What about ErrorT monad transformer? Well, if it's not what you really want, you can define your own one without ugly "Error" class, so it'd be something like execute :: FilePath -> [String] -> MyCoolErrorT ExecuteError IO ExitCode On 14 Apr 2009, at 23:29, brian@lorf.org wrote:
I'm finding it hard to write robust code that isn't really ugly. Suppose I want to write
execute :: FilePath -> [String] -> IO (Either ExecuteError ExitCode)
where 'ExecuteError' is a data type representing all the ways 'execute' could fail and that 'execute p args' is supposed to
* ensure p exists * get p's permissions * ensure p is readable * ensure p is executable * execute p and return its exit code
So 'ExecuteError' would have constructors corresponding to these ways to fail: * could not determine whether p exists * p does not exist * could not get p's permissions * p is not readable * p is not executable * could not execute p for some other reason
So if I start to code it, I 'tryJust' 'doesFileExist'. If an exception is thrown, I convert it to Left $AppropriateExecuteError. If not, we know whether p exists. If it doesn't, convert it to Left. Otherwise, 'tryJust' 'getPermissions', etc. It's starting to staircase.
I'm needing it to be easier to stop my computation and return specific things when exceptions are thrown or when certain requirements aren't met. Thanks for any help. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe