Module :: former

A flexible implementation of the Builder pattern supporting nested builders and collection-specific subformers. Comprehensive struct support with enum support under active development.

What is Former?

The former crate provides a powerful derive macro, #[ derive( Former ) ], that automatically implements the Builder pattern for your Rust structs and enums.

Its primary goal is to simplify the construction of complex objects, especially those with numerous fields, optional values, default settings, collections, and nested structures, making your initialization code more readable and maintainable.

Current Status: Struct support is fully functional and production-ready. Enum support is actively developed with 227 total tests passing, including functional unit variants, tuple variants, and multi-field patterns. Some advanced features like #[arg_for_constructor] are still under development.

Why Use Former?

Compared to manually implementing the Builder pattern or using other builder crates, former offers several advantages:

• Reduced Boilerplate: #[ derive( Former ) ] automatically generates the builder struct, storage, and setters, saving you significant repetitive coding effort.
• Fluent & Readable API: Construct objects step-by-step using clear, chainable methods (.field_name( value )).
• Comprehensive Struct Support: Fully implemented builder pattern for structs with automatic generation of setters, defaults, and subformers
• Effortless Defaults & Optionals: Fields automatically use their Default implementation if not set. Option< T > fields are handled seamlessly – you only set them if you have a Some( value ). Custom defaults can be specified easily with #[ former( default = ... ) ].
• Powerful Collection & Nested Struct Handling: former truly shines with its subformer system. Easily build Vec, HashMap, HashSet, and other collections element-by-element, or configure nested structs using their own dedicated formers within the parent's builder chain. This is often more complex to achieve with other solutions.

Installation

Add former to your Cargo.toml:

cargo add former

The default features enable the Former derive macro and support for standard collections, covering most common use cases.

When to Use Former

Former is designed for building complex, nested data structures with compile-time guarantees. Consider using Former when:

✅ Use Former when you need:

• Nested builders (Parent::former().child().field(x).end())
• Collection building (adding items one-by-one with dedicated subformers)
• Complex validation logic with custom mutators
• Subform composition for hierarchical data
• Compile-time type-safe construction

❌ Consider simpler alternatives when:

• Building simple flat structs (< 5 fields, no nesting)
• No custom defaults or validation needed
• Just need a basic .build() method

Decision Matrix

The ROI: Former's additional complexity pays off when you have ~5+ fields with nesting or need collection builders. For simple cases, the overhead may not be justified.

Known Limitations

Former has some architectural limitations you should be aware of:

• Lifetimes: Cannot use borrowed data (&'a T) in Former structs. Use owned types (String, Vec) or smart pointers (Arc, Cow). See workarounds.
• Generic Enums: Currently not supported due to parser limitations. Use concrete types instead. Details.
• Multi-Variant Enums: May encounter trait conflicts in complex scenarios. Single-variant enums work perfectly. Details.

📖 Full limitations documentation: limitations.md

Basic Usage

Derive Former on your struct and use the generated ::former() method to start building:

# #[ cfg( any( not( feature = "derive_former" ), not( feature = "enabled" ) ) ) ]
# fn main() {}
# #[ cfg( all( feature = "derive_former", feature = "enabled" ) ) ]
# fn main()
# {
  use former::Former;

  #[ derive( Debug, PartialEq, Former ) ]
  pub struct UserProfile
  {
    age : i32, // Required field
    username : String, // Required field
    bio : Option< String >, // Optional field
  }

  let profile = UserProfile::former()
  .age( 30 )
  .username( "JohnDoe".to_string() )
  // .bio is optional, so we don't *have* to call its setter
  .form();

  let expected = UserProfile
  {
    age : 30,
    username : "JohnDoe".to_string(),
    bio : None, // Defaults to None if not set
  };
  assert_eq!( profile, expected );
  dbg!( &profile );
  // > &profile = UserProfile {
  // >     age: 30,
  // >     username: "JohnDoe",
  // >     bio: None,
  // > }

  // Example setting the optional field:
  let profile_with_bio = UserProfile::former()
  .age( 30 )
  .username( "JohnDoe".to_string() )
  .bio( "Software Developer".to_string() ) // Set the optional bio
  .form();

  let expected_with_bio = UserProfile
  {
    age : 30,
    username : "JohnDoe".to_string(),
    bio : Some( "Software Developer".to_string() ),
  };
  assert_eq!( profile_with_bio, expected_with_bio );
  dbg!( &profile_with_bio );
  // > &profile_with_bio = UserProfile {
  // >     age: 30,
  // >     username: "JohnDoe",
  // >     bio: Some( "Software Developer" ),
  // > }
# }

Run this example locally | Try it online

Handling Optionals and Defaults

Former makes working with optional fields and default values straightforward:

• Option< T > Fields: As seen in the basic example, fields of type Option< T > automatically default to None. You only need to call the setter if you have a Some( value ).

