[Haskell-cafe] Re: New Hackage category: Error Handling
ok at cs.otago.ac.nz
Mon Dec 7 17:45:33 EST 2009
When I was working at Quintus, I came up with a classification
which I can simplify something like this:
operating system fault
Something bad happened (like a remote node going down) that was
entirely out of your control. There is nothing you can do to
your program to prevent this. Example: power failure. It's
still your problem to clean up on your way out.
your program tried to do something possibly meaningful but
the system ran out of some kind of resource (cpu limit,
memory limit, disc quota, &c)
You might respond to this by increasing the limit and trying
your program tried to do something meaningful but the system
was unable to represent the result (integer overflow, upper
case of ÿ in a Latin 1 system, floating point overflow on a
non-IEEE system, &c)
Your program isn't *wrong* but you will still have to change it.
Your program tried to do something with a (typically external)
resource that doesn't exist (missing file)
Your program could be wrong but probably isn't.
You will have to create the resource or provide a different name.
Your program tried to do something to a (typically external but
not always) resource that you do not have permission to do
(typically writing to a read-only file)
You may have named the wrong resource. If not, you may have to
get the permissions for the resource changed, or ask someone
else to run the program.
A precondition on the input arguments of an operation was not
satisfied (e.g., X/0, sqrt(-1), malformed file name, head ).
Your program is definitely wrong.
Your program invoked some operation and the precondition for
the operation was satisfied, but when it completed, the
postcondition was not.
The operation you invoked is broken. If it's yours, you will
have to fix it. If the precondition was not strong enough,
it may be your program at fault. Otherwise, until you can
get a fix from someone, you will have to program around it.
I didn't find a simple error/exception distinction helpful, and
Take the case of trying to write to "/dev/nul". This is a permission
error. If the program is responsible for the name being what it is,
it's a mistake in the program. If the user typed the name in, it's
the user's mistake. You really can't tell without tracing each value
to its origin.
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