Syntax

S-Expressions

sel uses a uniform S-expression syntax. Every piece of code is either:

• an atom — a number, string literal, or symbol
• a list — zero or more expressions enclosed in ( … )

The first element of a list is the operator (a special form or a function name); the remaining elements are the arguments.

(+ 1 2)          ; call operator +, two arguments
(println "hi")   ; call println with one string argument
(if (= x 0) "zero" "nonzero")

Tokens

Symbols

Any sequence of non-whitespace, non-parenthesis, non-quote characters that does not parse as a number is a symbol. Symbols are case-sensitive.

x  foo  my-var  +  =  <<  some-thing-123

Numbers

An optional leading -, at least one digit, optionally one . and more digits, optionally an exponent e/E followed by an optional sign and digits.

42        ; integer
-7        ; negative integer
3.14      ; float
1.5e10    ; scientific notation
-2.0e-3   ; negative float with negative exponent

Integers that fit in a 64-bit long are stored as tagged longs (no allocation). Integers that overflow are automatically promoted to bigint (arbitrary precision decimal strings).

Floats that fit in a double are stored as tagged doubles. Floats with more than 17 significant digits are stored as bigfloat decimal strings and handled with arbitrary-precision arithmetic.

String Literals

sel has two string delimiters:

'...' escape sequences:

'hello'           ; ASCII string
'line1\nline2'    ; newline in single-quoted string
'\x41\x42\x43'   ; "ABC"
"café ☕"         ; raw UTF-8 — any Unicode, no escaping
"C:\Users\name"  ; backslashes are literal

Comments

A semicolon ; begins a line comment that runs to the end of the line.

; this is a comment
(+ 1 2)   ; inline comment

There are no block comments.

Whitespace

Spaces, tabs, and newlines are all interchangeable whitespace. They separate tokens and are otherwise insignificant. Expressions can be broken across lines freely.

(let result
  (if (> x 0)
    (* x 2)
    0))

nil

The symbol nil represents the empty list and falsy zero value. It is equivalent to 0 and compares equal to it.

nil        ; same as 0
(= nil 0)  ; 1 (true)