haskellfunction-compositionwriter-monad

Function composition with the Writer monad?

I'm studying the Writer monad and have the following: 

myFunction :: Int -> Int -> Writer String Int
myFunction e1 e2 
   | e1 > e2 = do
      tell ("E1 greater")
      return (e1)
   | otherwise = do
      tell ("E2 greater")
      return (e2)

main = do
-- execWriter :: Writer w a -> w 
   print $ execWriter . myFunction 1 2

Error:

"Couldn't match type ‘WriterT String Data.Functor.Identity.Identity Int’with ‘a0 -> Writer c0 a1’
  Expected type: a0 -> Writer c0 a1
    Actual type: Writer String Int"

Why does this computation error with . and not $? Perhaps my understanding of function composition is incorrect?

Solution

  • Function composition with . means that the resulting composition will receive an argument.

    This portion:

    execWriter . myFunction 1 2

    Can be written more explicitly like this:

    (\x -> execWriter (myFunction 1 2 x))

    Since myFunction only takes two arguments, you are getting a compile error.

    Had you used $ in your code, like this:

    execWriter $ myFunction 1 2

    The resulting code expanded is equivalent to this:

    execWriter (myFunction 1 2)

    Which is valid.