Question

In this Haskell code the constructor arguments(length and width) can be of any data type (like Int,Char etc.). Is there a way by which i can explicitly specify the data type of length and width? How can i specify the return type of function getLength?

{-# 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
     widthRef <- newIORef width
     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

The main should work (which it is) if it is written like:

main = do
   c1 <- mfix $ rectangle 0 0
   c2 <- mfix $ rectangle 0 0
   c3 <- mfix $ rectangle 0 0
   c1# setWidth $ 3
   c2# setWidth $ 2
   c3# setWidth $ 2
   c1# incr
   c2# incr
   c1# incr
   c1# show'
   c3# show'

But main is working even if it is written this way which is undesirable( as width cannot be a character).

main = do
   c1 <- mfix $ rectangle 0 'a'
   c2 <- mfix $ rectangle 0 'b'
   c3 <- mfix $ rectangle 0 'c'
   c1# setWidth $ 'd'
   c2# setWidth $ 'e'
   c3# setWidth $ 'f'
   c1# incr
   c2# incr
   c1# incr
   c1# show'
   c3# show'

Answer 1

Assuming this is all type-safe, the return type of getLength will match the contents of lengthRef, so you just need to add a type signature somewhere that constrains that type, and the type of getLength should be inferred correctly from that.

For example, you can specify the type when creating lengthRef:

lengthRef <- newIORef (length :: Int)