gistfile1.hs

import Control.Applicative
import Control.Exception
import Control.Monad
import Data.Maybe
import System.IO

withHandle :: Handle -> IO [String]
withHandle h = do
  s <- hGetLine h
  (s:) <$> (withHandle h)

withHandleT :: Exception e => Handle -> IO [String]
withHandleT h = (either (\_ -> []) id) <$> (try $ do
  s <- hGetLine h
  (s:) <$> (withHandleT h))

[1 of 1] Compiling Main ( Main.hs, interpreted )

Main.hs:12:16:
Could not deduce (Exception e0)
arising from the ambiguity check for ‘withHandleT’
from the context (Exception e)
bound by the type signature for
withHandleT :: Exception e => Handle -> IO [String]
at Main.hs:12:16-51
The type variable ‘e0’ is ambiguous
In the ambiguity check for:
forall e. Exception e => Handle -> IO [String]
To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
In the type signature for ‘withHandleT’:
withHandleT :: Exception e => Handle -> IO [String]
Failed, modules loaded: none.