[plt-scheme] style question

2009/9/19 Ryan Culpepper <ryanc at ccs.neu.edu>:
> Todd O'Bryan wrote:
>>
>> I'm writing a text converter that goes through a text file and
>> replaces parts of it with place-holders that I can fill back in later.
>>
>> As a result, I have to keep track of the number of parts I've replaced
>> and increment the count each time I replace a new substring. If I were
>> doing this as an OO program, I'd create a Converter class and have the
>> number of parts so far be a field. Doing the same thing functionally,
>> it seems like I have two choices--either have the counter be a
>> module-level variable that all the functions have access to, or pass
>> it around as a parameter to every function that needs it or calls
>> other functions that need it.
>>
>> So, here's my question--which is better style in Scheme? If I don't
>> provide the variable, it's not really *global* in the scary sense that
>> global variables are bad, but suppose two different processes both use
>> the same module at the same time. Does each get its own copy of the
>> variable, or is there really only one copy that the two processes
>> would share (and possibly confuse each other as a result)? Passing
>> around the counter seems cleaner conceptually--all the dependencies
>> are explicit--but it just feels clunky.
>
> Module-level variables are shared across threads, so lifting local state to
> a global level is generally a bad idea.
>
> In general, make sure the mechanisms you use to represent state in your
> program match the state you're trying to represent. Anything else requires
> careful thinking. For example, it might be safe to represent *task-specific*
> state using a *thread-local* storage mechanism. But you'd better be sure
> that tasks really match up with threads; that is, that tasks never hop
> threads, and that different tasks never nest within a single thread.
>
> Ryan

Then again, you could use a parameter...

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