For example:
newfile :: FilePath -> IO Bool
newfile x | length x <= 0 = return False
| doesFileExist x == True = return False
| otherwise = return True
Can this be made to work?
views:
208answers:
4
+4
A:
No, there's no way to do this (short of unsafe tricks which would be completely inappropriate here).
BTW doesFileExist x == True would be better written as doesFileExist x were it possible at all.
Ganesh Sittampalam
2010-01-26 10:57:46
+8
A:
You're already in the IO monad, so why not use the following?
newfile :: FilePath -> IO Bool
newfile x | length x <= 0 = return False
| otherwise = do exists <- doesFileExist x
return $ not exists
For applicative goodness:
import Control.Applicative
newfile :: FilePath -> IO Bool
newfile x | length x <= 0 = return False
| otherwise = not <$> doesFileExist x
As you can see, the applicative route is even more concise than the guards you'd like to use in your question!
Greg Bacon
2010-01-26 16:46:19
+1
A:
The type of guard clauses must be Bool. The type of doesFileExist x is IO Bool. The type mismatch means you can't do that.
Justice
2010-01-26 16:52:12
This doesn't really mean anything. The type of `getLine` is `IO String`. The type of `(++ "foo")` is `String -> String`. But you can still feed `getLine` into `(++ "foo")` with the right combinators. (Namely `(++ "foo") <$> getLine` instead of `(++ "foo") $ getLine`. So the answer to this question is not as simple as your post may lead someone to believe. (I think the word "No." would be more correct than your post, actually.)
jrockway
2010-01-27 05:52:15
I'm not sure I understand your point. I'm not sure how you expect to get a `Bool` out of an `IO Bool` using combinators. While you can use `unsafePerformIO` to get a `Bool` out of an `IO Bool`, please don't. Other than that, guard clauses require `Bool` expressions, and not an expression of any other type. So what did I miss? How is the single word "No" more correct, rather than an explanation of what is the expected type and what was the given type?
Justice
2010-01-27 15:14:39
A:
This works and does what's needed:
newfile :: FilePath -> IO Bool
newfile fn = do
x <- runErrorT $ do
when ((length fn) <= 0) (throwError "Empty filename")
dfe <- liftIO $ doesFileExist fn
when (dfe) (throwError "File does not exist")
return True
return $ either (\_ -> False) id x
me2
2010-01-27 21:29:42
`\\_ -> False` can be written `const False`. Stylistically, I believe the latter carries the intent better, but it's effectively equivalent.
ephemient
2010-01-29 20:23:04