Atomic EHR Codegen


Table of Contents

- Atomic EHR Codegen
- Features
- Versions & Release Cycle
- Installation
- Quick Start
- Usage Examples
- Architecture
- Input - FHIR packages & resolves canonicals
- Load Local StructureDefinitions & TGZ Archives
- Intermediate - Type Schema
- Tree Shaking
- Field-Level Tree Shaking
- Generation
- 1. Writer-Based Generation (Programmatic)
- 2. Mustache Template-Based Generation (Declarative)
- Roadmap
- Support
- Footnotes

A powerful, extensible code generation toolkit for FHIR (Fast Healthcare Interoperability Resources) that transforms FHIR specifications into strongly-typed code for multiple programming languages.

Guides:

- Writer Generator Guide - Build custom code generators with the Writer base class
- Mustache Generator Guide - Template-based code generation for any language
- TypeSchemaIndex Guide - Type Schema structure and utilities
- Testing Generators Guide - Unit tests, snapshot testing, and best practices
- Contributing Guide - Development setup and workflow

Features

- 🚀 High-Performance - Built with Bun runtime for blazing-fast generation
- 🔧 Extensible Architecture - Three-stage pipeline:
- FHIR package management & canonical resolution
- Optimized intermediate FHIR data entities representation via Type Schema
- Generation for different programming languages
- 📦 Multi-Package Support - Generate from a list of FHIR packages
- 🎯 Type-Safe - Generates fully typed interfaces with proper inheritance
- 🛠️ Developer Friendly - Fluent API

Versions & Release Cycle

- canary channel - Latest development version from main branch
- latest channel - Latest stable version, changelog: Releases
- All versions: NPM: @atomic-ehr/codegen

Installation

``bash


Using npm

npm install @atomic-ehr/codegen

Using bun

bun add @atomic-ehr/codegen

Using yarn

yarn add @atomic-ehr/codegen
`

Quick Start

1. Write SDK generation script (generate-types.ts):

`typescript
import { APIBuilder, prettyReport } from '@atomic-ehr/codegen';

const builder = new APIBuilder()
.fromPackage("hl7.fhir.r4.core", "4.0.1")
.typescript({})
.outputTo("./examples/typescript-r4/fhir-types")
.introspection({ typeTree: "./type-tree.yaml" });

const report = await builder.generate();
console.log(prettyReport(report));
`

2. Run the script with:

- npm exec tsx generate-types.ts
- bun run generate-types.ts
- pnpm exec tsx generate-types.ts

$3

See the examples/ directory for working demonstrations:

- typescript-r4/ - FHIR R4 type generation with resource creation demo and profile usage
- typescript-ccda/ - C-CDA on FHIR type generation
- typescript-sql-on-fhir/ - SQL on FHIR ViewDefinition with tree shaking
- python/ - Python/Pydantic model generation with simple requests-based client
- python-fhirpy/ - Python/Pydantic model generation with fhirpy async client
- csharp/ - C# class generation with namespace configuration
- mustache/ - Java generation with Mustache templates and post-generation hooks
- local-package-folder/ - Loading unpublished local FHIR packages

For detailed documentation, see examples/README.md.

Architecture

The toolkit uses a three-stage architecture (details: link):

1. Input - FHIR packages & resolves canonicals
2. Intermediate representation - TypeSchema provides a universal representation for FHIR data entities and processing utilities
3. Generation - Generate code for TypeScript, Python, etc.

The APIBuilder provides a fluent interface for configuring and generating code:

`typescript
const builder = new APIBuilder()

// Input sources (choose one or combine)
.fromPackage("hl7.fhir.r4.core", "4.0.1") // NPM registry package
.fromPackageRef("https://...package.tgz") // Remote TGZ file
.localStructureDefinitions({ ... }) // Loose JSON files

// Type Schema processing
.typeSchema({
treeShake: { ... }, // Include only specified types
promoteLogical: { ... }, // Process logical models as resources
})

// Code generator (choose one)
.typescript({ // TypeScript generator
generateProfile?: boolean,
withDebugComment?: boolean,
openResourceTypeSet?: boolean,
})
.python({ // Python generator
allowExtraFields?: boolean,
fieldFormat?: "snake_case" | "camelCase",
staticDir?: string,
})
.csharp("NameSpace", "staticFilesPath") // C# generator

// Output configuration
.outputTo("./generated/types") // Output directory
.cleanOutput(true) // Clean before generation

// Optional: Introspection & debugging
.throwException() // Throw on errors (optional)
.introspection({
typeSchemas: "./schemas", // Export TypeSchemas
typeTree: "./tree.yaml", // Export type tree
fhirSchemas: "./fhir-schemas", // Export FHIR schemas
structureDefinitions: "./sd" // Export StructureDefinitions
})

// Execute generation
.generate(); // Returns GenerationReport
`

Each method returns the builder instance, allowing method chaining. The generate() method executes the pipeline and returns a report with success status and generated file details.

