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 }}
• 🔁 <include> tag for nesting other .fml files
• 🧪 Simple, testable, and transformable prompt formats

📦 Installation

npm install @devgertschi/fml-sdk

Syntax

Tags

FML uses angle-bracketed XML-style tags:

| Tag | Purpose | | -- | -- | | <include src="..."/> | Imports other .fml files |

<system>
  You are a helpful assistant.
</system>

<user>
  Hello, what can you do?
</user>

<assistant>
  I can help with a variety of tasks including answering questions and writing code.
</assistant>

Variables

Use {{ variable_name }} syntax for dynamic substitution:

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

File Composition

Modularize prompts using <include>:

<include src="common/system-prompt.fml" />
Hello there!

BBCode Tag Style

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

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

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.