CODEWITHSUNDEEP
haskelliomonads
Danstahr
Danstahr

Reputation: 4319

Haskell monad return type

I'm trying to do a bit of programming and I can't get into monads. I've advanced a bit with IO functions, but now I'm definitely lost...

I've got a XML string that was loaded from the network (so it's "stored" in IO String). Because of this, I need a do block to load to common String. 

foo :: IO String -> Nothing
foo xmlio = do
    xmlio <- xml
    -- do some magic

I've already implemented another function that takes pure string and returns pure XML element. It does a bit more, but let's simplify that as it's not important for this question.

import Text.XML.Light
import Text.XML.Light.Input

toxml :: String -> Maybe Element
toxml s = parseXMLDoc s

Now I tried to combine these two to accept an IO String and to return an "IO Element".

toxmlio :: IO String -> ??? Maybe Element
toxmlio s = do
    pures <- s
    let toReturn = parseXMLDoc s
    return s

The return function's type is (according to ghci) : 

let foo = "<foo>bar</foo>"
:t return (parseXMLDoc foo)
    >> return (parseXMLDoc foo) :: Monad m => m (Maybe Element)

But if I change the header of my function to 

toxmlio :: IO String -> Monad (Maybe Element)
toxmlio s = do
    pures <- s
    let toReturn = parseXMLDoc s
    return s

I get a compilation error

Kind mis-match
The first argument of `Monad' should have kind `* -> *',
but `Maybe Element' has kind `*'
In the type signature for `xmlio':
  xmlio :: String -> Monad (Maybe Element)

Does anyone have any idea how to solve this issue? Or am I completely lost and haskell does this another way? Thank you for your answer.

Upvotes: 0

Views: 630

Answers (1)

Lee
Lee

Reputation: 144206

It looks like you want:

toxmlio :: IO String -> IO (Maybe Element)
toxmlio s = do
    pures <- s
    let toReturn = parseXMLDoc pures
    return toReturn

however you can use liftM:

toxmlio :: IO String -> IO (Maybe Element)
toxmlio = liftM parseXMLDoc

You could make it even more general, since liftM does not depend on a particular monad type:

toxmlio :: Monad m => m String -> m (Maybe Element)
toxmlio = liftM parseXMLDoc

Upvotes: 3

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