The way I do it is pretty dumb. We have files like src/test/_template.test.ts that are valid TS, that our codegen commands like `notion new test` regex-replace to “instantiate” the template. The convention we follow is to name replacement targets with double underscores. No need for some kind of specialized template library - just regex & replaceAll.

The scripts that consume our templates are mostly very simple ~15 line affairs but there are a few very complex, 1000+ line affairs that use the Typescript type checker API to codegen model classes and test data generators from hand-written interface types. Those things read ASTs but writing ASTs seems like a huge waste of time to me. AST is only nice if you need to very carefully morph existing code, and even then I usually end up doing a string replacement of an AST node’s position in its original file. In all that code I think the only thing that writes using an AST is import statement stuff.

Luckily that super complex stuff runs on every commit so it can’t break. It’s the one-off codegen commands like “make me a new component” or “make me an integration test” that are in danger of going stale, and even those we could write some CI tests for like “make a new thingy, check that it typechecks”.

Anyways here’s a full example template file:

    /*__FILE_METADATA__*/
    /* ================================================================================
    
     __TEMPLATE_CLASS_NAME__.
     Docs: https://dev.notion.so/notion/Record-Framework-1f41f97e1cc746628a6af57ba75d75ad
    
     THIS IS FILE IS PARTIALLY GENERATED. ONLY EDIT THE EDITABLE REGION BELOW.
     __GENERATED_BY__
     __GENERATED_FROM__
    
    ================================================================================ */
    
    import type {
     /*__VALUE_IMPORT__*/ BlockValue as __VALUE_TYPE__,
     /*__TABLE_IMPORT__*/ BlockTable as __TABLE__,
    } from /*__SCHEMA_IMPORT__*/ "../schemas/Block"
    
    import { Model } from "./Model"
    
    /*__REMOVE_LINE__*/ // : Value <Value> as __MODEL__<Value> | undefined <- Conform to template args in RecordStore.
    
    /**
     * This class is generated from {@link __VALUE_TYPE__}.
     * To customize, edit the section in {@link __TEMPLATE_CLASS_NAME__} below.
     */
    abstract class Generated__TEMPLATE_CLASS_NAME__<
     Value extends __VALUE_TYPE__ = __VALUE_TYPE__
    > extends Model<Value, __TABLE__> {
     /*__REMOVE_LINE__*/
     __BODY__: undefined // Put the goods here.
    }
    
    /**
     * Read and interpret the data of a __RECORD__ record.
     */
    export class __TEMPLATE_CLASS_NAME__<
     Value extends __VALUE_TYPE__ = __VALUE_TYPE__
    > extends Generated__TEMPLATE_CLASS_NAME__<Value> {
     /*__USER_EDITABLE_SECTION__*/
    }