LCML -- JSON with expressions

Low-Code Markup Language (DSL) presents values with Dynamic Expressions. It is a superset of human readable JSON.

[ 👯 Try it Now | 💻 GitHub | 📓 LCML Syntax | 📓 Integrating LCML ]

Highlights
🤓 loose mode	💪 error-tolerant (recover from errors)
🌲 parse and output AST	👨‍🎓 type information is inferred
🎼 output formatted JavaScript	🔨 expression processing hooks

Have a Glimpse

Written in LCML	Output JavaScript	Inferred Type Information
3.14159	3.14159	number
[1, 2, {{ dice() }}]	[1, 2, dice()]	array with 2 numbers + 1 expression
"hello {{ user.name }}"	"hello" + toString(user.name)	string
{{ foo.bar }}	foo.bar	expression

Here is a loooong LCML example:

// this whole text is written in LCML
// human-readable JSON with {{ expression }} inside:
{
  name: "John {{ lib.genLastName() }}",  // in string
  info: {{ lib.genInfo() }},  // as property value
  tags: ["aa", 123, false, {{ location.href }}],

  {{ lib.sym }}: "wow",  // as property key
}

Compiled with default options:

// output valid JavaScript code
{
  name: "John" + toString(lib.genLastName()),  // wrapped by "toString"
  info: lib.genInfo(),
  tags: ["aa", 123, false, location.href],

  [lib.sym]: "wow",
}

And every part's type information is inferred:

[] object
  [name] string
  [info] unknown
  [tags] array
    [0] string
    [1] number
    [2] boolean
    [3] unknown
  [[( lib.sym )]] string

LCML Syntax

LCML syntax is based on JSON syntax, with {{ expression }} and comments supported.

We support /* block comment */ and // line-comment

You can use {{ expression }} in many places:

• in string
• as array item
• as property value
• as property key
• as the whole LCML

Loose Mode

When { loose: true } is passed to parse, these invalid LCML will become valid strings:

LCML	Default Mode	Loose Mode (loose: true)
{{ user.name }}, Welcome	Error: unexpected remainder	treated as string "{{ user.name }}, Welcome"
Hello, {{ user.name }}	Error: invalid input (at "H")	treated as string "Hello, {{ user.name }}"
/* corrupted */ {{ user	Error: expect end of expression	treated as string "{{ user"

Loose Mode Rules:

1. leading comments are ignored, then the remainder might be treated as string

2. if the beginning of LCML input looks like a string, array or object, the loose mode will NOT work!

3. { ignoreUnparsedRemainder: true } will not work, unless loose mode is suppressed (see rule #2)

4. due to rule #2, corrupted input like { hello: will cause a Error, not string.

   • (dangerous) to treat it as string, set { onError: 'as-string' } -- this can be confusing! the parser still outputs a string but it is NOT Loose Mode's credit!

5. if Loose Mode actually has functioned, parser will return { looseModeEnabled: true }

Some rarely-used notices related to the rule #4, FYI:

• if { onError: 'as-string' } is set, to tell whether it has functioned, you shall check !!parseResult.error && parseResult.ast.type === 'string' && !parseResult.ast.quote instead of parseResult.looseModeEnabled

Integrating LCML

import { compile, CompileOptions } from 'lcml';
// compile = parse + toJS

const options: CompileOptions = {

  // loose: false,
  // onError: 'throw' | 'recover' | 'as-string',
  // ignoreUnparsedRemainder: false,
  // treatEmptyInput: 'as-undefined',
  
  // compact: false,
  // processExpression: (node, parents) => { return node.expression },
  // globalToStringMethod: "toString",
  
};

const result = compile('"Hello, {{ user.name }}"', options);

console.log(result.body);
// => 'Hello, ' + toString(user.name)

console.log(result.ast);
// => { type: "string", start: 0, end: 24 }

console.log(result.expressions);
// => [
//      {
//         start: 8,
//         end: 23,
//         expression: " user.name ",
//      }
//    ]

Global toString Method

In the generated code, you might see toString(something).

This happens when user use {{ expression }} inside a string literal.

Therefore, to ensure the generated JavaScript runnable, you shall compose your function like this:

function composedGetter() {
  // declare the toString method
  const toString = x => (typeof x === 'string' ? x : JSON.stringify(x));

  // provide other variables that user might reference
  const user = getUserInfo();
  const state = getState();

  // return (/* put the generated code here! */);
  return 'Hello, ' + toString(user.name);
}

You can set option globalToStringMethod in order to use other name instead of toString.

Process Embedded Expressions

As presented above, option processExpression can be a callback function receiving node and its parents, returns a JavaScript expression.

You can read node.expression, transpile it and return new expression. For example, use Babel to transpile the fashion ESNext syntax like pipeline operator.

The generated JavaScript code will be affected.

Beware: in the received node.expression, leading and trailing spaces are NOT trimmed.

processExpression: (node, parents) => {
    return node.expression;
}