Haskell: No instance for (Monoid Int) when using writer monad with haskell 2013 2.0.0 framework

module Main where
import Data.Char
import Control.Monad
import Control.Monad.Trans.Writer
import Control.Applicative
import Data.Monoid

wt :: Int -> Writer Int [String]
wt x=writer(["num:"++ show x],x)


addw::Writer Int [String]
addw = do
        a <- wt 2
        b <- wt 3
        return (a*b)

      

        
        
        
      

    

There are 2 errors:

No instance for (Monoid Int) arising from a do statement
    Possible fix: add an instance declaration for (Monoid Int)
    In a stmt of a 'do' block: a <- wt 2
    In the expression:
      do { a <- wt 2;
           b <- wt 3;
           return (a * b) }
    In an equation for `addw':
        addw
          = do { a <- wt 2;
                 b <- wt 3;
                 return (a * b) }


No instance for (Num [String]) arising from a use of `*'
    Possible fix: add an instance declaration for (Num [String])
    In the first argument of `return', namely `(a * b)'
    In a stmt of a 'do' block: return (a * b)
    In the expression:
      do { a <- wt 2;
           b <- wt 3;
           return (a * b) }

      

        
        
        
      

    

I am using eclipse 4.4 juno with newest eclipsefp, haskell 2013 2.0.0 platform includes ghc 7.6.3, this code snippet is to find out you haskell for great quality