Exception warning in Haskell, but not with OldException

Question

Why does this code work with

import qualified Control.OldException as E

but not with

import qualified Control.Exception as E

Here is the code:

    fileContents <- (readFile "shpm.txt") `E.catch` (\_ -> return "")

Here is the error I get with the "new" Exception

Ambiguous type variable `e0' in the constraint:
  (E.Exception e0) arising from a use of `E.catch'
Probable fix: add a type signature that fixes these type variable(s)
In a stmt of a 'do' block:
  fileContents <- (readFile "shpm.txt")
                  `E.catch` (\ _ -> return "No Tasks")

Answer 1

Because the types have changed. Specifically:

• OldException: catch :: IO a -> (Exception -> IO a) -> IO a
• Exception: catch :: Exception e => IO a -> (e -> IO a) -> IO a

The new model needs to know what the value of e is for the Exception e type. What this means practically is that you need to tell the compiler which exception you are catching. Your example with OldException catches everything, which is now discouraged (see Catching All Exceptions for more info).

A simple fix to your function would be something like this:

foo = (readFile "foo") `E.catch` (\e -> const (return "") (e :: E.IOException))

Or the lambda-less version:

bar = (readFile "foo") `E.catch` myHandler

myHandler :: E.IOException -> IO String
myHandler _ = return ""

Answer 2

You need to provide an explict type for the exception being caught. For example:

fileContents <- (readFile "shpm.txt") `E.catch` ((\_ -> return "") :: E.SomeException -> IO String)