Re: [Chicken-users] Re: tcp-read-timeout: procedure or parameter?

[Tobia Conforto]

William Xu wrote:
> I read the srfi-39 carefully.  It appears that it can be more or  
> less expressed by simple lexical scoping.  So what's the extra  
> benifits of using `parameter'?

First of all, the macro you refer to is called `parameterize', not  
`parameter', and it's just a shorthand for setting a new value before  
the body and then resetting the old value at the end of the body.

The benefits of parameters are dynamic scoping and thread safety.

Contrary to function arguments and let forms, parameters are  
dynamically scoped, much like global variables, or "special variables"  
in other lisp dialects.  This is very handy for setting and querying  
generic, global configuration items, without "passing state"  
explicitly between function calls, which is what parameters (and  
global variables) are commonly used for.

Contrary to global variables, parameters are thread-local, so every  
thread has its own set of parameter values.

The combination of these two features makes parameters very useful.


Tobia