From: Will Duquette <will@wjduquette.com>
Date: May 9, 2004 4:45:50 PM PDT
To: snit@lists.wjduquette.com
Subject: Snit Reconsidered #4: Default Methods

(This is the fourth in a series of essays I'll be writing in preparation
for the design and implementation of Snit 1.0.  Please comment!)

Snit 1.0 will likely include a mechanism to define a type's "default
method".  The default method can be called by name, just as any
method can; in addition, it will be called when the instance command is
invoked without a method.  It practice, it might look like this:

    % snit::type friendlydog {

         method bark {{times 1}} {
              puts "Woof!"
         }

         defaultmethod wagtail {
              puts "Wag, wag, wag."
         }
    }
    ::friendlydog
    % friendlydog spot
    ::spot
    % spot bark 2
    Woof!
    Woof!
    % spot wagtail
    Wag, wag, wag.
    % spot
    Wag, wag, wag.
    %

As shown in the last command, entering "spot" with no arguments
automatically invoked the default method.  If no default method had been
defined, then invoking "spot" by itself would get the same response Snit
gives now in similar circumstances.

I find this idiom useful in cases where the object represents a single
(possibly complex) piece of data that can be represented as a Tcl string
but that has a number of things that can be done to it.  In this case,
the object's methods represent the valid operations, and the default
method can return the value as a string.  Consider the following partial
type definition:

    % snit::type complex {
        variable real   ;# The real part
        variable imag   ;# The imaginary part

        constructor {initialValue} {
            # Omitted: checks value, which must have form "A+Bi" where
            # "A" and "B" are decimal strings, and sets real and imag.
        }

        defaultmethod value { return "$real+${imag}i" }
        }

        method := {newValue} {
            # Omitted: checks value, which must have form "A+Bi" where
            # "A" and "B" are decimal strings, and sets real and imag.
        }

        method real {} { return $real }

        method imag {} { return $imag }
    }
    % complex x 5+3i
    5+3i
    % x
    5+3i
    % x value
    5+3i
    % x := 2+7i
    2+7i
    % x real
    2
    % x imag
    7
    % complex y [complex expr {([x] + [x]) * ([x] - 5i)}]
    ....

This idiom allows the value to be validated on set, and queried in a
variety of interesting ways, while still making the most typical query
very easy.

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