[Haskell-cafe] cannot catch exception

[Viktor Dukhovni]

On Mon, Mar 04, 2019 at 09:26:41PM +0100, Kees Bleijenberg wrote:

> readCDFile :: FilePath -> FilePath -> IO (Either String T.Text)
> readCDFile baseDir fn = do
>   catch ( do
>             buffer <- B.readFile (combine baseDir fn)  --reads strict the whole file
>             let bufferStrict = B.toStrict buffer
>             return $ Right $! TE.decodeUtf8 bufferStrict        -- this doesn’t work
>             Right <$> evaluate (TE.decodeUtf8 bufferStrict)     –- this does
>             liftM Right $ evaluate (TE.decodeUtf8 bufferStrict) -- this works too
>          ) exceptionHandler

Take a close look at the three increasingly strict examples below:

    ghci> do { x <- return $ Right $ undefined ; putStrLn "delayed..."; case x of { Right _ -> return 1; _ -> return 0 } }
    delayed...
    1

    ghci> do { x <- return $ Right $! undefined; putStrLn "delayed..."; case x of { Right _ -> return 1; _ -> return 0 } }
    delayed...
    *** Exception: Prelude.undefined
    CallStack (from HasCallStack):
      error, called at libraries/base/GHC/Err.hs:78:14 in base:GHC.Err
      undefined, called at <interactive>:8:29 in interactive:Ghci3

    ghci> do { x <- return $! Right $! undefined ; putStrLn "delayed..."; case x of { Right _ -> return 1; _ -> return 0 } }
    *** Exception: Prelude.undefined
    CallStack (from HasCallStack):
      error, called at libraries/base/GHC/Err.hs:78:14 in base:GHC.Err
      undefined, called at <interactive>:9:30 in interactive:Ghci3

While "Right $! x" is strict in x, "return $ Right $! x" is not!
You need "return $! Right $! x" to make that happen.

-- 
	Viktor.