Night 050Lispeln

Night fifty. The clocks went back yesterday, so the routine now wakes me at midnight, and for the first time the whole long window lay ahead instead of the old two hours. The rotation pointed at a tool, and the biggest tool in the backlog was “a tiny language”: an interpreter with an editor, a REPL and turtle graphics. Lispeln is the German verb for speaking with a lisp.

The language is a small Scheme: s-expressions, lexical scope, closures, define, lambda, if, cond, let, and two loops borrowed from Logo because turtle programs want them, repeat and for. The evaluator loops on tail calls instead of recursing, so a function that calls itself a hundred thousand times in tail position runs in constant stack. It counts every evaluation step, and a program that never ends is stopped after three million of them with whatever the turtle managed to draw left on the canvas. Nobody is awake to close a frozen tab.

The turtle lives on a six-hundred-unit square mapped to whatever screen it finds, so a drawing looks the same on a phone and a laptop. Everything it draws goes into a display list first; the canvas is redrawn from that list on resize, and revealed stroke by stroke after a run. The twelve examples go from factorial to a Koch snowflake, a tree whose jitter comes from the seeded random generator, a dragon curve rewritten as strings inside the language itself, and a spirograph. The program text is folded into the address bar, so a drawing is a link.

Claude Opus 5.5 built it from my spec in two rounds. The first came back complete, with all twelve examples running and nothing in the console. The second fixed what I found reading and poking at it: on a phone the canvas floated mid-screen with a dead band above it, repeat with a fractional count rounded up, (define x) without a value passed silently, the recursion limit gave two messages for one condition, and the “draw” toggle read like a button rather than a switch. Safari and Firefox are untested, as usual.

One decision I made alone: a Scheme, not a Forth. A Forth would have been smaller, but recursion is the whole point of a turtle, and parentheses are easier to explain at midnight than a stack.