Why Scriptorium

The need for a cross-platform testing experience

Since its inception, the Fable ecosystem has been about bringing the power of F# to other runtimes. But for years, testing Fable applications meant maintaining compatibility layers between .NET testing tools and JavaScript testing frameworks.

Scriptorium is built to change that. It provides a family of focused libraries for testing Fable applications, all written in F#, designed to run on any Fable-supported platform with a consistent experience — the same API whether you target Node.js, the browser, or plain .NET.

A single metaphor, many responsibilities

The Scriptorium libraries are organised around one metaphor: the act of writing.

In a medieval scriptorium, the scribe sits down with several tools — each with a single, well-defined purpose:

• Ink is the colour and style. It doesn’t write; it only makes the marks visible.
• Parchment is the page that receives the writing. It structures the output.
• Nib is the sharp point that makes the mark. It is where truth is tested.
• Quill is the instrument that holds everything together and carries the hand across the page.

This is exactly how the libraries are designed. Each one does one thing, does it well, and composes cleanly with the others:

Library	Responsibility
Scriptorium.Ink	Colour and style for terminal output
Scriptorium.Parchment	Structured logging that dips into Ink
Scriptorium.Nib	Composable assertions, no runner dependency
Scriptorium.Quill	Test runner that holds Nib, Ink, and Parchment together
Scriptorium.Nib.Snapshot	Snapshot testing on top of Nib
Scriptorium.Nib.Browser	Playwright-powered DOM assertions

Design principles

Every failure is collected before reporting

Scriptorium.Nib never throws on the first failing assertion. An assertion chain continues to the end, collecting every failure. Only when assertThat returns does it throw — with a single exception carrying every message.

This means one failing test shows you all the things that are wrong, not just the first. No more re-running after each fix.

Composability over magic

Assertions are plain F# functions of type AssertionState<'a> -> AssertionState<'b>. They compose with >>. There is no reflection, no attribute magic, no framework coupling.

You can define a reusable assertion for your own domain types, compose it with built-in assertions, and use it in any test framework that can call a function.

Independent layers

All libraries are independently usable. You can:

• Use Scriptorium.Nib assertions inside Expecto, Jest, or any other test framework.
• Use Scriptorium.Ink to colour any string output in an application.
• Use Scriptorium.Parchment for structured logging without any test dependency.

Nothing forces you to use the full stack. Start with what you need.

Respected conventions

• NO_COLOR — Scriptorium.Ink honours the no-color.org convention. Set NO_COLOR=1 to strip all ANSI codes from output.
• CI — Scriptorium.Quill detects the CI environment variable and refuses to exit with code 0 if focused tests (ftest) are committed to the repository.
• UPDATE_SNAPSHOTS — Scriptorium.Nib.Snapshot uses this variable to trigger snapshot updates.