014 at 5:50 PM, Baojun Wang javascript:_e(%7B%7D,'cvml','wangbj@gmail.com');> wrote:

...

Hi List,

Per my understanding, return x would make a new copy of the object. What if the returned object is mutable? Will this make a new (mutable) object?

My concern is if I created a very large mutable object, does return mutable make a full copy of the original mutable data, or just copy a reference (pointer?)?

Thanks baojun

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