Example REPL session

>>> (car '(x))
x
>>> (eq 'foo (car '(foo)))
t
>>> ((lambda (x) (cons x '(b))) 'a)
(a b)
>>> (eval '((lambda (x) (cons x '(b))) 'a) '())
(a b)

Short-term plan

Make a REPL work in a web page
Change the 'eval' evaluator to compare values, not symbols
Change the 'eval' evaluator to pass function values, not lambda exprs
Change the 'eval' evaluator to accept varargs

Three surprises on the way to the interpreter

Reynolds warns in his "Definitional Interpreters" that it's very easy for features of the implementing language to "leak through" into the language or interpreter being implemented. The risk is especially high when you're using something like Common Lisp as the metalanguage; you might be using a feature without noticing.

By defining a CEK machine, I head off such implicit usages of features. I need to explicitly support things that the Ipso evaluator on top will then use. If I don't, then the evaluator simply won't work.

The following three things surprised me along the way, things that "The Roots of Lisp" swept under the rug but I needed to address in order to build the evaluator.

Surprise 1: First-class functions are quoted lambdas

This first one was a surprise, but it's not wrong as such. It's just surprising from a modern, Scheme-enlightened perspective. It's about what functions are when you pass them around in the program.

Let me quote the relevant part of "The Roots of Lisp":

If an expression has as its first element an atom f that is not one of the primitive operators

(f a₁ ... a_n)

and the value of f is a function (lambda (p₁ ... p_n) e) then the value of the expression is the value of

((lambda (p₁ ... p_n) e) a₁ ... a_n)

In other words, parameters can be used as operators in expressions as well as arguments:
> ((lambda (f) (f '(b c)))
   '(lambda (x) (cons 'a x)))
(a b c)

It goes by really quickly, so let me just mention what threw me off this time: it was the apostrophe/quote sign before (lambda (x) (cons 'a x)) in that final example evaluation.

It means that, when we pass a function (as the lambda in that example to the f parameter) we literally quote a lambda form and pass it, unevaluated. Later, we look up f, find the lambda form, and substitute it in directly in place of the f. That is, evaluation of (f '(b c)) looks up f and delegates to evaluation of ((lambda (x) (cons 'a x)) '(b c)).

If we were to understand program composition on the lexical level only, there would be no apparent reason why the text representing a procedure couldn't replace the text representing a variable before the compound expression is executed.

— "Sans-Papiers as First-Class Citizens", Julian Rohrhuber

Let's call this approach the textual approach to first-class functions. It came as a surprised to me only because I've internalized and gotten used to the modern alternative, which we might call the esoteric approach. Let me show them side by side to compare them:

Textual approach:
- On the passing side, quote the lambda (delaying its evaluation).
- On the usage side, when the operator is not in the closed set of primitives, look up its value and substitute it in, re-evaluating.
- Consistent principle: quote is used whenever we are passing data.
- Consistent principle: lambda is only (sensibly) evaluated in operator position; that is, first in a form.
Esoteric approach:
- On the passing side, evaluate the lambda. This results in a previously-unseen type of value which we'll call a function object. The internal representation of a function object is not so constrained; in particular, it may or may not be just a lambda form.
- On the usage side, always look up the operator name (choosing whether or not to hard-code the built-ins and special forms first); if the value found is a function object, invoke it — that is, evaluate the operands, extend the environment with the resulting arguments, and evaluate the function value's body in the extended environment.
- Consistent principle: just as there's a distinction between the numeral "5" and the number 5, there's a distinction between how you write a function in code, and the underlying value it evaluates to.
- Consistent principle: lambda itself already delays evaluation, by wrapping code inside an abstraction. Using quote is superfluous.

The esoteric approach has more moving parts (a new type of value) and takes more explaining, but as an idea it also has longer reach. The reason has to do with lexical variable scope.

Each formalism implies a specific distinction between objects and the system of their combination. Thereby, the concept of function has a peculiar role: it governs how objects interact, and is also an object of computation. Over more than a century, this intermediary status has broached the problem of how exactly a formalism should admit functions as first-class citizens.

— "Sans-Papiers as First-Class Citizens", Julian Rohrhuber

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Surprise 2: `defun` is not simply sugar for `label` + `lambda`

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Surprise 3: Global scope and mutually recursive definitions

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License

masak/ipso

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