Anticipated changes of parameters

Parameters are Scheme's way to realize dynamic bindings, and have
standardized in RnRS.  However, RnRS doesn't specify interactions between
parameters and threads.  Implementations, including Gauche, have extended
semantics of parameters in their own way.

SRFI-226, currently in draft, attempts to define interactions of
parameters, continuations and threads.  It defines a clean model and we
plan to adopt it.  However, there are some incompatibilities between
current Gauche's parameters and the srfi's.

This mail explains the planned changes, and also solicits user's
opinion about the matter---if the change will break your code seriously, we
want to find a better way to make transition smooth.

We'll have some transition period so that the current code will work for
some time.

1. Parameters and thread local storages will be splitted.

Srfi-226 parameters are not thread-specific.  This is an inevitable
consequence: Dynamic environment is captured in a continuation.  A
continuation can be invoked with a thread different from the one that
captured it.  If parameters are thread-specific, the continuation won't see
the same environment as when it is invoked in the same thread.

From the next release, we provide thread locals, an object that realizes
thread-specific storage.  The interface follows srfi-226.   If you're using
parameters for thread-specific storage, please consider moving to thread
locals.

2. Parameter-like objects won't be supported.

Currently, you can use something "that works like a parameter bot not a
<parameter>" in the parameterize form.  This "something" is a procedure
which returns the current value when called without an argument, and update
its value when called with one argument (and returns the previous value).
 This is Gauche's extension.

This is a byproduct of the specifc way we implement 'parametize'.  Once we
move to the srfi-226 model, however, the way the dynamic binding is
represented will be changed and it will no longer be the case.

If your code uses this kind of "non-parameter parameters", it needs to be
changed to use a proper parameters.

3. Hooks will be dropped.

Gauche's parameter can have hooks that are called whenever parameter values
are changed.  I added this feature just because "I could".  Personally I
never used this feature.

Does anybody use this feature and need to be supported in future?

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