HeyDSL

HeyDSL is a lightweight local web UI for editing custom DSLs with live HTML preview.

If you've built a domain-specific language that produces visual output (diagrams, formatted text, HTML, or graphics), HeyDSL gives you an interactive editor to develop and test it.

Give HeyDSL a preview function, a compile function, and syntax highlighting rules, and it provides:

• Live preview as you type
• Syntax highlighting for your DSL
• Compile & save output to disk

It's designed for local development and can be deployed in your own applications.

Links

• GitHub: https://github.com/smuj/heydsl
• PyPI: https://pypi.org/project/heydsl

Installation

pip install heydsl

Quick Start

from heydsl import DSLDefinition, HeyDSLApp, Syntax, LineCommentStyle

def preview_fn(code: str) -> str:
    """Convert DSL to HTML for preview"""
    return f"<pre>{code}</pre>"

def compile_fn(code: str) -> bytes:
    """Compile DSL to output format"""
    return code.encode()

app = HeyDSLApp(
    DSLDefinition(
        syntax=Syntax.from_lists(
            name="MyDSL",
            keywords=["if", "else", "function"],
            line_comment_style=LineCommentStyle.HASH,
        ),
        preview_fn=preview_fn,
        compile_fn=compile_fn,
        sample_code="# Hello World",
    )
)
app.run()

Visit http://127.0.0.1:5000 in your browser to start editing.

Features

• Live preview: See output as you edit with no refresh needed
• Custom syntax highlighting: Built-in support for CodeMirror 5; define your syntax highlighting rules via keyword lists or custom JavaScript
• Compile & save: Built-in file dialogs for saving output, or bring your own handlers
• Zero setup: Single Python app instance is the entire server
• Customizable UI: Swap editor themes, header text, and assets
• Local-first: Runs on localhost by default; deploy with your app if needed

Configuration

Defining Syntax

Create syntax highlighting in two ways:

From keyword/type/operator lists:

syntax = Syntax.from_lists(
    name="MyDSL",
    keywords=["if", "else", "for", "return"],
    types=["int", "string", "bool"],
    operators=["+", "-", "*", "/", "=", "=="],
    line_comment_style=LineCommentStyle.HASH,
)

From a custom CodeMirror 5 mode file:

syntax = Syntax.from_file("MyDSL", Path("my_mode.js"))

If creating from a file, make sure the name matches the mode name within the file.

Customizing the UI

from heydsl import UIConfig, curated_cm5_themes

ui = UIConfig(
    header_text="My DSL Editor",
    code_themes=curated_cm5_themes(),  # or provide your own
)

app = HeyDSLApp(dsl_definition, ui_config=ui)

Save Handlers

By default, saving opens a file dialog. You can customize this:

from heydsl import save_file, save_compiled
from datetime import datetime

def custom_save(code: str) -> str:
    """Custom save handler with timestamp."""
    timestamp = datetime.now().strftime("%Y%m%d_%H%M%S")
    # Use the built-in helper
    return save_file(code, default_name=f"my_dsl_{timestamp}.dsl")

def custom_compile_save(compiled: bytes) -> str:
    """Custom compiled save handler with timestamp."""
    timestamp = datetime.now().strftime("%Y%m%d_%H%M%S")
    # Use the built-in helper
    return save_compiled(compiled, default_name=f"output_{timestamp}.bin")

app = HeyDSLApp(
    DSLDefinition(
        syntax=my_syntax,
        preview_fn=my_preview_fn,
        compile_fn=my_compile_fn,
        save_fn=custom_save,
        save_compiled_fn=custom_compile_save,
    )
)

Or skip the dialogs entirely:

def save_to_disk(code: str) -> str:
    path = Path("/tmp/dsl_output.txt")
    path.write_text(code)
    return str(path)

app = HeyDSLApp(
    DSLDefinition(
        syntax=my_syntax,
        preview_fn=my_preview_fn,
        compile_fn=my_compile_fn,
        save_fn=save_to_disk,
    )
)

HTML Preview & Security

The clean_preview setting controls how preview HTML is handled.

