Hyper Lisp Markup Language

Introduction

HLML is a expirmental new markup language/scripting environment

Lisp Concepts

First, some basic concepts of writing code in a lisp:

1. Atoms: Things like 3 "hello" evaluate to themselves, and are basic atoms.
2. Symbols: Things written f are the “symbol” f. Evaluating a symbol causes it to be looked up in the enviornment and replaced with their definiton.
3. Lists: Writing (f 2 3) is the list containing 1, 2, & 3. Evaluating this list causes f to be interpreted as a function, and 2, 3, to be interpreted as arguments.

   (f 2 3) is equivalent to writing f(2,3) in most languages.

   Example: (+ 1 2) evaluates to 3.

   This means you cannot write the list consisting of 1,2,3 as (1 2 3), as when evaluated, this will treat 1 as a function, which it’s not, and will thus cause a type error.

   If you want to produce a list instead of calling a function, use the list function: (list 1 2 3) evaluates to (1 2 3).

4. Quoting: This is were things get lispy. There are is a builtin form called quote that prevents its arguments from being evaluated. So evaluating (quote x) gets x, regardless of what x is defined as, or if it’s defined at all. This can be used to make lists: (quote (1 2 3)) is equivalent to (list 1 2 3), as quote stops the evaluation, so 1 does not get treated as a fucntion. However (quote (1 (+ 1 1) 3)) is not equivalent to (list 1 (+ 1 1) 3). quote stops all evaluation, so the former evalutes to (1 (+ 1 1) 3), whereas normal functions evaluate their aguments, producing: (1 2 3).

   In addition to quote, there are the forms quasiquote and unquote, shortcutted to ` and , respectivly. These allow you to conditionally resume evaluation. For example: `(1 ,(+ 2 3) 3) evaluates to (1 5 3).

5. Useful builtins:
   • define -> Defines a variable: (define x 3) (+ x 2) evaluates to 5
   • define -> Define a fucntion (define (add1 x) (+ 1 x)) (add1 3) evaluates to 4
   • list -> produce a list
   • map -> maps a function over a list: (map add1 (list 1 2 3)) evaluates to (2 3 4)

The Markup Language

HLML looks fairly similar to HTML but using lisp syntax. So to use the b tag, you’d write: (b Here is my text in bold). Tag attributes are specified by following a tag with the following list: (attr (name value) ...), for example: (a (attr (href "google.com")) Link to google).

Scripting support is enabled by the script tag. Scripts can create hlml objects by just making symbols with quote.

An example hlml file is provided in test.hlml

Prototype Implementation

The file hlml.rkt is a prototype implementation of the language.

To use it, run racket hlml.rkt <file>, which will evaluate the script calls and produce the final hlml represesntation.

Run racket hlml.rkt -o <file> to compile hlml to html, to view in a browser.

Todo

Features remaining to implemented: Mutation, scripts not allowed to mutate the page yet. Big feature. Attribute syntax: Does this syntax work nicely, or is it annoying? (Maybe it should be ((a (href "example.com")) click)) CSS?