The whole Felix library has now been converted to use type classes.
**************************************************************************
No "modules" anywhere. The only semantic change: at this stage classes
can't contain initialised variables, there's no _init_() function as there is
for
non-polymorphic modules. (Bug to be fixed perhaps?)
A couple of variables had to be moved into
global scope to cope with this issue: these are nasties: the system job queue
and clock.
The typeclass keyword is gone.
**********************************
Please use "class" to fool newbies into thinking
Felix has a weird and wonderful kind of class system. It's the truth!
Effects: mainly, this will make Felix easier to maintain. Modules still exist,
but now I can think about taking them out, at least of the front end.
One upside is the following clever idea:
Instead of:
module IO { string readln(file) {
if posix do
return posix_read(file);
elif windows do
return windows_read(file);
else
throw "Unsupported OS"
done
}
which uses conditional compilation and looks messy, we do this instead:
class IO { virtual string read(file); }
if posix do
instance IO { string read(f:file)=>read_posix(f); }
elif windows do
instance IO { string read(f:file)=>read_windows(f); }
done
We're still using conditional compilation BUT now we can still add
an instance for OSX. Furthermore, the instances can be moved into
platform specific places and many of them conditionally selected
at once. In fact with code generators we could do it without conditional
compilation (at config time) .. if we wanted.
The important thing is we can provide platform independent code
(the IO class) with platform specific "holes" in it that can be filled
in elsewhere, providing a much better way to separate the platform
independent and platform specific code than before.
(We could even use a type parameter instead)
******************************
There is another possibility here: we provide an instance which uses
a record of functions, dynamically installed in the record. This
is real dynamic dispatch. It's almost OO, except it is more like
OO with multi-methods because the dispatcher handles a collection
of types. Haskell does something like this already, passing dictionaries.
If something works along these lines we might provide compiler and
run time support.
--
john skaller
skal...@users.sourceforge.net
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