FML: Freeform Markup Language v0.1

Overview

Freeform Markup Language (FML) is a lightweight, expressive markup language designed to write prompts for large language models (LLMs) using structured XML-style tags and Mustache-style variables. It’s designed for developers who want readable, reusable, and modular prompts without the chaos of raw strings.

Features

- 🧩 Mustache-style variable interpolation: {{ name }}
- 🔁 tag for nesting other .fml files
- 🧪 Simple, testable, and transformable prompt formats

📦 Installation

``bash npm install @devgertschi/fml-sdk`

`Syntax`

`$3`


FML uses angle-bracketed XML-style tags:

| Tag | Purpose | | -- | -- | | | Imports other .fml files |

`xml You are a helpful assistant.

Hello, what can you do?

I can help with a variety of tasks including answering questions and writing code.`

`$3`

Use

{{ variable_name }} syntax for dynamic substitution:

`Tell me a story about {{ animal }} who learns to {{ skill }}.`

`$3`


Modularize prompts using

:

`xml Hello there!`

`$3`

You can configure the parser to use BBCode-style tags (e.g., [include src="..." ]) instead of XML-style tags by passing the tagStyle option:

`ts import { parseFML } from '@devgertschi/fml-sdk';

const message = await parseFML('bbcode-include.fml', undefined, { tagStyle: 'bbcode' });`

Example BBCode-style FML:

`[include src="common/system-prompt.fml"] Hello there!`

`Example`

`ts import { parseFML } from 'fml-sdk'

const message = await parseFML('onboarding-prompt.fml', { variables: { name: 'David' } })`

`File Format`

- Extension: .fml`
- UTF-8 encoding

---

Built with ❤️ by David Pichsenmeister to make prompting a bit more human.

Heard about in a talk @viennajs, tried, forked and probably improved by devgertschi.

Syntax

$3

FML uses angle-bracketed XML-style tags:

| Tag | Purpose |
| -- | -- |
|

| Imports other .fml files |

xml

  You are a helpful assistant.


  Hello, what can you do?


  I can help with a variety of tasks including answering questions and writing code.

$3

Use

syntax for dynamic substitution:


Tell me a story about {{ animal }} who learns to {{ skill }}.

$3

Modularize prompts using

:

`xml Hello there!`

`$3`

You can configure the parser to use BBCode-style tags (e.g., [include src="..." ]) instead of XML-style tags by passing the tagStyle option:

`ts import { parseFML } from '@devgertschi/fml-sdk';

const message = await parseFML('bbcode-include.fml', undefined, { tagStyle: 'bbcode' });`

Example BBCode-style FML:

`[include src="common/system-prompt.fml"] Hello there!`

`Example`

`ts import { parseFML } from 'fml-sdk'

const message = await parseFML('onboarding-prompt.fml', { variables: { name: 'David' } })`

`File Format`

- Extension: .fml`
- UTF-8 encoding

---

Built with ❤️ by David Pichsenmeister to make prompting a bit more human.

Heard about in a talk @viennajs, tried, forked and probably improved by devgertschi.