Question

returnIO is used in OOHaskell to return the object as a record of methods. But even on use of return instead of returnIO there is no difference in the output or object behaviour. The OOHaskell code is:

{-# LANGUAGE EmptyDataDecls, DeriveDataTypeable, TemplateHaskell #-}
{-# OPTIONS_GHC -fcontext-stack=100 #-}

module Rectangle where

import OOHaskell

$(label "getLength")
$(label "getWidth")
$(label "incr")
$(label "lengthenBy")
$(label "setLength")
$(label "setWidth")
$(label "show'")

rectangle length width self
 = do
     lengthRef <- newIORef length :: IO (IORef Int)
     widthRef <- newIORef width :: IO (IORef Int)
     return $
          getLength      .=. readIORef lengthRef
      .*. getWidth      .=. readIORef widthRef
      .*. setLength      .=. writeIORef lengthRef
      .*. setWidth      .=. writeIORef widthRef
      .*. lengthenBy    .=. (\dl ->
              do
             length <- self # getLength
             (self # setLength) (length + dl))
      .*. incr          .=. (self # lengthenBy) (1)
      .*. show'         .=. printLn ("Length : "<< self # getLength<<" Width : "<< self # getWidth)
      .*. emptyRecord

Can anyone please explain why is this happening? What makes the object behave in same way in both cases?

Answer 1

returnIO is return, but specialised to IO types.

That is:

return   :: Monad m => a ->  m a
returnIO ::            a -> IO a

See lines 75-76 of OOHaskell.hs:

returnIO :: a -> IO a
returnIO = return