[gambit-list] How to handle and access and interact with object members host code

Marc Feeley feeley at iro.umontreal.ca
Mon Dec 7 12:00:08 EST 2015

Here’s a simple example creating a JavaScript Date object and extracting the number of milliseconds since January 1, 1970:

(define (new-Date)
  (##inline-host-expression "new Date()"))

(define (.getTime obj)
  (##inline-host-expression "@1 at .getTime()" obj))
(define o (new-Date))

(println (.getTime o))

This is possible because host objects can be used as Scheme objects (they are opaque, but you can return them from functions, pass them as function parameters and store them in data-structures).

Does that clarify things?

BTW, the above code can be improved quite a bit to make it more general.  I’m just giving this example to show it is simple to interface to host objects.


> On Dec 7, 2015, at 11:44 AM, ben yakawp <ben.lists at yakawp.com> wrote:
> Just to be a little more precise. Expressing OO style in Scheme was not
> my primary concern here. The problem I see is that the all of the host
> languages (except the original C backend) access their library functions
> through an OO interface. It's not like the library approach in scheme
> were you can directly use a function after you imported it.
> That means if you want to use a function from a host language you first
> have to create an object. But now what is this object in Scheme and what
> can it do? In the best case, it would check the member calls at compile
> time. In the worst case it add runtime cost in the backend.
> But thats just stuff I was thinking about. 
> What I wanted to know in the first place is how I should accessing
> backing library functions / ffing that are "hidden" behind OO apis. 
> How is this done?
> On Mon, Dec 7, 2015, at 05:13 PM, Marc Feeley wrote:
>> At the Scheme level, you can express the OO style in various ways.  Here
>> are 2 that come to mind:
>> 1) Your “new” operator could return a closure which receives as a first
>> parameter the name of the method to be called.  So you could:
>> (define o (new Lib))
>> (o print: "hello")
>> or
>> (define .print '.print)
>> (o .print "hello")
>> 2) You could make .print a function that receives as a first parameter
>> the object on which the method applies.  For example:
>> (define .print (lambda (self text) …))
>> (define o (new Lib))
>> (.print o "hello")
>> You can of course use macros to automate these styles.
>> Marc
>>> On Dec 7, 2015, at 8:31 AM, ben yakawp <ben.lists at yakawp.com> wrote:
>>> hi
>>> Sorry if this too obvious, but I don't understand how gambit handles
>>> objects from the hosts backend.
>>> For example: if one would like to construct a 'Node' library for the JS
>>> backend, how would you do that? The whole node api seems to be object
>>> oriented.  But also the Python standard libaries (batteries) are layed
>>> out class based. You had to do something like
>>> (define o (new Lib))
>>> (o.print "hello")
>>> This looks wrong. What am I missing?
>>> Regards
>>> ben
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