Re: [Haskell-cafe] Webdriver: Couldn't match type `Element' with `WD Element'

9 Jul 2014


      I've posted this question to stackoverflow:
https://stackoverflow.com/questions/24650813/find-all-text-inputs-on-webpage...

On Tue, Jul 8, 2014 at 6:16 PM, Cody Goodman
 wrote:
...
That gave a syntax error, however this was valid:
let textElems = filter ((==) "text" . (`attr` "type")) inputs
It then gave me some ambiguous type errors:
{-# LANGUAGE OverloadedStrings #-}
import           Control.Monad
    import           Control.Monad.IO.Class
    import qualified Data.Text                    as T
    import           Test.WebDriver
    import           Test.WebDriver.Classes       (WebDriver (..))
    import           Test.WebDriver.Commands
    import           Test.WebDriver.Commands.Wait
main = do
      runSession defaultSession capabilities $ do
          openPage "http://www.appnitro.com/demo/view.php?id=1"
          inputs <- findElems $ ByTag "input"
          let textElems = filter ((==) ("text") . (`attr` "type"))
inputs :: [Element]
          -- wait 20 seconds
          waitUntil 20 (getText <=< findElem $ ByCSS ".doesnotexist")
            `onTimeout` return ""
      liftIO $ putStrLn "done"
        where
          capabilities = allCaps { browser=firefox }
-- Prelude Control.Monad Control.Monad.IO.Class
Control.Monad.Trans.Class> :r
    -- [1 of 1] Compiling Main             ( src/Main.hs, interpreted )
-- src/Main.hs:15:31:
    --     No instance for (Eq (wd0 (Maybe T.Text)))
    --       arising from a use of ‘==’
    --     The type variable ‘wd0’ is ambiguous
    --     Note: there are several potential instances:
    --       instance Eq a => Eq (GHC.Real.Ratio a) -- Defined in ‘GHC.Real’
    --       instance Eq (Test.WebDriver.Common.Profile.PreparedProfile b)
    --         -- Defined in ‘Test.WebDriver.Common.Profile’
    --       instance Eq (Test.WebDriver.Common.Profile.Profile b)
    --         -- Defined in ‘Test.WebDriver.Common.Profile’
    --       ...plus 27 others
    --     In the first argument of ‘(.)’, namely ‘(==) ("text")’
    --     In the first argument of ‘filter’, namely
    --       ‘((==) ("text") . (`attr` "type"))’
    --     In the expression:
    --         filter ((==) ("text") . (`attr` "type")) inputs :: [Element]
-- src/Main.hs:15:37:
    --     No instance for (Data.String.IsString (wd0 (Maybe T.Text)))
    --       arising from the literal ‘"text"’
    --     The type variable ‘wd0’ is ambiguous
    --     Note: there is a potential instance available:
    --       instance a ~ Data.ByteString.Internal.ByteString =>
    --                Data.String.IsString
    --
(attoparsec-0.12.1.0:Data.Attoparsec.ByteString.Internal.Parser a)
    --         -- Defined in
‘attoparsec-0.12.1.0:Data.Attoparsec.ByteString.Char8’
    --     In the first argument of ‘(==)’, namely ‘("text")’
    --     In the first argument of ‘(.)’, namely ‘(==) ("text")’
    --     In the first argument of ‘filter’, namely
    --       ‘((==) ("text") . (`attr` "type"))’
I then remembered that the type of attr is  WebDriver wd => Element ->
Text -> wd (Maybe Text) and made this change:
- let textElems = filter ((==) ("text") . (`attr` "type")) inputs
:: [Element]
    + let textElems = filter ((==) (Just "text" :: Maybe T.Text) .
(`attr` "type")) inputs :: [Element]
and got what I believe to be a monadic error:
Prelude Control.Monad Control.Monad.IO.Class Control.Monad.Trans.Class> :r
    [1 of 1] Compiling Main             ( src/Main.hs, interpreted )
src/Main.hs:15:69:
        Couldn't match type ‘Maybe T.Text’ with ‘T.Text’
        Expected type: Element -> Maybe T.Text
          Actual type: Element -> Maybe (Maybe T.Text)
        In the second argument of ‘(.)’, namely ‘(`attr` "type")’
        In the first argument of ‘filter’, namely
          ‘((==) (Just "text" :: Maybe T.Text) . (`attr` "type"))’
        In the expression:
            filter
              ((==) (Just "text" :: Maybe T.Text) . (`attr` "type")) inputs ::
              [Element]
    Failed, modules loaded: none.
This led me to make this change:
-let textElems = filter ((==) (Just "text" :: Maybe T.Text) .
(`attr` "type")) inputs :: [Element]
    +let textElems = filter ((==) (return $ Just "text" :: Maybe
T.Text) . (`attr` "type")) inputs :: [Element]
and I got the following error:
src/Main.hs:15:78:
        Couldn't match type ‘Maybe T.Text’ with ‘T.Text’
        Expected type: Element -> Maybe T.Text
          Actual type: Element -> Maybe (Maybe T.Text)
        In the second argument of ‘(.)’, namely ‘(`attr` "type")’
        In the first argument of ‘filter’, namely
          ‘((==) (return $ Just "text" :: Maybe T.Text) . (`attr` "type"))’
        In the expression:
            filter
              ((==) (return $ Just "text" :: Maybe T.Text) . (`attr` "type"))
              inputs ::
              [Element]
    Failed, modules loaded: none.
On Tue, Jul 8, 2014 at 6:02 PM, Brandon Allbery  wrote:
...
On Tue, Jul 8, 2014 at 6:52 PM, Cody Goodman 
wrote:
...
textElems <- filterM (liftM $ ((==) "text" . (`attr` "type")))
inputs
Are you sure this shouldn't be:
let textElems = filter ((==) "text" . `attr` "type") inputs
? The type in the error suggests this is more appropriate.
--
brandon s allbery kf8nh                               sine nomine associates
allbery.b@gmail.com                                  ballbery@sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net