On Sun, 11 Mar 2012, Brandon Allbery wrote:
Is looking up a missing key in a Map an exception?
So you will insist that lookup when the key doesn't exist throw an exception? If not, why not?
If we're going to make interfacing with the system as difficult and minetrap-strewn as possible, then let's cut to the chase and switch to Agda. Or we need to change a bunch of IO (Maybe x) to throw approved exceptions and make them suitably difficult to use for our walled garden instead of thinking about whether that's appropriate.
Or, just maybe, we just need to stop thinking of it's-in-IO-it-should-throw-an-exception-at-every-opportunity-instead-of-being-*usable*. IO's enough of a sewer as it is without turning it into don't-think-about-it-just-hork-a-hairball too.
"exception" in what sense? Certainly not an exception in IO, since I do not like the implicitness of exceptions in IO. No I think, Maybe is a perfect return type for 'lookup'. What I want to say is, that Nothing is also an exceptional value.
(So, you just tossed in <stdrant.h> C-style without looking at the context?)
The context is that getEnv currently throws an IO exception if the name isn't found, and we'd like it to be IO (Maybe String) because it's really a keyed store where absence of the key isn't really justification for an IO exception, but is a reasonable use for a Maybe. It's the cranky IO exception that needs to go away, not the notion of an exceptional condition in general.