Obviously without knowing the details I am speculating, but would it not be possible to do a first pass of the XML and build a list of files to read (a pure function) this returns its result to the IO monad where the files are read and concatenated together, and passed to a second (pure functional) processing function. If written well this can take advantage of lazy execution, so both functions end up running concurrently. It seems to me that as unsafePerformIO is not in the standard and only implemented on some compilers/interpreters, that you limit the portability of code by using it, and that it is best avoided. Also as any safe use of unsafePerformIO can be refactored to not use it I could certainly live without it. Keean. Graham Klyne wrote:
[Switching to Haskell-cafe]
I have used it once, with reservations, but at the time I didn't have the time/energy to find a better solution. (The occasion of its use was accessing external entities within an XML parser; by making the assumption that the external entities do not change within any context in which results from a program are compared, I was able to satisfy the "proof obligation" of not causing or being sensitive to side effects.)
The reason this was important to me is that I wanted to be able to use the parser from code that was not visibly in the IO monad. For me, treating Web data transformations as pure functions is one of the attractions of using Haskell.
(Since doing that, I had an idea that I might be able to parameterize the entity processing code on some Monad, and use either an Identity monad or IO depending on the actual requirements. This way, I could keep pure XML processing out of the IO monad, but use IO when IO was needed.)
In short: I think it's usually possible to avoid using unsafePerformIO, but I'd be reluctant to cede it altogether, if only for sometimes quick-and-dirty pragmatic reasons.
#g
------------ Graham Klyne For email: http://www.ninebynine.org/#Contact