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Late binding for record–type methods

Posted on September 15, 2015

“Late binding”... how elegant, how mysterious. When you call it “run–time dispatching”, which is the same at some level of abstract reasoning, it does not sound as elegant; it sounds vulgar, it tastes of look–up tables. I have implemented late binding for record–type methods in Vicare; the code is in the current head of the master branch.

Here is how it works. First, let’s create a file containing this library:

#!vicare
(library (subdemo)
  (options tagged-language)
  (export greetings)
  (import (vicare))
  (define (greetings lang)
    (display (.greetings lang))
    (newline)))

and compile it:

$ vicare --compile ./subdemo.sls --output ./subdemo.fasl

the form:

(.greetings lang)

is transformed by the Scheme reader into:

(method-call greetings lang)

the expander does not know the type of the lang argument to function; so the syntax method-call will expand to an application of the function method-call-late-binding:

(method-call-late-binding 'greetings lang)

Now let’s create a file containing this program, which imports the (subdemo) library and calls the greetings function:

#!vicare
(program (demo)
  (options tagged-language)
  (import (vicare)
    (subdemo))

  (define-record-type <lang>
    (fields greetings))

  (define {it <lang>}
    (new <lang> "ciao"))

  (define {en <lang>}
    (new <lang> "hello"))

  (define {jp <lang>}
    (new <lang> "ohayo"))

  (greetings it)
  (greetings en)
  (greetings jp)

  (flush-output-port (current-output-port))

  #| end of program |# )

and compile it:

$ vicare -L . --compile ./demo.sps --output ./demo

this program defines the record–type <lang> that was non–existent when the library was compiled. The expansion of the define-record-type syntax use automatically creates methods to access and mutate record–type fields; in this case: an accessor method for the field ‘greetings’ is defined. Let’s run the program:

$ vicare -L . --binary-program ./demo
ciao
hello
ohayo

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