$3

The input stage leverages Canonical Manager to handle FHIR package management and dependency resolution. It processes FHIR packages from multiple sources (registry, local files, TGZ archives) and resolves all canonical URLs to their concrete definitions, ensuring all references between resources are properly linked before transformation.

The Register component wraps Canonical Manager specifically for codegen purposes, providing:

- Multi-package indexing for fast canonical URL lookups across package boundaries
- Package-aware resolution with automatic dependency tree traversal
- FHIR-to-TypeSchema conversion using the @atomic-ehr/fhirschema translator
- Element snapshot generation that merges inherited properties from base resources

#### Load Local StructureDefinitions & TGZ Archives

Use the new localPackage helper to point the builder at an on-disk FHIR package folder (for example, an unpublished implementation guide). If you only have loose StructureDefinition JSON files, group them under a folder and pass it to localStructureDefinitions. Canonical Manager handles copying, indexing, and dependency installation in both scenarios, so the API builder only needs to describe where the files live and what upstream packages they depend on.

`typescript
.localStructureDefinitions({
package: { name: "example.local.structures", version: "0.0.1" },
path: "./custom-profiles",
dependencies: [{ name: "hl7.fhir.r4.core", version: "4.0.1" }],
})
.localTgzPackage("./packages/my-custom-ig.tgz")
`

The example above points Canonical Manager at ./custom-profiles, installs the HL7 R4 core dependency automatically, and then limits generation to the custom ExampleNotebook logical model plus the standard R4 Patient resource via tree shaking. The localTgzPackage helper registers .tgz artifacts that Canonical Manager already knows how to unpack.

$3

Type Schema serves as a universal intermediate representation that bridges FHIR's complex hierarchical structure with programming language constructs. It transforms FHIR StructureDefinitions into a flattened, code-generation-friendly format that:

- Unifies all FHIR elements (Resources, Types, ValueSets) into a consistent structure
- Flattens nested paths for direct field access without complex traversal
- Enriches definitions with resolved references, value set expansions, and type dependencies
- Simplifies FHIR concepts like choice types and extensions for easier code generation

This approach enables generating idiomatic code for any programming language while preserving FHIR semantics and constraints. Learn more: Type Schema specification.

#### Tree Shaking

Tree shaking optimizes the generated output by including only the resources you explicitly need and their dependencies. Instead of generating types for an entire FHIR package (which can contain hundreds of resources), you can specify exactly which resources to include:

`typescript
.typeSchema({
treeShake: {
"hl7.fhir.r4.core#4.0.1": {
"http://hl7.org/fhir/StructureDefinition/Patient": {},
"http://hl7.org/fhir/StructureDefinition/Observation": {}
}
}
})
`

This feature automatically resolves and includes all dependencies (referenced types, base resources, nested types) while excluding unused resources, significantly reducing the size of generated code and improving compilation times.

##### Field-Level Tree Shaking

Beyond resource-level filtering, tree shaking supports fine-grained field selection using selectFields (whitelist) or ignoreFields (blacklist):

`typescript
.typeSchema({
treeShake: {
"hl7.fhir.r4.core#4.0.1": {
"http://hl7.org/fhir/StructureDefinition/Patient": {
selectFields: ["id", "name", "birthDate", "gender"]
},
"http://hl7.org/fhir/StructureDefinition/Observation": {
ignoreFields: ["performer", "note"]
}
}
}
})
`

Configuration Rules:

- selectFields: Only includes the specified fields (whitelist approach)
- ignoreFields: Removes specified fields, keeps everything else (blacklist approach)
- These options are mutually exclusive - you cannot use both in the same rule

Polymorphic Field Handling:

FHIR choice types (like multipleBirth[x] which can be boolean or integer) are handled intelligently. Selecting/ignoring the base field affects all variants, while targeting specific variants only affects those types.

##### Logical Promotion

Some implementation guides expose logical models (logical-kind StructureDefinitions) that are intended to be used like resources in generated SDKs. The code generator supports promoting selected logical models to behave as resources during generation.

Use the programmatic API via APIBuilder:

`typescript
const builder = new APIBuilder({})
.fromPackage("my.custom.pkg", "4.0.1")
.typeSchema({
promoteLogical: {
"my.custom.pkg": [
"http://example.org/StructureDefinition/MyLogicalModel"
]
}
})
`

$3

The generation stage transforms Type Schema into target language code using two complementary approaches:

#### 1. Writer-Based Generation (Programmatic)

For languages with built-in support (TypeScript, Python, C#), extend the Writer class to implement language-specific generators:

- FileSystemWriter: Base class providing file I/O, directory management, and buffer handling (both disk and in-memory modes)
- Writer: Extends FileSystemWriter with code formatting utilities (indentation, blocks, comments, line management)
- Language Writers (TypeScript, Python[^py], CSharp): Implement language-specific generation logic by traversing TypeSchema index and generating corresponding types, interfaces, or classes

[^py]: For details on Type Schema: Python SDK for FHIR

Each language writer maintains full control over output formatting while leveraging high-level abstractions for common code patterns. Writers follow language idioms and best practices, with optimized output for production use.

