psil

Grammar

This document describes Psil’s grammar in a somewhat formal way, which roughly follows EBNF.

The actual parser may do something slightly different (but equivalent) than this, based on convenience.

The Grammar

• program ::= expr_list? end
• expr_list ::= expr+

• expr ::= atom

  invocation

• atom ::= number

  boolean

  string

  symbol

  lambda

  identifier

  special_form

• invocation ::= ( (identifier

  special_form) expr_list? )

• lambda ::= { params? expr }

• params ::= **

  ** identifier+ **

  **

• special_form ::= if

  cond

  define

  do

  and

  or

Tokens

Above, all words/characters in bold are token literals, while all words in italics are “specially defined tokens”:

• end is the end of the input stream.
• number is any “word” that can be parsed as a Rust f64.

• boolean ::= true

  false

• string ::= “ c* “, where c is any character except ". There are also the escape sequences \\, \n, \r, \t, \". Strings such as "\" and “\z” are considered invalid tokens.
• symbol ::= #c+, where c is any character excluding whitespace.
• identifier is pretty much any word that doesn’t conflict with another token.

Comments

Not specified in the formal grammar is Psil’s “comments”. Currently, only single-line comments are supported, which start with ;, and are terminated with the new line character.

; This is an example of a comment

This site is open source. Improve this page.