Tag Archives: BOOL

BOOL Exceptions

When BOOL gets into trouble at run-time it throws an Exception. Such problems include resource issues (like a missing file) or code issues (such as when a Model can’t find an Action to handle a received Message).

There are a some native Exceptions, but user code often extends those for the sake of the application (following a common model used in Java, Python, and many other languages).

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More about “new”

Picking up the topic of Model implementation two years later, some further thoughts about the new method as well as about the set and get methods.

The summary here is that all three are virtual methods built into the implementation. They have to be to handle the under-the-hood aspects of Instances. They can be “extended” by user code, but not over-ridden.

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Array Redux

Strange how my thoughts turn to BOOL in the early part of even-numbered years. (I wonder if it has something to do with the San Francisco Giants baseball team, although I don’t know why, and I’ve been hitting even-numbered BOOL version years longer than they’ve been winning World Series, plus I hope they don’t win this year.)

But in any event, I found myself pondering the *array type (again)…

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BOOL Message Center

It’s been a while since I’ve waded in BOOL waters. As always, there are periods of strong interest and periods — often quite long — of dormancy. Finally solving most of the implementation problems of the language triggered a period of disinterest. That and some uncertainties about exactly how to implement Action Methods, especially the “new” method.

But I recently did have some fun with a BOOL/OS subsystem, the Message Center Service. I roughed out a crude working version in Python that provides a flavor of the real thing’s architecture.

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Implementing “new”

One of the last hurdles involves the implementation of the new behavior. Models manage Instance value objects and provide new data objects (used by Instance objects to populate Call Frame slots). Model objects must provide an API to support creating and destroying Instance objects as well as manipulating their value objects.

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Binding Instances and Models

There is a design choice involved with regard to how Instance objects are bound to their Model object. A similar choice exists concerning Actor objects and their Action objects. (Actors can be called “instances” of an Action.)

The choice is between a BOOL-like design where the Instance sends a message to the Model, or an implementation design that lets the Instance directly call the Model. This post documents the two choices and the resolution to use the latter.

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Singleton Messages and Actions

A recommended idiom involves defining Actions to implement Singleton Messages. This cleans up source code by removing the need to use the semi-colon, and shifts possible “Unknown Message” errors to data type errors, some of which can be detected at compile time.

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Action Walkthrough, part 1

This post series explores an Action in detail with emphasis on the Instance objects, especially the dynamic (temporary) ones. Posts are broken up mainly for size considerations.

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Casting Values

There is the situation when an operation takes multiple operands, and one (or more) of those operands are inappropriate for the operation. For example, suppose an integer add operation was passed an integer object and an open file object? There is generally no “add” semantic for an integer and a file.

Total incompatibility results in a run-time error, but sometimes values are compatible enough for the operation to proceed. How the system accomplishes that often involves casting one value into another.

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Message Arity (revisited)

I recently determined to use a trailing marker — a semicolon — to close the list of arguments due a Message. But after using it for a while, I’ve decided that this:

.   set:x add: mul:a b; div:c d; ;;

Is butt-ugly and has to change. Since I still think registering Message arity is an even worse option, I’ve had to take the only other real option: assuming Message arity.

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