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ccgoto: fix overflows

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mehbark 2026-02-19 19:15:38 -05:00
parent a36fd874ec
commit 13f1113536
Signed by: mbk
GPG key ID: E333EC1335FFCCDB
1 changed files with 13 additions and 3 deletions
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16
site-src/ccgoto.mdx
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@ -146,7 +146,7 @@ operator.
The `k` that `call/cc` calls its argument with represents, roughly, the rest of the computation.
The “current continuation” is what will be executed next at the point that `call/cc` is called.
Incidentally, this helps me understand scheme’s multiple return values; `(values v1 v2 ...)` is just `(call/cc (lambda (k) (k v1 v2 ...)))`.
Incidentally, this helps me understand scheme’s multiple return values; `(values v1 v2 ...)` is just <code class="long">(call/cc (lambda (k) (k v1 v2 ...)))</code>.
I recommend reading about continuations in Dybvig’s [<i>The Scheme Programming Language</i>](https://www.scheme.com/tspl4/further.html#g63)
if you’re (justly) dissatisfied with my explanation or just want to learn more precisely how they work and their applications.
@ -223,7 +223,12 @@ done
I’ll explain this macro one part at a time.
First, `(define-syntax goto (syntax-rules () [...]))` defines `goto` as a syntax transformer (more precise name for a macro) using the `syntax-rules` pattern-matching language.
First,
```scheme
(define-syntax goto
(syntax-rules () [...]))
```
defines `goto` as a syntax transformer (more precise name for a macro) using the `syntax-rules` pattern-matching language.
The `()` after `syntax-rules` is the empty list of literals; we don't have any special words here, so it doesn't apply.
You can read more about how `syntax-rules` works in [TSPL](https://scheme.com/tspl4/syntax.html#./syntax:s14), but we'll only be using the most basic features here.
The important thing is to know that matched names are replaced in the output and that `x ...` matches/splices zero or more expressions.
@ -292,7 +297,12 @@ We wrap the body of `with-goto` in `(call/cc (lambda (k) ...))`.
Inside the body, if we call `k` like `(k (label))` we effectively replace the body with the result of calling `label`.
We accomplished a jump!
`(set! goto (lambda (label) (k (label))))` makes `goto` do exactly this (note that function arguments have to be evaluated before the procedure call takes place).
```scheme
(set! goto
(lambda (label)
(k (label))))
```
makes `goto` do exactly this (note that function arguments have to be evaluated before the procedure call takes place).
We use `(define goto #f)` combined with a `set!` because the labels we defined earlier need to be able to see the `goto` function.
This is what our first `with-goto` looks like when we expand it: