This page is still under construction and will be gradually filled in.


Traditionally, programming languages define whitespace and comment. Whitespaces and comments, also referred to as layout, can appear anywhere in a program, and in many programming languages, these parts of the program have no effect on the program execution, and therefore, insignificant. For example, in Java one can type:

1 + 2 * 3 // equal to 7!

instead of 1+2*3.

In traditional, two phase parsing, a lexer (the lexing phase) produces a stream of tokens throwing out the layout, and the grammar (parser) is written if no layout exists. In single-phase parsing, there is no separate lexing phase, and a parser has to explicitly deal with layout. For example, a nonterminal defining layout, say LAYOUT, can be inserted between each two symbols in the grammar rules:

    | Num

Such layout insertion can also be done automatically. Our library supports automatic layout insertion, which can be conveniently used for the most parts of a grammar, manual layout insertion, and no layout insertion.

To support automatic layout insertion, our binary sequence combinator ~ declares an implicit parameter of type Layout, so that a parser for layout, defined in the scope of (a part of) the grammar, can be implicitly passed to ~. Sequence combinator ~ uses this parser and a more basic sequence combinator ~~ to compose two argument parsers inserting layout between them.

To define a parser for layout, function layout (which is similar to syn) can be used as follows:

implicit val L = layout { """\s?""".r }

val E: Nonterminal = syn ( E ~ "*" ~ E
                     | E ~ "+" ~ E
                     | Num )

In this case, we use scala regular expression to define layout as optional whitespace. The result of layout is of type Layout. Also note that L is declared as an implicit value, and nonterminal parser E can be defined as if no layout exists.

Our library provides a default layout definition (LAYOUT), which uses the following scala regular expression


That is, the default layout can recognize whitespace, single line comment and C-style multiline comment.

If no layout is defined in the scope of the grammar, the default layout will be passed to ~. Note that the default layout will also be used if syn is accidentally used in place of layout, or implicit keyword is missing.