• Custom Defaults: For required fields that don't implement Default, or when you need a specific default value other than the type's default, use the #[ former( default = ... ) ] attribute:

# #[ cfg( any( not( feature = "derive_former" ), not( feature = "enabled" ) ) ) ]
# fn main() {}
# #[ cfg( all( feature = "derive_former", feature = "enabled" ) ) ]
# fn main()
# {
  use former::Former;

  #[ derive( Debug, PartialEq, Former ) ]
  pub struct Config
  {
    #[ former( default = 1024 ) ] // Use 1024 if .buffer_size() is not called
    buffer_size : i32,
    timeout : Option< i32 >, // Defaults to None
    #[ former( default = true ) ] // Default for bool
    enabled : bool,
  }

  // Only set the optional timeout
  let config1 = Config::former()
  .timeout( 5000 )
  .form();

  assert_eq!( config1.buffer_size, 1024 ); // Got default
  assert_eq!( config1.timeout, Some( 5000 ) );
  assert_eq!( config1.enabled, true ); // Got default

  // Set everything, overriding defaults
  let config2 = Config::former()
  .buffer_size( 4096 )
  .timeout( 1000 )
  .enabled( false )
  .form();

  assert_eq!( config2.buffer_size, 4096 );
  assert_eq!( config2.timeout, Some( 1000 ) );
  assert_eq!( config2.enabled, false );
# }

See full example code

Building Collections & Nested Structs (Subformers)

Where former significantly simplifies complex scenarios is in building collections (Vec, HashMap, etc.) or nested structs. It achieves this through subformers. Instead of setting the entire collection/struct at once, you get a dedicated builder for the field:

Example: Building a Vec

# #[ cfg( not( all( feature = "enabled", feature = "derive_former", any( feature = "use_alloc", not( feature = "no_std" ) ) ) ) ) ]
# fn main() {}
# #[ cfg( all( feature = "enabled", feature = "derive_former", any( feature = "use_alloc", not( feature = "no_std" ) ) ) ) ]
# fn main()
# {
  use former::Former;

  #[ derive( Debug, PartialEq, Former ) ]
  pub struct Report
  {
    title : String,
    #[ subform_collection( definition = former::VectorDefinition ) ] // Enables the `.entries()` subformer
    entries : Vec< String >,
  }

  let report = Report::former()
  .title( "Log Report".to_string() )
  .entries() // Get the subformer for the Vec
    .add( "Entry 1".to_string() ) // Use subformer methods to modify the Vec
    .add( "Entry 2".to_string() )
    .end() // Return control to the parent former (ReportFormer)
  .form(); // Finalize the Report

  assert_eq!( report.title, "Log Report" );
  assert_eq!( report.entries, vec![ "Entry 1".to_string(), "Entry 2".to_string() ] );
  dbg!( &report );
  // > &report = Report {
  // >     title: "Log Report",
  // >     entries: [
  // >         "Entry 1",
  // >         "Entry 2",
  // >     ],
  // > }
# }

See Vec example | See HashMap example

former provides different subform attributes (#[ subform_collection ], #[ subform_entry ], #[ subform_scalar ]) for various collection and nesting patterns.

Standalone Constructors

For scenarios where you want a direct constructor function instead of always starting with YourType::former(), former offers standalone constructors.

• Enable: Add #[ standalone_constructors ] to your struct or enum definition.
• Function Name: A function named after your type (in snake_case) will be generated (e.g., my_struct() for struct MyStruct). For enums, functions are named after variants (e.g., my_variant() for enum E { MyVariant }).
• Arguments: By default, all fields become constructor arguments.
• Exclude Arguments: Mark specific fields with #[ former_ignore ] to exclude them from constructor arguments.
• Return Type Logic:
  • If no fields are marked with #[ former_ignore ], the standalone constructor takes all fields as arguments and returns the instance directly (Self).
  • If any fields are marked with #[ former_ignore ], the standalone constructor takes only non-ignored fields as arguments and returns the Former type.

