Guillemets 0.1.0

Guillemets

A markdown-aware template engine for non-technical authors.

«» — guillemets, pronounced ghee-uh-MAY — were chosen for readability: they never collide with markdown syntax and stand out visually in prose. That's a deliberate trade against writability — neither delimiter sits on a standard keyboard, so typing either takes a shortcut (see below), not a keystroke. Templates get read far more often than typed by hand, so that trade favors the reader.

<details> <summary>How to type «»</summary>

On Windows/macOS, Turkish (Q) keyboards have no dedicated key for either character — use the row above for your OS instead. If Alt+0171/ Alt+0187 doesn't work in a given Windows app, try Alt+174/Alt+175 (same numpad requirement, no leading zero) — a different Alt-code table that some apps read instead.

On macOS, you can also set up Text Replacement once so <</>> expand to «/» automatically in any app: System Settings → Keyboard → Text Input → Text Replacements… → + → add << → «, then >> → ».

</details>

Hi «first name»,

««is member
Welcome back — thanks for being a member!
~
Thanks for placing your first order with us!
»»

**Order #«order id»** — placed «order date / date: MMMM d, yyyy»

««items
| Item   | Qty        | Price              | Total                    |
| ------ | ---------- | ------------------ | ------------------------ |
| «name» | «quantity» | «price / currency» | «total / currency»       |
|        |            | **Order total**    | «order total / currency» |
»»

You ordered «items: name / join last:  and  / join: , ».

Shipping to «shipping address: city», «shipping address: state».

Thanks for shopping with us!

rendered against

{
  "FirstName": "Priya",
  "IsMember": true,
  "OrderId": "A-1042",
  "OrderDate": "2026-03-04",
  "OrderTotal": 87.50,
  "Items": [
    { "Name": "Wireless Mouse", "Quantity": 1, "Price": 24.99, "Total": 24.99 },
    { "Name": "USB-C Hub", "Quantity": 1, "Price": 39.99, "Total": 39.99 },
    { "Name": "Mousepad", "Quantity": 2, "Price": 11.26, "Total": 22.52 }
  ],
  "ShippingAddress": { "City": "Austin", "State": "TX" }
}

produces

Hi Priya,

Welcome back — thanks for being a member!

Order #A-1042 — placed March 4, 2026

You ordered Wireless Mouse, USB-C Hub and Mousepad.

Shipping to Austin, TX.

Thanks for shopping with us!

<details> <summary>Raw output</summary>

Hi Priya,

Welcome back — thanks for being a member!

**Order #A-1042** — placed March 4, 2026

| Item   | Qty        | Price              | Total                    |
| ------ | ---------- | ------------------ | ------------------------ |
| Wireless Mouse | 1 | $24.99 | $24.99       |
| USB-C Hub | 1 | $39.99 | $39.99       |
| Mousepad | 2 | $11.26 | $22.52       |
|        |            | **Order total**    | $87.50 |

You ordered Wireless Mouse, USB-C Hub and Mousepad.

Shipping to Austin, TX.

Thanks for shopping with us!

</details>

One template covers both the member and first-time-buyer greeting (an if / else block, inferred from IsMember being a boolean — no keyword needed), repeats the order table's one row per item straight from a plain markdown table, and turns the item list into a natural "A, B and C" sentence with the join / join last filters. See docs/specs.md for the full language.

Usage

using Guillemets;

var template = Template.Create("Dear «full name»,");
var output = template.RenderObject(new { FullName = "Alice Smith" });
// => "Dear Alice Smith,"

Render/RenderObject also accept System.Text.Json.JsonElement (template.Render(jsonElement)) and Newtonsoft.Json.Linq.JToken.

A Template is immutable and stateless once created — parse it once and reuse the same instance for every Render call, including concurrently across threads, rather than re-parsing per request.

Custom filters

Template.Create's optional configure callback exposes ParseOptions.Filters, the registry Register(instance) adds a filter to alongside the built-ins (join, date, upper, ...) — re-registering an existing name (e.g. Register(new CurrencyFilter("TL"))) replaces it, and Remove<TFilter>() drops one entirely. A filter's template name drops the Filter suffix and lowercases the rest — ReverseFilter becomes reverse. Apply maps over the current sequence: a single-value filter like date returns one string per input; a collapsing filter like join returns fewer strings than it received.

using Guillemets.Filters;

public class ReverseFilter : IFilter
{
    public IEnumerable<string> Apply(IEnumerable<string> values, string? arg) =>
        values.Select(value => new string(value.Reverse().ToArray()));
}

var template = Template.Create(text,
    options => options.Filters.Register(new ReverseFilter())
);
// «text / reverse» reverses each value

The same configure callback also sets options.Localizer, an IStringLocalizer that bridges a template's business vocabulary to model property names when they don't already match — see docs/specs.md's Glossary & Localization section.

Documentation

• docs/specs.md — full syntax reference and behavior spec.
• docs/implementations/dotnet.md — this .NET implementation's own behavior, including its runtime-specific filters.
• docs/architecture.md — how the engine is built.

License

MIT