Question

I have this function:

onData :: IO ([Float]) -> IO ()
onData vals =
    do
    let res = liftM fsum vals
    putStrLn " * Processing ... "
    putStrLn res
    putStrLn " * Sum : "
    putStrLn " * Done Processing"
    return ()

    fsum :: [Float] -> Float
    fsum []     = 0
    fsum (x:xs) = x + fsum(xs)

And this function, I am getting an error at the ‘fsum’ call. What am I missing? I just want the value returned.

HaskellParseData.hs:20:14:
    Couldn't match expected type `[Char]' with actual type `IO Float'
    Expected type: String
      Actual type: IO Float
    In the first argument of `putStrLn', namely `res'
    In a stmt of a 'do' expression: putStrLn res

Answer 1

liftM fsum vals has type IO Float. You’re giving it a name with let res = ..., but later trying to use it as a Float. You should bind it instead, producing a Float as desired, using <-:

res <- liftM fsum vals

It would be more idiomatic for onData to take a [Float] instead; you can sequence the action that produces this data elsewhere. Then you could simply use:

let res = fsum vals