Syntax

A Swan program is one expression, made of operands (names, literals and bracketed groups) joined by operators. This chapter describes the operands; Operators describes the rest.

Whitespace and comments

Spaces, tabs and line breaks separate tokens and are otherwise ignored, so an expression can span several lines.

A comment starts with # and runs to the end of the line:

x = 10     # bind 10 to x
x + 1      # => 11

Names

A name starts with a letter or _ and continues with letters, digits and _. Names are case-sensitive.

total = 1
_tmp = 2
item2 = 3
Total = 4
total                # => 1

Swan has no keywords. TRUE, FALSE and the built-in namespaces are ordinary names bound in the global scope, so you can shadow them (but you rarely want to). A name that is not bound evaluates to Nothing, not to an error:

not_bound_anywhere   # => ()

Numbers

A number literal is a sequence of digits, with an optional fraction and an optional exponent. All numbers are 64-bit floating-point values.

42                   # => 42
3.14                 # => 3.14
1.5e3                # => 1500
2E-2                 # => 0.02

A number cannot start or end with a dot, and cannot contain _. A negative number is the - operator applied to a positive literal.

.5                   # => Undefined
1_000                # => Undefined
-0.5                 # => -0.5

Text

A text literal is enclosed in double quotes. It can span several lines.

"hello"              # => "hello"
greeting = "Dear reader,
  welcome."
text.size greeting   # => 23

Text literals have no escape sequences: a backslash is an ordinary character, and a literal cannot contain a double quote. Single quotes do not delimit text.

"a\nb"               # this is the four characters a, \, n, b
text.size "a\nb"     # => 4
'single'             # => Undefined

In a template, write quotes and other special characters in the text around the tags instead.

Brackets

(1 + 2) * 3          # => 9
()                   # => ()
[1, 2, 3]            # => [1, 2, 3]
{a: 1, b: 2}         # => {a: 1, b: 2}

Syntax errors

Swan never rejects a program. A syntax error, such as a missing closing bracket or a misplaced operator, produces an Undefined value in the place where the error is, and the rest of the expression is evaluated as usual.

(1 + 2               # => Undefined
1 + * 2              # => Undefined

See Undefined values to find out what went wrong.