Handling Haskell exceptions with a single case clause?

Question

Consider the following problem.

I am writing wrapper for some string functions; one of them being chr (for example) from Data.Char . Now, if the input to my API is a bad input, I want to throw an error otherwise, I simply want to return the integer output. I want to keep the algorithm simple —

getChr someInput = do
  x <- chr someInput -- this doesn't evaluate unless we evaluate ourselves
  -- handle exception here
  result = <_some_evaluation_>
  case result of
    Left  _ -> custom error throw
    Right _ -> return something

This is obviously not any Haskell code but a description of what I want the algorithm to look like. I have seen the try (evaluate _) examples but those return IO (Either SomeException a) type values that I am unsure can be dealt with simple case statements. I want something very simple so I can case it according to my needs. Is there a way?

Answer 1

try works well for this.

getChr someInput = do
  x <- chr someInput
  result = try (some_evaluation x)
  case result of
    Left  (SomeException _) -> print "Oh no!"
    Right val               -> print val

Answer 2

I guess that your problem might be that you want to catch only some specific exception, right? Then you need to make sure that you cast your the exception somewhere so that it's type is something more special than SomeException . For example:

do
  result <- try something
  case result of
    Left  e  -> print (e :: IOException)
    Right x  -> return x

Or if you want to discard the exception, cast it within const :

do
  result <- try something
  case result of
    Left  e  -> const (print "whatever") (e :: IOException)
    Right x  -> return x