Confused about Monad output

[Shawn P. Garbett]

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I've been fiddling with the example in section 10.2 of Bird's book, 
_Introduction_to_Functional_Programming_. I can't seem to make sense of why 
the output appears as it does. It's as if the final evaluation through Show 
doesn't take place, is there some critical piece of syntax missing?

I expect:

Main> evalOut(answer)
term: Con 1972, yields 1972
term: Con 2, yields 2
term: Div (Con 1972) (Con 2), yields 986
term: Con 23, yields 23
term: Div(Div(Con 1972)(Con 2))(Con 23), yields 42
value: 42

but I get: 

Main> evalOut(answer)
("term: Con 1972, yields 1972\nterm: Con 2, yields 2\nterm: Div (Con 1972) 
(Con 2), yields 986\nterm: Con 23, yields 23\nterm: Div (Div (Con 1972) (Con 
2)) (Con 23), yields 42\n",42)
(1149 reductions, 3134 cells)
Main> 


Here's the program:

data Term = Con Int | Div Term Term
            deriving(Show)

- -- 10.2.5 Monadic evaluator 
eval            :: Monad m => Term -> m Int
eval (Con x)    =  return x
eval (Div t u)  =  do x <- eval t
                      y <- eval u
                      return (x `div` y)

- -- Examples to try

answer, wrong :: Term
answer = Div ( Div ( Con 1972 ) (Con 2)) (Con 23)
wrong  = Div (Con 2) (Div (Con 1)(Con 0))

- -- Output monad
newtype Out a = MkOut (String, a)

instance Monad Out where 
  return x  =  MkOut ("", x)
  p >>= q   =  MkOut (ox ++ oy, y)
               where MkOut (ox, x) = p
                     MkOut (oy, y) = q x

instance Show a => Show (Out a) where
  show (MkOut (x,y)) = show x ++ "value: " ++ show y

- -- operation to generate output


out     :: String -> Out()
out ox  =  MkOut (ox, ())


line      :: Term -> Int -> String
line t x  =  "term: " ++ show t ++ ", yields " ++ show x ++ "\n"

evalOut          :: Term -> Out Int
evalOut (Con x)  =  do out (line(Con x) x)
                       return x
evalOut (Div t u) = do x <- evalOut t
                       y <- evalOut u
                       out(line(Div t u) (x `div` y))
                       return (x `div` y)


Shawn
- -- 
You're in a maze of twisty little statements, all alike.
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