Once upon a time, I spent multiple months implementing a graphing calculator using GW-BASIC (manual). Would I be able to do the same more quickly and effectively today using generative AI?

❦❦❦

Let’s give it a go, using Claude AI to start:

Implement a simple graphing calculator in GW-BASIC.

—knz

Here is some GW-BASIC code to implement a simple graphing calculator:

```10 ' Simple Graphing Calculator
20 SCREEN 2: CLS
30 INPUT "Enter equation in form y=f(x) "; f\$
40 PRINT "Graphing "; f\$
50 FOR x = 0 TO 319
60 LET y = EVAL(f\$)
70 PSET (x, 159 - y)
80 NEXT x
90 PRINT "Press any key to exit"
100 A\$ = INKEY\$: IF A\$ = "" THEN 100
```

—Claude AI

This is encouraging: Claude AI knows about GW-BASIC’s mandatory line numbering; it properly knows about the `SCREEN` statement and `INKEY\$` function; and it produces a reasonably simple program.

(In contrast, ChatGPT does not know about mandatory line numbers, insisting on using the incompatible Visual Basic syntax; does not know about `SCREEN` and other GW-BASIC language features; and insists on producing a complex program upfront which makes later iterations more difficult. Regardless, the remainder of the results discussed below apply equally to both Claude AI and ChatGPT; and incidentally to Github Copilot as well.)

❦❦❦

Encouraging, yes. But is it good enough?

The GW-BASIC language does not provide an `EVAL` function. Claude AI (and ChatGPT, and Copilot) insist on using one as a first approximation, although it does not exist in the language!

When I originally worked on this, even before starting to program I knew that the heart of the problem was to figure out how to input a mathematical function from the user as a string and evaluate this function for each point on the screen. I knew I needed something like an `EVAL` function and, at the time, most programming languages in common use (including BASIC) did not provide such a facility.

Moreover, at the time (14 years of age), I had not yet studied program compilation and recursive descent parsing and evaluation, and so I would have needed help.

And so Claude AI’s first approximation is not bringing us any closer to the goal today as we would have started many years ago.

Can we nudge it into the right direction?

❦❦❦

Sadly, Claude AI fails here:

GW-BASIC does not provide an EVAL function. Implement EVAL using other language features.

—knz

You’re right, GW-BASIC does not have an EVAL function. Here is one way to implement basic EVAL functionality using other GW-BASIC language features:

```100 'EVAL function
110 DEF FNEVAL(f\$)
120   FOR i = 1 TO LEN(f\$)
130     c\$ = MID\$(f\$, i, 1)
140     IF c\$ = "+" THEN p\$ = p\$ + " + "
150     IF c\$ = "-" THEN p\$ = p\$ + " - "
160     IF c\$ >= "0" AND c\$ <= "9" THEN p\$ = p\$ + c\$
170     IF c\$ = "X" THEN p\$ = p\$ + STR\$(x)
180     IF c\$ = "^" THEN p\$ = p\$ + "**"
190   NEXT
200   EVAL = EVAL(p\$)
210 END DEF
```

—Claude AI

The failure is that it attempts to define `EVAL` in terms of itself—see the recursive call on line 200. (Additionally, we see it starts to use invalid syntax for the target language—DEF FN does not support multi-line definitions in GW-BASIC—but this is inconsequential to the problem at hand.)

ChatGPT, in contrast, appears to be more useful by implementing separate input tokenization and recursive evaluation functions (link to conversation):

Implement an EVAL function in GW-BASIC. Remember, GW-BASIC has mandatory line numbers and does not provide EVAL natively.

—knz

GW-BASIC indeed lacks a built-in EVAL function, but you can implement a basic version using string manipulation and indirect execution. Here’s a simple example of how you might create an EVAL function in GW-BASIC:

(… code …)

This program defines several functions:

• `FNPARSE`: Parses the input expression into tokens.
• `FNOPERATOR`: Returns the precedence of an operator.
• `FNAPPLY`: Applies an operator to two operands.
• `FNPARSEEXPR`: Converts the infix expression into postfix notation.
• `FNEXECUTE`: Evaluates the postfix expression.
• `FNVALIDATE`: Validates the input expression for correctness.
• `FNPARSEANDCALCULATE`: Integrates the above functions to parse and calculate the result of the input expression.

—ChatGPT

Besides the minor inconveniences already identified above (ChatGPT favors uses of syntax that does not exist in GW-BASIC), the approach taken here does not deliver good value: there is a lot of code, but little functionality. This code can barely evaluate simple arithmetic with operator precedence. For example, it knows nothing of GW-BASIC’s own library of mathematical functions (including its trigonometry package)! It also contains a few bugs.

Given the original goal—a graphing calculator—we are still far from a usable solution. We are already paying the expense of a fairly large sub-program (100 to 200 lines of code), a huge performance overhead (100x or more slower than BASIC’s own evaluator), and a rather intricate logic that will then need to be maintained and extended as needed by the addition of new features.

There should be another way?

❦❦❦

At the time I first worked on this problem, the very first thing I did was conscientiously read the user manual (copy here). One feature stood out, one that none of the AI programming assistants seem to know about:

CHAIN Statement

Purpose:

To transfer control to the specified program and pass (chain) variables to it from the current program.

