Table of Contents

• Projections
  • Marketplace Gpack Examples
    • CLI Subcommands
    • API Endpoints
    • SQL DDL from Ontology
    • Edge Function Stubs
    • Documentation from Graph Annotations
  • Local Template Examples
    • Custom Projections
  • Discovering Projection Gpacks
    • By Category
    • By Language
    • By Framework
  • Cross-Language Projections
    • Same RDF Ontology
    • Deterministic Output
  • Projection Patterns
    • Standard Pattern
    • Marketplace Pattern
    • Local Pattern
  • Best Practices
    • Gpack Selection
    • Version Management
    • Custom Projections

Projections

Targets are just templates. ggen supports both marketplace gpacks and local templates for generating code projections.

Marketplace Gpack Examples

CLI Subcommands

# Install multi-language CLI gpacks
ggen add io.ggen.rust.cli-subcommand
ggen add io.ggen.python.cli-subcommand
ggen add io.ggen.bash.cli-subcommand

# Generate across languages
ggen gen io.ggen.rust.cli-subcommand:cli/subcommand/rust.tmpl name=status
ggen gen io.ggen.python.cli-subcommand:cli/subcommand/python.tmpl name=status
ggen gen io.ggen.bash.cli-subcommand:cli/subcommand/bash.tmpl name=status

API Endpoints

# Install API gpacks
ggen add io.ggen.rust.api-endpoint
ggen add io.ggen.python.api-endpoint
ggen add io.ggen.typescript.api-endpoint

# Generate REST endpoints
ggen gen io.ggen.rust.api-endpoint:api/endpoint/rust.tmpl name=users
ggen gen io.ggen.python.api-endpoint:api/endpoint/python.tmpl name=users
ggen gen io.ggen.typescript.api-endpoint:api/endpoint/typescript.tmpl name=users

SQL DDL from Ontology

# Install SQL generation gpacks
ggen add io.ggen.sql.schema
ggen add io.ggen.sql.migrations

# Generate database schema
ggen gen io.ggen.sql.schema:schema/postgres.tmpl name=users
ggen gen io.ggen.sql.migrations:migration/postgres.tmpl name=add_users_table

Edge Function Stubs

# Install edge function gpacks
ggen add io.ggen.vercel.edge-function
ggen add io.ggen.cloudflare.worker

# Generate edge functions
ggen gen io.ggen.vercel.edge-function:edge/function.ts.tmpl name=api
ggen gen io.ggen.cloudflare.worker:worker/index.js.tmpl name=api

Documentation from Graph Annotations

# Install documentation gpacks
ggen add io.ggen.docs.api-reference
ggen add io.ggen.docs.user-guide

# Generate documentation
ggen gen io.ggen.docs.api-reference:docs/api.md.tmpl name=users
ggen gen io.ggen.docs.user-guide:docs/guide.md.tmpl name=getting-started

Local Template Examples

Custom Projections

# Create custom template
mkdir -p templates/custom/projection
cat > templates/custom/projection/rust.tmpl << 'EOF'
---
to: src/{{ name }}.rs
vars:
  name: example
---
pub struct {{ name | pascal }} {
    // Custom projection logic
}
EOF

# Generate custom projection
ggen gen custom projection --vars name=user

Discovering Projection Gpacks

By Category

# Browse categories
ggen categories

# Search by projection type
ggen search cli subcommand
ggen search api endpoint
ggen search sql schema
ggen search docs reference

By Language

# Search by language
ggen search rust
ggen search python
ggen search typescript
ggen search go

By Framework

# Search by framework
ggen search clap
ggen search fastapi
ggen search express
ggen search gin

Cross-Language Projections

Marketplace gpacks enable consistent projections across multiple languages:

Same RDF Ontology

All language-specific gpacks use the same semantic model:

• Consistent variable binding
• Identical SPARQL queries
• Unified frontmatter structure

Deterministic Output

Version locking ensures reproducible results:

• Same gpack versions → identical outputs
• Cross-language consistency
• Deterministic generation

Projection Patterns

Standard Pattern

1. Describe object in RDF - Define semantic model
2. Bind vars via SPARQL - Extract variables from graph
3. Render per target - Generate language-specific code

Marketplace Pattern

1. Search for gpacks - Find language-specific templates
2. Install dependencies - Get gpacks and dependencies
3. Generate consistently - Use same RDF across languages

Local Pattern

1. Create templates - Build custom projection logic
2. Define RDF model - Specify semantic structure
3. Generate outputs - Render to target formats

Best Practices

Gpack Selection

• Choose gpacks with active maintenance
• Verify compatibility with your ggen version
• Check dependency requirements
• Review example outputs

Version Management

• Pin versions for production use
• Test updates before applying
• Use semantic versioning
• Maintain lockfile consistency

Custom Projections

• Start with marketplace gpacks
• Extend with local templates
• Share via gpack publishing
• Document projection patterns