Warts

Where Elle surprises a programmer from another Lisp: the intentional differences, then the known limitations.

Intentional differences

The three that shape whole programs are in AGENTS.md and QUICKSTART.md; this is the full list.

nil and () are distinct. nil is falsy; () is the empty list and is

truthy. Use empty? for end-of-list. empty-list.md gives the rationale.

Only nil and false are falsy. 0, "" and () are truthy.
# starts a comment, and ; splices. ;[1 2 3] spreads into the

surrounding form.

assign mutates; set builds a set.
let is sequential. Bindings are flat pairs, and each sees the ones

before it.

A bare literal is immutable; @ makes it mutable. [1 2] is an array,

@[1 2] an @array.

Strings count grapheme clusters. length, indexing and slicing work in

grapheme clusters, not bytes or code points.

Integers wrap. Integer arithmetic is 64-bit two's complement, and / on

two integers truncates.

print and println write their arguments with no separator. A string

prints without quotes and a keyword without its colon.

(assert (if () true false))
(assert (= (length "café") 4))
(assert (= (+ 9223372036854775807 1) -9223372036854775808))
(assert (= (/ 7 2) 3))
(assert (= (set 1 2) |1 2|))
(let [a 1 b (+ a 1)] (assert (= b 2)))

Known limitations

Non-tail recursion stops at depth 200 (#1207). Tail calls, mutual ones

included, run in constant stack.

A macro cannot be exported from a module. A module's defmacro is not a

value its export struct can name (macros.md).

A macro cannot return an improper list. The expander converts a macro's

result to syntax, and syntax has no improper-list form.

A mutable container stored into itself leaks its region (#1228). Other

mutable cycles free with their owner where the ownership forest can own them (regions/semantics.md).