Question

Here is a simplified version of my Code:

data Bookmark = Bookmark {
   url :: String
 , label :: String
 } deriving (Show)

genBookmark :: String -> String -> IO Bookmark
genBookmark u l = return ( Bookmark { url = u, label = l } )

But the Strings (url, label) are in the data base, so I have to deal with IO String. There must me a very easy solution, but I don’t see it (and an extensive web search didn’t get me anywhere.)

Basically I want to change my code to:

genBookmark :: IO String -> IO String -> IO Bookmark

Here is another version illustrating the problem:

genBookmark2 :: String -> String -> Bookmark
genBookmark2 u = return ( Bookmark { url = u, label = newlabel } )
                   where newlabel = getLine

With the error “Expected type: String, Actual type: IO String”.

====== EDIT =======

Here is the “real code” that had the error (with the solution that I got from the answer):

getSkosConceptRight :: String -> IO SkosConcept
getSkosConceptRight catName = do
                      suConcepts <- getSubConcepts catName
                      concept <- getMainConcept catName 
                      return ( concept { subConcepts = suConcepts })

getSkosConceptWrong :: String -> IO SkosConcept
getSkosConceptWrong catName = return ( concept { subConcepts = suConcepts })
                        where suConcepts = getSubConcepts catName
                              concept = getMainConcept catName 

getMainConcept :: ShortUrl -> IO SkosConcept
getSubConcepts :: ShortUrl -> IO [SkosConcept]

Answer 1

You’ve left out the part of your code that gets the string from the db. Either way, you need to “unpack” the String from your IO String.

In general, you should get rid of the IO as early as possible. So you should really try for a type of String -> String -> Bookmark

Here’s a simple example:

-- These will represent your 'get from the db' calls
ioUrl :: IO String
ioUrl = return "http://..."

ioLabel :: IO String
ioLabel = return "my label"

genBookmark :: String -> String -> Bookmark
genBookmark url label = Bookmark { url = url, label = label }

Now, from main which is in IO, we make our “db” calls to get our strings, unpack the values, and send to our genBookmark function.

main = do
    url <- ioUrl     -- here, we're removing the "http://..." from the IO String
    label <- ioLabel -- same here
    print (genBookmark url label)

---

The reason why your genBookmark2 is failing, is because you are trying to use getLine, whose type is IO String in a pure function; hence the expected String, got IO String error message. Also, the type mentions two Strings as inputs, but you only take one.