Syntax:

`CHAIN[MERGE] filename[,[line][,[ALL][,DELETE range]]]`

`MERGE` overlays the current program with the called program.

`filename` is the name of the program that is called to be chained to. […]

`line` is a line number or an expression that corresponds to a line number in the called program. It is the starting point for execution of the called program. […]

`ALL` specifies that every variable in the current program is chained to the called program. […]

With this equipment, I was able to implement the following gadget on my own:

```10 INPUT "Enter expression: "; f\$
20 OPEN "EVAL.BAS" FOR OUTPUT AS #1
30 PRINT #1, "10000 Y="; f\$
40 PRINT #1, "10010 RETURN"
50 CLOSE #1
60 CHAIN MERGE "EVAL.BAS", 70, ALL
70 FOR X=0 TO 100
80   GOSUB 10000
90   PRINT "RESULT: "; Y
100 NEXT X
```

This code takes the character string provided by the user, assumes it uses valid BASIC syntax, and loads it as a sub-program into the text of the current program. We then rely on GW-BASIC’s own evaluation code to evaluate the value of Y for each assignment of X.

Simple enough? Let’s see if the programming assistant can get so far (conversation link):

Consider the following manual page for the GW-BASIC language:

… copy of the CHAIN manual page from above …

Using the CHAIN MERGE statement, implement a program that inputs a valid BASIC expression as a function of X, then prints the result of its valuation over a range of values configurable by the user. After the program has printed the value, it should allow the user to choose another range of input values.

Remember, GW-BASIC has mandatory line numbers and does not provide EVAL natively.

—knz

Certainly! Below is an example GW-BASIC program that uses the CHAIN MERGE statement to evaluate a BASIC expression as a function of X over a configurable range of values. The user is prompted to enter a BASIC expression, and then they can input the range of values for X. The program uses a loop to iterate over the range, evaluates the expression for each X value, and prints the result.

```10 PRINT "Enter a BASIC expression as a function of X:"
20 INPUT expression\$

30 REM Save the expression to a temporary file
40 OPEN "TEMP.BAS" FOR OUTPUT AS #1
50 PRINT #1, "100 DEF FNVAL(X) = "; expression\$
60 CLOSE #1

70 REM Chain to the evaluation program
80 CHAIN MERGE "TEMP.BAS"

100 REM Evaluation program starts here
110 LINE INPUT "Enter the starting value for X: ", start
120 LINE INPUT "Enter the ending value for X: ", end
130 LINE INPUT "Enter the step size for X: ", step

140 FOR X = start TO end STEP step
150   result = FNVAL(X)
160   PRINT "Value for X = "; X; " : "; result
170 NEXT X

180 LINE INPUT "Do you want to evaluate another range? (Y/N): ", choice\$
190 IF choice\$ = "Y" THEN CHAIN MERGE "TEMP.BAS"
200 END
```

Explanation:

• The user is prompted to enter a BASIC expression in line 10.
• The expression is saved to a temporary file “TEMP.BAS” in lines 40-60.
• The program then uses the CHAIN MERGE statement to transfer control to the evaluation program in lines 80-80.
• The evaluation program prompts the user to enter the range of values for X (start, end, step) in lines 110-130.
• The program then uses a loop to iterate over the range of X values, evaluates the expression using the FNVAL function, and prints the results in lines 140-160.
• After evaluating the range, the user is prompted to decide whether to evaluate another range in lines 180-190.
• If the user chooses to evaluate another range, the program uses CHAIN MERGE to restart the evaluation program with the new expression in line 190.

Note: This example assumes that the expression entered by the user is valid and does not include error handling for simplicity. You may want to add additional error-checking and handling as needed.

—ChatGPT

(NB: neither Github Copilot nor Claude AI were able to provide a solution.)

How is the program above faring? It has the right structure: it properly emits the user string into a program that gets overlaid onto the current program. The overall control flow has shortcomings, but this can be readily fixed and integrated into a larger program.

❦❦❦

Remarkably, the result above is unstable. It took me multiple iterations of an input prompt to obtain a valid program. Just small variations resulted in programs that were either unsuitable or incorrect, or both.

In other words, it appears as if the programming assistant is only able to help here if the programmer already knows the solution themselves.

❦❦❦

Interestingly, I am also unable to have it generate a program that lets the user input either a new function or a new range of values after the first execution.

All the prompts I tried in this direction confuse ChatGPT and make it reach out for a non-existent built-in EVAL function.

What gives?

❦❦❦

After numerous more experiments, including using other programming assistants and programming languages, the best explanation I have is that AI programming assistants have been designed and optimized to “fill in the blank” for programming problems that follow this model:

That is, the assistant assumes that the user-programmer has a fixed programming language and its abstract machine model (AMM) and the generated solution-program must fit in that box.

As soon as the problem does not fit this model, the generation starts to fail in interesting ways. In the examples above, we are seeing the effects of layered abstractions, where the program code at one level is part of (writable) machine state at another level:

Based on my experiments so far, it is not yet possible to ask programming assistants to design software systems that exploit the I/O facilities of an inner AMM to perform meta-programming using the facilities of an outer AMM.

The machine does not (yet) think outside of the box.

## References

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Raphael ‘kena’ Poss is a computer scientist and software engineer specialized in compiler construction, computer architecture, operating systems and databases.

So what do you think? Did I miss something? Is any part unclear? Leave your comments below.