Syntax
Comments
Like many lisps, Urn uses ; to mark a line comment. This will result in the parser ignoring all input until the next
new line. There is no block comment syntax, meaning you will have to comment each line individually.
Number literals
Numbers follow a similar format as Lua, with additional support for binary literals.
- Binary digits start with
#b, then one or more0or1s. Using an invalid digit (such as 2, will result in a syntax error). - Hexadecimal digits start with
#x, then one or more hexadecimal digits, ([0-9a-zA-Z]). Unlike Lua, you cannot have fractions or exponents in hexadecimal digits. - Decimal digits: these follow the same format as Lua: an optional integer part, an optional fractional part, all
followed by an optional exponent (marked
eorE).
For example, 2, #x2, #b10, 2.e0, .2e1 and 20e-1 are all the same value.
Strings
Strings are delimited by double quotes. Characters can be escaped using the \ character, with the following escape
codes defined:
\a: Bell\b: Backspace\f: Form feed\n: New line\r: Carriage return\t: Horizontal tab\v: Vertical tab\\: Backslash\": Literal".
Note: Using single quotes may not always be a syntax error. However, it will definitely not give you want you want. For instance,
'foo'is a quoted version of thefoo'symbol.
Urn also allows decimal and hexadecimal escape codes. You can specify up to three decimal digits, or x followed by two
hexadecimal digits. It is a parser error for the resulting value to exceed 255 (so \300 will not parse). Consequently,
"A", "\65" and "\x41" are all equivalent strings.
Strings can also span multiple lines. In this case, all successive lines must be aligned to the character after the opening quote mark. This is especially convenient for documentation strings.
(print! "Hello,
World!")
Should you wish to split a string over multiple lines, but not insert a new line character, you can terminate each line with \:
;; Equivalent to (print! "Hello, World!")
(print! "Hello, \
World!")
Symbols
Symbols represent a reference to a specific variable. Symbols can be composed of almost any character:
- The first character of a symbol must be any letter or one of
!#$%&*+-./<=>?\^_`|. - The remaining characters can be any letter, number or one of
!"#$%&'*+,-./:<=>?@\^_`|>.
It is worth noting that symbols are case sensitive: foo is not the same as Foo. You should try to keep variable
names case sensitive, using kebab-case should you need to separate words.
Urn also has a couple of naming conventions:
- Predicates should end with
?. For instance,list?is a function which tests whether its argument is a list. - Impure functions should end with
!. For example,push-cdr!mutates its first argument by adding an element to it. - When dealing with modules,
/is used to separate child packages or components.
Booleans and nil.
The standard values true, false and nil take no special meaning with Urn: they are just another variable which can
be shadowed at will.
Keys
Keys are a rather weird type. They follow the same syntactic form as symbols, though must be prefixed with
:.
(print! :foo)
Keys primary purpose is as indexes in structures. For instance, you will commonly see code like (.> foo :bar), which
is equivalent to Lua’s foo.bar. Generally, you can consider keys to be equivalent to strings. However, when quoted,
keys are compiled to an object with the type key.
Lists
Lists are delimited by matching pairs of () or []. They can contain any number of elements, as long as the
list is terminated.
() ;; The empty list
(foo bar 23 "foo" [:baz]) ;; Another list, including a series of constants and nested lists.
Structs
Structs are Urn’s equivalent of tables, holding key value pairs. They are by specifying key value pairs inside {}. For
more information, see the const-struct documentation.
{ :foo 2
:bar 3 }
“Shortcut” characters
Urn offers several shortcut characters, which expand into a list which calls a given variable with the next form. These are as follows:
': Expands toquote.`: Expands tosyntax-quote.~: Expands toquasiquote.,: Expands tounquote.,@: Expands tounquote-splice.@: Expands tosplice.
'foo ;; A quoted symbol, which expands to (quote foo)
`(foo bar) ;; A syntax-quoted list (expands to (syntax-quote (foo bar)) ).
,23 ;; An unquoted number, which expands to (unquote 23).