XL (programming Language) - Syntax

Syntax

Syntax is defined at the XL0 level. The XL0 phase of the compiler can be configured using a syntax description file, where properties like the text representation and precedence of operators are defined. A basic syntax file defines common mathematical notations, like + for addition, with the usually accepted order of operations.

The parse tree consists of 7 node types, 4 leaf node types (integer, real, text and symbol) and 3 internal node types (infix, prefix and block).

• integer nodes represent an integer literal, such as 2. The # sign can be used to specify a base other than 10, as in (2#1001). A separating underscore can be used to improve readability, as in 1_000_000.
• real nodes represent non-integral numbers, such as 2.5. Based-notations and separators can be used, as for integer nodes, for example 16#F.FFF#E-10 is a valid real literal.
• text nodes represent textual contents. They are normally surrounded by simple or double quotes, like "Hello" or 'a', but the syntax file can be used to add other separators, including for multi-line textual contents.
• symbol nodes represent names or operators. Names are sequence of alphanumeric characters beginning with a letter, like Hello. XL0 preserves case, but XL1 ignores case and underscores, so that JohnDoe and john_doe are the same name. Operators are sequences of non-alphanumeric characters, like * or =/=.
• infix nodes represent two nodes related by an infix symbol, like A+1 or 2 and 3. Infix nodes are in particular used to separate lines, with an infix "new-line" symbol.
• prefix nodes represent two consecutive nodes, like Write "Hello". It is also used for postfix notations, like 3! or Open?.
• block nodes represent a node surrounded by grouping symbols, like (A), . Indentation is internally represented by a block node.

With the default syntax file, the following is valid XL0, irrespective of any semantics.

A = B + "Hello"

It parses as:

infix("=", symbol("A"), infix("+", symbol("B"), text("Hello")))