Support proper cons cells (dotted pairs) #60

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opened 2026-02-07 17:11:24 +00:00 by navicore · 0 comments

navicore commented

2026-02-07 17:11:24 +00:00

(Migrated from github.com)

Summary

Relax the cons constraint to allow any value as the second argument, enabling proper dotted pairs and improper lists.

Motivation

Currently seq-lisp's cons requires a list as the second argument. This is non-standard — in Scheme/Common Lisp, (cons 1 2) produces the dotted pair (1 . 2).

We hit this exact limitation during the document symbols LSP work, where (cons #f pos) failed because pos is a number. We had to use list as a workaround.

seq-lisp-2 implements this correctly with SCons { car, cdr } where cdr can be any value.

Scope

Update cons in src/eval.seq to accept any value as second argument
Add dotted pair display support to the printer ((1 . 2))
Add dotted pair syntax to the reader/parser
Update car/cdr to work on dotted pairs
Ensure list operations (map, filter, length, etc.) handle improper lists gracefully
Add tests for dotted pairs

Notes

This makes the Lisp more standard and eliminates a class of surprising runtime errors.

Inspired by seq-lisp-2/src/types.seq SCons type.

## Summary Relax the `cons` constraint to allow any value as the second argument, enabling proper dotted pairs and improper lists. ## Motivation Currently seq-lisp's `cons` requires a list as the second argument. This is non-standard — in Scheme/Common Lisp, `(cons 1 2)` produces the dotted pair `(1 . 2)`. We hit this exact limitation during the document symbols LSP work, where `(cons #f pos)` failed because `pos` is a number. We had to use `list` as a workaround. seq-lisp-2 implements this correctly with `SCons { car, cdr }` where `cdr` can be any value. ## Scope - Update `cons` in `src/eval.seq` to accept any value as second argument - Add dotted pair display support to the printer (`(1 . 2)`) - Add dotted pair syntax to the reader/parser - Update `car`/`cdr` to work on dotted pairs - Ensure list operations (`map`, `filter`, `length`, etc.) handle improper lists gracefully - Add tests for dotted pairs ## Notes This makes the Lisp more standard and eliminates a class of surprising runtime errors. Inspired by `seq-lisp-2/src/types.seq` `SCons` type.