Example: Struct Standalone Constructors

# #[ cfg( any( not( feature = "derive_former" ), not( feature = "enabled" ) ) ) ]
# fn main() {}
# #[ cfg( all( feature = "derive_former", feature = "enabled" ) ) ]
# fn main()
# {
  use former::Former;

  #[ derive( Debug, PartialEq ) ] // Former not yet implemented for standalone_constructors
  // #[ standalone_constructors ] // Enable standalone constructors
  pub struct ServerConfig
  {
    host : String,     // Will be constructor arg
    port : u16,        // Will be constructor arg  
    #[ former_ignore ] // This field is NOT a constructor arg
    timeout : Option< u32 >,
  }

  // Some fields ignored, so `server_config` returns the Former
  let config_former = server_config( "localhost".to_string(), 8080u16 ); // Added u16 suffix

  // Set the ignored field and form
  let config = config_former
  .timeout( 5000u32 ) // Added u32 suffix
  .form();

  assert_eq!( config.host, "localhost" );
  assert_eq!( config.port, 8080u16 ); // Added u16 suffix
  assert_eq!( config.timeout, Some( 5000u32 ) ); // Added u32 suffix

  #[ derive( Debug, PartialEq, Former ) ]
  #[ standalone_constructors ]
  pub struct Point
  {
    x : i32,  // Will be constructor arg
    y : i32,  // Will be constructor arg
  }

  // NO fields ignored, so `point` returns Self directly
  let p = point( 10, 20 );
  assert_eq!( p.x, 10 );
  assert_eq!( p.y, 20 );
# }

Example: Enum Standalone Constructors

Vocabulary & Terminology

Understanding the terminology used in former will help you leverage its full potential, especially when working with enums and variants:

Core Concepts

• Former: A builder object that accumulates field values and produces the final instance via .form().
• Storage: Internal structure that holds the building state, containing options for each field.
• Subformer: A specialized former for building nested structures, collections, or complex field types.
• FormingEnd: A mechanism that controls what happens when .form() is called on a (sub)former.

Variant Types (for Enums)

• Unit Variant: An enum variant with no associated data (e.g., Status::Active).
• Tuple Variant: An enum variant with unnamed fields in parentheses (e.g., Message::Error(String), Point::Coords(i32, i32)).
• Struct Variant: An enum variant with named fields in braces (e.g., Request::Get { url: String, headers: Vec<String> }).

Variant Field Categories

• Zero-Field Variant: A variant with no fields - can be unit (Status::Active) or empty tuple (Status::Active()).
• Single-Field Variant: A variant with exactly one field (e.g., Message::Text(String) or User::Profile { name: String }).
• Multi-Field Variant: A variant with multiple fields (e.g., Point::Coords(i32, i32) or Request::Post { url: String, body: String }).

Constructor Types

• Scalar Constructor: A method that takes direct values and immediately returns the enum instance (e.g., Message::text("hello") → Message::Text("hello")).
• Subform Constructor: A method that returns a former/builder for constructing the variant step-by-step, useful for complex variants.
• Direct Constructor: Simple constructor for variants with no fields (e.g., Status::active() → Status::Active).

Enum Constructor Patterns

• Method-style Constructor: Instance methods on the enum type (e.g., MyEnum::variant_name(...)).
• Standalone Constructor: Top-level functions generated when #[standalone_constructors] is used (e.g., variant_name(...)).

Variant Attributes

• #[scalar]: Forces generation of a scalar constructor that takes field values directly and returns the enum instance.
• #[subform_scalar]: For single-field variants where the field type implements Former - generates a method returning the field's former.
• #[standalone_constructors]: Applied to the enum itself, generates top-level constructor functions for each variant.
• #[former_ignore]: Applied to individual fields, excludes them from being parameters in standalone constructors.

Advanced Concepts

• Implicit Variant Former: An automatically generated former for variants with multiple fields, providing individual field setters.
• End-of-forming Logic: Custom behavior when a former completes, enabling advanced patterns like validation or transformation.
• Context Propagation: Mechanism for passing data through nested formers in complex builder hierarchies.