When to use: Full control needed, complex generation logic, performance-critical, language has a dedicated writer, production-grade output

#### 2. Mustache Template-Based Generation (Declarative)

For custom languages or formats, use Mustache templates to define code generation rules without programming:

- Template Files: Declarative Mustache templates that describe output structure
- Configuration: JSON config file controlling type filtering, naming, and post-generation hooks
- ViewModels: Type Schema automatically transformed into template-friendly data structures

Templates enable flexible code generation for any language or format (Go, Rust, GraphQL, documentation, configs) by describing the output format rather than implementing generation logic.

When to use: Custom language support, quick prototyping, template-driven customization, non-code output

---

$3

When generating TypeScript with generateProfile: true, the generator creates profile wrapper classes that provide a fluent API for working with FHIR profiles (US Core, etc.). These classes handle complex profile constraints like slicing and extensions automatically.

`typescript
import { Patient } from "./fhir-types/hl7-fhir-r4-core/Patient";
import { USCorePatientProfileProfile } from "./fhir-types/hl7-fhir-us-core/profiles/UscorePatientProfile";

// Wrap a FHIR resource with a profile class
const resource: Patient = { resourceType: "Patient" };
const profile = new USCorePatientProfileProfile(resource);

// Set extensions using flat API - complex extensions are simplified
profile.setRace({
ombCategory: { system: "urn:oid:2.16.840.1.113883.6.238", code: "2106-3", display: "White" },
text: "White",
});

// Set simple extensions directly
profile.setSex({ system: "http://hl7.org/fhir/administrative-gender", code: "male" });

// Get extension values - flat API returns simplified object
const race = profile.getRace();
console.log(race?.ombCategory?.display); // "White"

// Get raw FHIR Extension when needed
const raceExtension = profile.getRaceExtension();
console.log(raceExtension?.url); // "http://hl7.org/fhir/us/core/StructureDefinition/us-core-race"

// Get the underlying resource
const patientResource = profile.toResource();
`

Slicing Support:

Profile classes also handle FHIR slicing, automatically applying discriminator values:

`typescript
import { Observation } from "./fhir-types/hl7-fhir-r4-core/Observation";
import { USCoreBloodPressureProfileProfile } from "./fhir-types/hl7-fhir-us-core/profiles/UscoreBloodPressureProfile";

const obs: Observation = { resourceType: "Observation", status: "final", code: {} };
const bp = new USCoreBloodPressureProfileProfile(obs);

// Set slices - discriminator is applied automatically
// No input needed when all fields are part of the discriminator
bp.setSystolic({ valueQuantity: { value: 120, unit: "mmHg" } });
bp.setDiastolic({ valueQuantity: { value: 80, unit: "mmHg" } });

// Get simplified slice (without discriminator fields)
const systolic = bp.getSystolic();
console.log(systolic?.valueQuantity?.value); // 120

// Get raw slice (includes discriminator)
const systolicRaw = bp.getSystolicRaw();
console.log(systolicRaw?.code?.coding?.[0]?.code); // "8480-6" (LOINC code for systolic BP)
`

See examples/typescript-us-core/ for complete profile usage examples.

Roadmap

- [x] TypeScript generation
- [x] FHIR R4 core package support
- [x] Configuration file support
- [x] Comprehensive test suite (72+ tests)
- [x] Value Set Generation - Strongly-typed enums from FHIR bindings
- [x] Profile & Extension Support - Basic parsing (US Core in development)
- [ ] Complete Multi-Package Support - Custom packages and dependencies
- [ ] Smart Chained Search - Intelligent search builders

`typescript
// Intelligent search builders
const results = await client.Patient
.search()
.name().contains('Smith')
.birthdate().greaterThan('2000-01-01')
.address().city().equals('Boston')
.include('Patient:organization')
.sort('birthdate', 'desc')
.execute();
`

- [ ] Operation Generation - Type-safe FHIR operations

`typescript
// Type-safe FHIR operations
const result = await client.Patient
.operation('$match')
.withParameters({
resource: patient,
onlyCertainMatches: true
})
.execute();
``

- [x] Python generation
- [x] C# generation
- [ ] Rust generation
- [ ] GraphQL schema generation
- [ ] OpenAPI specification generation
- [ ] Validation functions
- [ ] Mock data generation

Support

- 🐛 Issue Tracker

---

Built with ❤️ by the Atomic Healthcare team