处理此特定的Haskell异常

Question

I understand that in Control.Exception , you need the evaluate function to make sure that the expression first evaluates into something and then handle the exception using the handle function.

Consider the following code below:

import Control.Exception
import System.Locale
import Data.Time

parseDay :: String -> String -> (Integer, Int, Int)
parseDay formatString dateString = toGregorian $ readTime defaultTimeLocale formatString dateString

Above is some code to parse dates in strings as its clear from the definition above. This returns type (Integer, Int, Int) .

So, I am using the following line of code to see if there was an exception:

x = parseDay "%m" "-"
result = try (evaluate $ x) :: IO (Either SomeException (Integer, Int, Int))

In this case, the input is bad, so this throws the following exception:

( * Exception: readsTime: bad input "-"

But the result is as follows:

Right ( * Exception: readsTime: bad input "-"

which is something I cannot understand; it should be Left <error_message> as shown in all the examples I've seen. I am unsure, what is being lazy evaluated here and why this isn't working as desired.

Answer 1

I don't think evaluate is evaluating x deeply enough to reach the exception, so try is returning a Right result and the exception is only really evaluated when result is printed.

Try replacing the $ in parseDay with the strict $! and see if it works.