Key Features Overview

• Automatic Builder Generation: #[ derive( Former ) ] for structs (enums under development).
• Fluent API: Chainable setter methods for a clean construction flow.
• Production-Ready Struct Support: Complete implementation with all features working:
  • Field setters: Individual setter methods for each field
  • Default handling: Automatic use of Default trait or custom defaults
  • Optional fields: Seamless Option<T> support
  • Subformers: Nested builders for complex field types
• Defaults & Optionals: Seamless handling of Default values and Option< T > fields. Custom defaults via #[ former( default = ... ) ].
• Collection & Nested Struct Support: Powerful subformer system for building complex structures:
  • #[ subform_scalar ]: For fields whose type also derives Former
  • #[ subform_collection ]: For collections like Vec, HashMap, HashSet, etc., providing methods like .add() or .insert()
  • #[ subform_entry ]: For collections where each entry is built individually using its own former
• Enum Support (Active Development): Comprehensive implementation with working functionality:
  • Unit variants: Direct constructors (e.g., MyEnum::variant()) - Fully functional
  • Tuple variants: Scalar constructors and subformers based on field count and attributes - Core patterns working
  • Struct variants: Subformers with individual field setters or scalar constructors - Core patterns working
  • Flexible attributes: #[scalar], #[subform_scalar], #[standalone_constructors] for fine-grained control
  • Known limitations: Single-field tuple variants with primitives require explicit #[scalar] attribute, #[former_ignore] not yet implemented
• Customization:
  • Rename setters: #[ scalar( name = ... ) ], #[ subform_... ( name = ... ) ]
  • Disable default setters: #[ scalar( setter = false ) ], #[ subform_... ( setter = false ) ]
  • Define custom setters directly in impl Former
  • Specify collection definitions: #[ subform_collection( definition = ... ) ]
• Advanced Control:
  • Storage-only fields: #[ storage_fields( ... ) ].
  • Custom mutation logic: #[ mutator( custom ) ] + impl FormerMutator.
  • Custom end-of-forming logic: Implement FormingEnd.
  • Custom collection support: Implement Collection traits.

Troubleshooting

Common Issues

"Missing Former types" Error

• Symptom: Errors like BreakFormer not found or RunFormerDefinition not found
• Cause: Required struct types don't have #[derive(Former)] enabled
• Solution: Check for commented-out // #[derive(Debug, Clone, PartialEq, former::Former)] and uncomment them
• Note: Historical "trailing comma issue" has been resolved - Former derive works correctly now

Raw Identifier Compilation Errors

• Symptom: Panic with error like "KeywordVariantEnumr#breakFormerStorage" is not a valid identifier
• Cause: Bug in enum variant handling with raw identifiers (e.g., r#break, r#move)
• Workaround: Use explicit #[scalar] attribute on variants with keyword identifiers
• Status: Known issue with utility functions available but not fully integrated

Inner Doc Comment Errors (E0753)

• Symptom: inner doc comments are not permitted here when compiling tests
• Cause: Files with //! comments included via include!() macro
• Solution: Replace //! with regular // comments in included test files

Test Import/Scope Issues

• Symptom: TestEnum not found or similar import errors in test files
• Solution: Update import paths to use full crate paths (e.g., use crate::inc::module::TestEnum)
• Architecture: *_only_test.rs files are included by derive.rs/manual.rs, not standalone modules

Enum Field Method Not Found

• Symptom: Method like .field_name() not found on enum variant former
• Cause: Current enum Former implementation uses positional setters, not field delegation
• Workaround: Use positional setters like ._0(value) instead of .field_name(value)
• Alternative: Mark complex variants as #[scalar] for direct construction

Standalone Constructor Conflicts

• Symptom: "Old behavior conflicts" in manual implementations
• Cause: Manual implementations following outdated patterns
• Solution: Update standalone constructors to return Self directly when no fields are marked with #[former_ignore]

Where to Go Next

• Technical Specification: Complete behavioral specification defining the Former macro's rules and expected behavior.
• Advanced Usage & Concepts: Dive deeper into subformers, customization options, storage, context, definitions, mutators, and custom collections.
• Examples Directory: Explore practical, runnable examples showcasing various features.
• API Documentation (docs.rs): Get detailed information on all public types, traits, and functions.
• Repository (GitHub): View the source code, contribute, or report issues.