Glynn Clements wrote:
Jyrinx wrote:Sure - and that's why I don't do *everything* in purely-functional-land. I suppose what I'm going for is separation of concerns: Anything with any business catching exceptions should be in the IO monad; calculations, transformations, etc., which depend on such a continuous stream of data couldn't deal with the exception if I wanted them to, but the IO code that invokes them can. In the Web browser example, I imagine (and this is off the top of my head) that a major functional part of the program would be a function that takes a bunch of HTML (presumably passed as a lazy stream from a server, achieved in the IO monad), processes it, and renders a bunch of graphical data for the screen (of which IO code could control the display). If a connection is refused, IO code catches the error before it can pass the stream to the rendering function; if a connection is cut off or something, the rendering code can't deal with that, and the exception gets caught back in I/O-land.
No, but with strict I/O, you are bound to be "within" the IO monad[...] andBut how does this differ from strict I/O? I mean, say there's a disk error in the middle of some big file I want to crunch. Under traditional I/O, I open the file and proceed to read each piece of data, process it, and continue to the next one, reading the raw data only as I need it. When I hit the error, an exception will be thrown in the middle of the operation. In lazy I/O, I might use getContents to get all the characters lazily; the getContents call will read each piece of data as it's needed in the operation - in other words, the data is read as the program uses it, just like with traditional I/O. And when the error occurs, the operation will be unceremoniously interrupted, again the same as by strict I/O. In mean, if an exception is thrown because of a file error, I can't hope to catch it in the data-crunching part of the program anyway ...
the inability to handle exceptions (the actual exception won't occur
until after e.g. getContents has returned).
when the exception is thrown, so you *can* catch it.
If you are just going to allow all exceptions to be fatal, and don't
need any control over I/O ordering, you may as well just use lazy I/O. However, if you are writing real software as opposed to just toy
programs, you have to handle exceptions; e.g. a web browser which died
every time that a server refused a connection wouldn't be of much use.
Luke Maurer
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