When clean_preview=True (default):

• Preview HTML is sanitized: only safe tags and attributes are allowed
• Content Security Policy is restrictive: no scripts, forms, or external requests
• Useful as a safety check while developing

When clean_preview=False:

• Preview HTML is rendered as-is
• CSP is permissive: scripts and forms can run
• It is your responsibility to ensure preview output is safe

For local development tools with trusted input, disabling is normal:

app = HeyDSLApp(
    DSLDefinition(
        syntax=my_syntax,
        preview_fn=my_preview_fn,
        compile_fn=my_compile_fn,
        clean_preview=False,  # You control the preview output
    )
)

If you're running in a shared environment or accepting untrusted DSL code without sanitising it, keep it enabled.

Deploying HeyDSL

HeyDSL runs as a Flask app. You can:

• Use the built-in run() method for development
• Integrate with any WSGI server (Gunicorn, uWSGI) for production
• Embed in a larger Flask/FastAPI application via the .app attribute

# Get the Flask app for WSGI deployment
flask_app = heydsl_app.app

---

API Reference

Core Classes

DSLDefinition

Configuration for your DSL. All fields except noted optional are required.

Field	Type	Description
syntax	Syntax	Syntax definition for editor highlighting
preview_fn	Callable[[str], str]	Function that converts code to HTML preview
compile_fn	Callable[[str], bytes]	Function that compiles code to output
save_fn	Callable[[str], str]	(Optional) Custom save handler; defaults to file dialog
save_compiled_fn	Callable[[bytes], str]	(Optional) Custom compiled save handler; defaults to file dialog
initial_file	Path | None	(Optional) File to load on startup
sample_code	str	(Optional) Code shown when no file is loaded
clean_preview	bool	(Optional) Enable HTML sanitization and strict CSP; default True

HeyDSLApp

Main application class.

HeyDSLApp(
    dsl_definition: DSLDefinition,
    ui_config: UIConfig = UIConfig(),
    server_config: ServerConfig = ServerConfig(),
    cm5_assets: list[ExternalAsset] = default_cm5_assets(),
)

Methods:

• run(open_browser: bool = True) - Start the server and optionally open in browser

Attributes:

• app - The underlying Flask application object

Syntax

Defines syntax highlighting rules.

Class Methods:

• from_lists(name, line_comment_style, keywords, types=[], operators=[]) - Create from keyword/type/operator lists
• from_file(name, path) - Load syntax definition from a JavaScript file

Attributes:

• name - Display name for the syntax
• definition - CodeMirror 5 mode JavaScript code

LineCommentStyle

Enum for comment syntax. Values: SLASH_SLASH (//), HASH (#), SEMI (;), PERCENT (%), NONE (disabled)

ServerConfig

Server settings.

ServerConfig(
    host: str = "127.0.0.1",
    port: int = 5000,
)

Methods:

• address() - Return the full http://host:port address

UIConfig

UI customization.

UIConfig(
    header_text: str = "HeyDSL Editor",
    code_themes: dict[str, ExternalAsset] = curated_cm5_themes(),
)

ExternalAsset

External stylesheet or script to load.

ExternalAsset(
    type: AssetType,  # AssetType.STYLESHEET or AssetType.SCRIPT
    url: str,
    integrity: str | None = None,  # SRI hash
)

AssetType

Enum for asset types: STYLESHEET, SCRIPT

Utilities

save_file(code, default_name="code.txt", initial_dir=None, filetypes=None) -> str

Opens a save dialog and writes code to the selected file. Returns the file path. Use within custom save_fn handlers.

save_compiled(compiled_bytes, default_name="output.bin", initial_dir=None, filetypes=None) -> str

Opens a save dialog and writes compiled bytes to the selected file. Returns the file path. Use within custom save_compiled_fn handlers.

curated_cm5_themes() -> dict[str, ExternalAsset]

Returns popular CodeMirror 5 themes: cobalt, dracula, eclipse, material, monokai, solarized.

default_cm5_assets() -> list[ExternalAsset]

Returns CodeMirror 5 core and simple mode assets.

HTTP API

The Flask app exposes these endpoints (for advanced use):

• GET / - Main editor UI
• GET /syntax-def.js - CodeMirror syntax definition
• POST /api/preview - Generate preview (JSON request: {"code": "..."})
• POST /api/compile - Compile and save (JSON request: {"code": "..."})
• POST /api/save-as - Save code (JSON request: {"code": "..."})