Adds defineHelper to allow custom Handlebars helpers.

docs/dotprompt.md

@@ -442,6 +442,34 @@ The name of the variant is included in the metadata of generation traces, so you
 can compare and contrast actual performance between variants in the Genkit trace
 inspector.
 
+## Defining Custom Helpers
+
+You can define custom helpers to process and manage data inside of a prompt. Helpers
+are registered globally using `defineHelper`:
+
+```ts
+import { defineHelper } from '@genkit-ai/dotprompt';
+
+defineHelper('shout', (text: string) => text.toUpperCase());
+```
+
+Once a helper is defined you can use it in any prompt:
+
+```none
+---
+model: vertexai/gemini-1.5-pro
+input:
+  schema:
+    name: string
+---
+
+HELLO, {{shout name}}!!!
+```
+
+For more information about the arguments passed into helpers, see the
+[Handlebars documentation](https://handlebarsjs.com/guide/#custom-helpers) on creating
+custom helpers.
+
 ## Alternate ways to load and define prompts
 
 Dotprompt is optimized for organization in the prompt directory. However, there

js/plugins/dotprompt/src/index.ts

@@ -25,7 +25,7 @@ import { basename } from 'path';
 import { defineDotprompt, Dotprompt } from './prompt.js';
 import { loadPromptFolder, lookupPrompt } from './registry.js';
 
-export { definePartial } from './template.js';
+export { defineHelper, definePartial } from './template.js';
 export { defineDotprompt, Dotprompt };
 
 export interface DotpromptPluginOptions {

js/plugins/dotprompt/src/template.ts

@@ -156,7 +156,6 @@ export function compile<Variables = any>(
       role: true,
       history: true,
     },
-    knownHelpersOnly: true,
   });
 
   return (
@@ -172,6 +171,10 @@ export function compile<Variables = any>(
   };
 }
 
+export function defineHelper(name: string, fn: Handlebars.HelperDelegate) {
+  Promptbars.registerHelper(name, fn);
+}
+
 export function definePartial(name: string, source: string) {
   Promptbars.registerPartial(name, source);
 }

js/testapps/prompt-file/prompts/recipe.prompt

@@ -3,10 +3,16 @@ model: googleai/gemini-pro
 input:
   schema:
     food: string
+    ingredients?(array): string
 output:
   schema: Recipe
 ---
 
 You are a chef famous for making creative recipes that can be prepared in 45 minutes or less.
 
-Generate a recipe for {{food}}.
+Generate a recipe for {{food}}.
+
+{{#if ingredients}}
+Make sure to include the following ingredients:
+{{list ingredients}}
+{{/if}}

js/testapps/prompt-file/src/index.ts

@@ -15,7 +15,7 @@
  */
 
 import { configureGenkit, defineSchema } from '@genkit-ai/core';
-import { dotprompt, prompt } from '@genkit-ai/dotprompt';
+import { defineHelper, dotprompt, prompt } from '@genkit-ai/dotprompt';
 import { defineFlow } from '@genkit-ai/flow';
 import { googleAI } from '@genkit-ai/googleai';
 import * as z from 'zod';
@@ -49,6 +49,13 @@ const RecipeSchema = defineSchema(
 // If it fails, due to the prompt file being invalid, the process will crash,
 // instead of us getting a more mysterious failure later when the flow runs.
 
+defineHelper('list', (data: any) => {
+  if (!Array.isArray(data)) {
+    return '';
+  }
+  return data.map((item) => `- ${item}`).join('\n');
+});
+
 prompt('recipe').then((recipePrompt) => {
   defineFlow(
     {