diff --git a/README.md b/README.md
index 2af0f79..3d837d8 100644
--- a/README.md
+++ b/README.md
@@ -78,12 +78,13 @@ failing arm of the result rather than as a throw; `ok` tells the two apart.
   - [Rendering a rule back](#rendering-a-rule-back): show an authored rule back to its author in a normalized form.
   - [Statement programs](#statement-programs): run a short script that assigns into its context.
   - [Writing a location](#writing-a-location): write an assignment's location into your own data without running a program.
-  - [Host functions](#host-functions): let a rule call functions your host supplies.
+  - [How to add host functions](https://github.com/riddler/predicator-ts/blob/main/docs/guides/how-to-add-host-functions.md): let a rule call a function your application supplies, and handle a call that fails.
 - Look up
   - The API reference: every exported function and type, built into `docs/api/` by `mise exec -- pnpm run docs` in a clone of this repository.
   - [The entry points](#the-entry-points): the two import paths and what each one exports.
   - [The value domain](#the-value-domain): each value type and its shape in TypeScript.
   - [Evaluation options](#evaluation-options): the options an evaluation takes, and their defaults.
+  - [Host functions](#host-functions): how a host function's arguments and answer cross into the language's values, and how its name shadows a builtin.
   - [The tagged subpath](#the-tagged-subpath): the encoding that keeps what a JSON round trip loses.
   - [The changelog](https://github.com/riddler/predicator-ts/blob/main/CHANGELOG.md): what changed in each version.
 - Understand
diff --git a/docs/guides/how-to-add-host-functions.md b/docs/guides/how-to-add-host-functions.md
new file mode 100644
index 0000000..7dd69a2
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@@ -0,0 +1,90 @@
+# How to add host functions
+
+This guide shows you how to let a rule call a function your application
+supplies, so the rule can ask about data the context does not carry.
+
+You start with `@riddler/predicator` installed and a renewal rule that needs
+your application's hold queue: a loan may be renewed when no other patron is
+waiting for the copy and the loan has been renewed fewer than twice.
+
+## Call the function from the rule
+
+1. Write the call into the rule's text, with the function's name and its
+   arguments, and compile it.
+
+   ```ts
+   import { compile } from "@riddler/predicator";
+
+   const compiled = compile("holds_waiting(copy.id) == 0 AND loan.renewals < 2");
+
+   if (!compiled.ok) {
+     throw new Error(compiled.error.message);
+   }
+   ```
+
+   `compiled.instructions` holds `["call", "holds_waiting", 1]`, a call with one
+   argument.
+
+## Supply the function
+
+1. Write the function. It takes the call's arguments as an array and answers
+   one value.
+
+   ```ts
+   import type { HostFunction } from "@riddler/predicator";
+
+   const holdQueues: Record<string, string[]> = {
+     "atlas-2": ["patron-17"],
+     "codex-1": [],
+   };
+
+   const holdsWaiting: HostFunction = ([copyId]) =>
+     typeof copyId === "string" ? (holdQueues[copyId] ?? []).length : 0;
+   ```
+
+   `holdsWaiting(["atlas-2"])` answers `1`.
+
+2. Pass it to `evaluate` under `functions`, keyed by the name the rule calls.
+
+   ```ts
+   import { evaluate } from "@riddler/predicator";
+
+   const renewable = evaluate(
+     compiled.instructions,
+     { copy: { id: "codex-1" }, loan: { renewals: 1 } },
+     { functions: { holds_waiting: holdsWaiting } },
+   );
+   ```
+
+   `renewable` is `{ ok: true, value: true }`: nobody is waiting for this copy.
+
+3. Evaluate it for a copy another patron is waiting for.
+
+   ```ts
+   const held = evaluate(
+     compiled.instructions,
+     { copy: { id: "atlas-2" }, loan: { renewals: 1 } },
+     { functions: { holds_waiting: holdsWaiting } },
+   );
+   ```
+
+   `held` is `{ ok: true, value: false }`.
+
+## Handle a call that fails
+
+1. Check `ok` before you read `value`. If you leave `functions` out, `ok` is
+   `false` and `error.reason` is `"Unknown function: holds_waiting"`.
+
+2. If your function throws, read its message from the result. For a function
+   that throws `new Error("the hold queue is unavailable")`, `ok` is `false`,
+   `error.type` is `"EvaluationError"` and `error.reason` is
+   `"the hold queue is unavailable"`.
+
+3. If your function answers a value the language has no type for, such as a
+   symbol, `ok` is `false` and `error.reason` is `"unsupported_host_value"`.
+
+For how arguments and answers cross into the language's values, and how a
+host function's name shadows a builtin of the same name, see
+[Host functions](https://github.com/riddler/predicator-ts/blob/main/README.md#host-functions)
+and
+[The value domain](https://github.com/riddler/predicator-ts/blob/main/README.md#the-value-domain).