why does the process dictionary have a bad rep?

[Ulf Wiger]

Den 2005-07-15 17:18:22 skrev ke.han <ke.han@REDACTED>:

> This thread is talking about two things: abuse of the proc dict and  
> abuse of message passing.
> I understand the example of using the proc dict to get around single  
> assignment.  You guys have lost me on the second part.
> Can someone give an example of message passing abuse?  What does message  
> passing have to do with variable assignment?
> thanks, ke han

The things listed in the Erlang book about the process dictionary
apply equally to message passing.

- if a computation is rolled back by a catch, messages
   sent during that computation cannot be rolled back

- if functions send messages (have side effects), the
   order in which functions are called can become very
   important.

- understanding a single function is much easier if
   it only depends on the input arguments, and not
   on message passing inside the function. Without
   side-effects, you know that the function will
   always return the same result each time it's
   called with a given set of arguments - regardless
   of timing issues.

- verifying code that doesn't have side-effects is
   also much easier, as the state space is radically
   reduced. This is related to the previous point.

In general, it's a very good idea to isolate all
side-effects as much as possible, and to keep
them clearly separated from the code that can be
side-effect free.

/Uffe