Ross Paterson cryptically writes:

Shouldn't IOError be identified with IOException rather than Exception?

I had to grovel through the code to understand what this question means. It seems that GHC.IOBase contains these definitions: type IOError = Exception data Exception = ArithException ArithException | ... | IOException IOException | ... data IOException = IOError { ioe_handle :: Maybe Handle, -- the handle used by the action flagging -- the error. ioe_type :: IOErrorType, -- what it was. ioe_location :: String, -- location. ioe_descr :: String, -- error type specific information. ioe_filename :: Maybe FilePath -- filename the error is related to. } I think the idea is that we want code protected using catch clauses to be safe against not just IOErrors but also all the exceptions (division by zero, etc) that can happen. Since the type of 'catch' is fixed by the report, we have to make IOError include all those types. The report deliberately specifies IOError in such a way that additional kinds of IOError can be easily added. This only leaves the question of whether division by 0 is an IO error or is a pure error. The argument is that the only thing which causes evaluation to happen is IO - if your program doesn't interact with the outside world, it might as well not do anything. So, in that sense, all errors are errors triggered by performing IO. [You can disagree with this argument if you like - the fact will remain that we'd like Prelude.catch to catch as many exceptions as possible even if the names of the types seem a little screwy when we do that.] (Well, there's also the other question of how much work it'll take to make Hugs fit this framework. I wrote a lot of that code but I don't honestly remember what it looks like.) -- Alastair