Introduction

MetaJS is Lisp compiled to Javascript. Final code runs in browser or on NodeJs plaform without runtime dependencies. More about MetaJS background you can find on coect.net/metajs/. Project Coect aims to replace outdated Email and XMPP with modern universal communication protocol. MetaJS for Coect plays the same role as Emacs Lisp for Emacs.

Bellow is an interactive overview of MetaJS language. You can change MetaJS code as you read docs and imediatelly see generated javascript code.

Destructuring

MetaJS supports destructuring in def, get, set forms:

(def actors ["Neo" "Trinity" "Morpheus"]
     [neo trinity morpheus] actors)

(let digits [1 2 3]
     [one two three] digits
  (set [one two three] digits))

Method and property hooks

For chaining methods calls and object property access there is also method and property hooks. Literal begining with . is treated as method of the previous form, next forms are parameters of the method. Literal begining with @ is property of previous form, for example (get obj 'prop) can be written as (obj @prop). Hooks are espesially useful when they are combined:

(d3 .selectAll "circle" .transition
    .duration 750
    .delay #(* %2 10)
    .attr "r" #(Math.sqrt (* %1 scale)))

Macros

To define a macro you can use standard Lisp syntax-quote, unquote and unquote-splicing:

(defmacro when (x & code)
  `(if ~x (do ~@code)))

(when earth-moves (log "And yet it moves!"))

String interpolation

You can embed variables inside interpolated strings that are started with #. Inside such strings $symbol is replaced with symbol's value and $=var replaced with pair name=value. For escaping $ itself use $$.

#"Hello, $name!"
#"$name, you got $$100."
#"Debug: $=name, $=last-value."

Functions

Functions can have optional parameters and parameters with default values. Each parameter can be passed positionally or as keyword. In addition function can accept variable number of parameters that are accessible as list.

(defn demo-fn (a b:Thing c:? d:2 & more)
 (log a b c d more))

;; an example of the function call
(demo-fn "Just A" "noumenon" c:42)

Smart compilation

MetaJS tries to generate optimized Javascript code. Same MetaJS code can be compiled to several forms of Javascript code. For example, let's look at the classical conditional form if in different contexts.

(defn if-return (a b)
  (if a b))

(defn if-statement (a b)
  (if a (set b a))
  b)

(defn if-expression (a b)
  (log (if a a b)))

Magic

It's not a joke that MetaJS compiler can guess your thoughts and generate missed parts of code. Let's look at example bellow where function named-id requires 2 parameters but it's called without any parameters. Notice that in the compiled Javascript code on the right function namedId called with both two required parameters obtained from local context. How is it done? It's magic.

(defn named-id (id name opt:"")
  (str id ":" name ":" opt))

(defn logos-demo (id)
  (let name "A name" opt 42
    (named-id)))

MetaJS will do what you mean only if you didn't say what to do. Compiler will never attempt to be smarter than you. MetaJS will try to resolve only required function parameters, i.e. do what you said it to do. It will not try to find optional parameters in the execution context.

In the above example local variable opt from logos-demo function isn't passed to named-id function, because opt argument declared as optional for named-id function (and has default value – empty string). If you missed a required variable and MetaJS can't find it in the call context or there more than one possible solution, you should see a warning.

Try more

For experiments with bigger code parts you can use Translate Eval Print Loop (TEPL) or install MetaJS locally.

Look at the discussions about MetaJS on Reddit and LOR.

You can find more information about semantic code transformations, examples of symbolic and entitative MetaJS to JavaScipt transformations in the metajs_semantic_code_transformations.pdf. Please look also at the high-level MetaJS language overview metajs_lisp.pdf.