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Fixed runaway recursion in cond. However, let is still segfaulting, and member

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does not work correctly.
This commit is contained in:
Simon Brooke 2026-02-25 11:17:40 +00:00
parent d34d891211
commit 8c63272214
12 changed files with 358 additions and 156 deletions
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105
docs/state-of-play.md
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@ -1,5 +1,110 @@
# State of Play
## 20260224
Found a bug in subtraction, which I hoped might be a clue into the bignum bug;
but it proved just to be a careless bug in the small integer cache code (and
therefore a new regression). Fixed this one, easily.
In the process spotted a new bug in subtracting rationals, which I haven't yet
looked at.
Currently working on a bug which is either in `let` or `cond`, which is leading
to non-terminating recursion...
H'mmm, there are bugs in both.
#### `let`
The unit test for let is segfaulting. That's a new regression today, because in
last night's buildv it doesn't segfault. I don't know what's wrong, but to be
honest I haven't looked very hard because I'm trying to fix the bug in `cond`.
#### `cond`
The unit test for `cond` still passes, so the bug that I'm seeing is not
triggered by it. So it's not necessarily a new bug. What's happening? Well,
`member` doesn't terminate.
The definition is as follows:
```lisp
(set! nil?
(lambda
(o)
"`(nil? object)`: Return `t` if object is `nil`, else `t`."
(= o nil)))
(set! member
(lambda
(item collection)
"`(member item collection)`: Return `t` if this `item` is a member of this `collection`, else `nil`."
(cond
((nil? collection) nil)
((= item (car collection)) t)
(t (member item (cdr collection))))))
```
In the execution trace, with tracing of bind, eval and lambda enabled, I'm
seeing this loop on the stack:
```
Stack frame with 1 arguments:
Context: <= (member item (cdr collection)) <= ((nil? collection) nil) <= (cond ((nil? collection) nil) ((= item (car collection)) t) (t (member item (cdr collection)))) <= "LMDA"
Arg 0: CONS count: 6 value: (member item (cdr collection))
Stack frame with 3 arguments:
Context: <= ((nil? collection) nil) <= (cond ((nil? collection) nil) ((= item (car collection)) t) (t (member item (cdr collection)))) <= "LMDA" <= (member item (cdr collection))
Arg 0: CONS count: 7 value: ((nil? collection) nil)
Arg 1: CONS count: 7 value: ((= item (car collection)) t)
Arg 2: CONS count: 7 value: (t (member item (cdr collection)))
Stack frame with 1 arguments:
Context: <= (cond ((nil? collection) nil) ((= item (car collection)) t) (t (member item (cdr collection)))) <= "LMDA" <= (member item (cdr collection)) <= ((nil? collection) nil)
Arg 0: CONS count: 8 value: (cond ((nil? collection) nil) ((= item (car collection)) t) (t (member item (cdr collection))))
Stack frame with 2 arguments:
Context: <= "LMDA" <= (member item (cdr collection)) <= ((nil? collection) nil) <= (cond ((nil? collection) nil) ((= item (car collection)) t) (t (member item (cdr collection))))
Arg 0: STRG count: 19 value: "LMDA"
Arg 1: NIL count: 4294967295 value: nil
Stack frame with 1 arguments:
Context: <= (member item (cdr collection)) <= ((nil? collection) nil) <= (cond ((nil? collection) nil) ((= item (car collection)) t) (t (member item (cdr collection)))) <= "LMDA"
Arg 0: CONS count: 6 value: (member item (cdr collection))
Stack frame with 3 arguments:
Context: <= ((nil? collection) nil) <= (cond ((nil? collection) nil) ((= item (car collection)) t) (t (member item (cdr collection)))) <= "LMDA" <= (member item (cdr collection))
Arg 0: CONS count: 7 value: ((nil? collection) nil)
Arg 1: CONS count: 7 value: ((= item (car collection)) t)
Arg 2: CONS count: 7 value: (t (member item (cdr collection)))
Stack frame with 1 arguments:
Context: <= (cond ((nil? collection) nil) ((= item (car collection)) t) (t (member item (cdr collection)))) <= "LMDA" <= (member item (cdr collection)) <= ((nil? collection) nil)
Arg 0: CONS count: 8 value: (cond ((nil? collection) nil) ((= item (car collection)) t) (t (member item (cdr collection))))
Stack frame with 2 arguments:
Context: <= "LMDA" <= (member item (cdr collection)) <= ((nil? collection) nil) <= (cond ((nil? collection) nil) ((= item (car collection)) t) (t (member item (cdr collection))))
Arg 0: STRG count: 19 value: "LMDA"
Arg 1: NIL count: 4294967295 value: nil
```
This then just goes on, and on, and on. The longest instance I've got the trace of wound up more than a third of a million stack frames before I killed it. What appears to be happening is that the cond clause
```lisp
((nil? collection) nil)
```
Executes correctly and returns nil; but that instead of terminating the cond expression at that point it continues and executes the following two clauses, resulting in (infinite) recursion.
This is bad.
But what's worse is that the clause
```lisp
((= item (car collection)) t)
```
also doesn't terminate the `cond` expression, even when it should.
And the reason? From the trace, it appears that clauses *never* succeed. But if that's true, how come the unit tests are passing?
Problem for another day.
I'm not going to commit today's work to git, because I don't want to commit something I know segfaults.
## 20260220
### State of the build