Lykn

Loops

Lykn inherits JavaScript’s loop forms as kernel passthrough. They compile to their JS equivalents with no transformation.

for-of — The Recommended Loop

For iterating over collections — arrays, strings, Maps, Sets, any iterable:

(for-of item items
  (console:log item))

for (const item of items) {
  console.log(item);
}

for-of is the loop that functional programmers reach for when map/filter/reduce don’t fit — typically when the iteration performs side effects or needs early exit via break.

while

(while (> remaining 0)
  (console:log remaining)
  (-- remaining))

while (remaining > 0) {
  console.log(remaining);
  --remaining;
}

for — C-Style

The traditional three-part loop: (for init test update body...).

(for (let i 0) (< i 10) (++ i)
  (console:log i))

for (let i = 0; i < 10; ++i) {
  console.log(i);
}

Note: the for initializer uses kernel let because the loop variable must be reassignable across iterations. This is one of the few places where let appears in Lykn code — a necessary concession to imperative iteration.

for-in — Property Enumeration

(for-in key obj
  (console:log key (get obj key)))

for (const key in obj) {
  console.log(key, obj[key]);
}

for-in includes inherited properties from the prototype chain — a perennial source of bugs. Prefer Object:keys or Object:entries with for-of:

(for-of entry (Object:entries obj)
  (console:log entry))

do-while

Executes the body at least once, then checks the condition:

(do-while (should-continue?)
  (console:log "at least once"))

do {
  console.log("at least once");
} while (shouldContinue());

Note: in Lykn’s do-while, the test comes first syntactically (for consistency with while), but the body executes first at runtime.

When to Loop, When Not To

Surface Lykn prefers functional patterns over imperative loops:

• Transforming data → map, filter, reduce (Ch 7)
• Chaining transformations → threading macros (Ch 13)
• Recursive structures → recursion (Ch 7)
• Side effects on each item → for-of
• Early exit needed → for-of with break
• Index-based iteration → for
• Condition-based repetition → while

If you’re reaching for for to transform an array, map is almost always cleaner. Loops exist because they’re sometimes the right tool — but in a functional language, that’s less often than you might expect.