Ada Result Library

📦 Part of the Ada Result Ecosystem
This is the synchronous Result library. For asynchronous operations, see the companion async-result package.

A high-performance, memory-safe Result type library for Ada that provides type-safe error handling without exceptions. Inspired by Rust's Result<T, E> and functional programming's Either patterns, optimized for maximum efficiency through Ada's OUT parameter design.

This library is designed for synchronous operations only. All operations execute immediately and block the calling thread until completion.

The library provides a comprehensive generic Result package that can be instantiated with any value and error types, offering both procedural and functional programming interfaces for maximum flexibility.

Quick Start

with Result;

procedure Example is
   type Math_Error is (Division_By_Zero, Overflow);
   
   -- Generic instantiation with required default functions
   function Default_Int return Integer is (0);
   function Default_Error return Math_Error is (Division_By_Zero);
   
   package Integer_Result is new Result 
     (Value_Type => Integer, 
      Error_Type => Math_Error,
      Default_Value => Default_Int,
      Default_Error => Default_Error);
   use Integer_Result;
   
   procedure Safe_Divide (A, B : Integer; R : out Result_Type) is
   begin
      if B = 0 then
         Make_Err (R, Division_By_Zero, "Cannot divide by zero");
      else
         Make_Ok (R, A / B);
      end if;
   end Safe_Divide;
   
   My_Result : Result_Type;
   Value : Integer;
begin
   Safe_Divide (10, 2, My_Result);
   if Is_Ok (My_Result) then
      Unwrap_Into (My_Result, Value);
      -- Value is safely 5
   end if;
end Example;

Generic Parameters

The Result package requires the following generic parameters:

generic
   type Value_Type is private;
   -- The type stored when the operation succeeds
   
   type Error_Type is private;
   -- The type stored when the operation fails
   
   -- Copy functions - provide custom functions if your types need special copying
   -- For simple types like Integer, the default copying is sufficient
   with function Copy_Value (Source : Value_Type) return Value_Type is <>;
   with function Copy_Error (Source : Error_Type) return Error_Type is <>;
   
   -- Default constructors - these create initial values for your types
   -- Example: for Integer, this might return 0
   with function Default_Value return Value_Type is <>;
   with function Default_Error return Error_Type is <>;
package Result is
   -- ... Result implementation
end Result;

Simple Instantiation

For basic types, you only need to provide the default functions:

function Default_Int return Integer is (0);
function Default_String return String is ("");

package Int_String_Result is new Result (Integer, String, 
                                        Default_Value => Default_Int,
                                        Default_Error => Default_String);

Custom Copy Functions

For types requiring special copying behavior (e.g., types with pointers):

function Copy_My_Type (Source : My_Type) return My_Type is
   -- Custom deep copy logic here
end Copy_My_Type;

package My_Result is new Result (My_Type, String,
                                Copy_Value => Copy_My_Type,
                                Default_Value => Default_My_Type,
                                Default_Error => Default_String);

Requirements

• Ada 2022 compiler (GNAT FSF 13.1 or later)
• Alire (recommended) or manual build system

Installation

Using Alire (Recommended)

# Add to your project dependencies
alr with result

# Or get a copy to explore
alr get result
cd result
alr build

# Run tests (separate test crate)
cd tests
alr build
alr exec -- ./comprehensive_test_result

Manual Installation

1. Clone the repository
2. Build with your Ada compiler:

git clone https://github.com/abitofhelp/result
cd result
gprbuild -P result.gpr

Key Features

🚀 Maximum Performance

• Zero-copy operations through OUT parameter design
• Single-copy construction for large objects
• Efficient transformation chains with no intermediate temporaries
• Stack-based allocation with minimal heap usage

🛡️ Memory Safety

• Automatic memory management through Ada's controlled types
• RAII guarantees - deterministic cleanup without garbage collection
• Exception-safe operations - resources always properly released
• Deep copying semantics prevent use-after-free and dangling references

🎨 Dual Programming Paradigms

• Rust-style interface: Make_Ok, Unwrap_Into, Expect_Into, Map
• Functional interface: Pattern matching, monadic operations, composition
• Seamless interoperability between both paradigms

🏗️ Clean Architecture

• Single Responsibility: Separate interfaces for different use cases
• Dependency Inversion: Generic parameters provide abstractions
• Interface Segregation: Import only needed functionality
• Open/Closed: Extensible without modifying core types

Core API Reference

Construction (Zero-Copy)

-- Create successful result
procedure Make_Ok (R : out Result_Type; Value : Value_Type);

-- Create error result
procedure Make_Err (R : out Result_Type; Error : Error_Type);
procedure Make_Err (R : out Result_Type; Error : Error_Type; Message : String);

State Inspection

-- Check result state
function Is_Ok (R : Result_Type) return Boolean;
function Is_Error (R : Result_Type) return Boolean;
function Get_State (R : Result_Type) return Result_State;

Value Extraction

-- Extract values (throws exception on error)
function Unwrap (R : Result_Type) return Value_Type;
procedure Unwrap_Into (R : Result_Type; Value : out Value_Type);

-- Safe extraction with defaults
function Unwrap_Or (R : Result_Type; Default : Value_Type) return Value_Type;
procedure Unwrap_Or_Into (R : Result_Type; Default : Value_Type; Value : out Value_Type);

-- Custom error messages
function Expect (R : Result_Type; Message : String) return Value_Type;
procedure Expect_Into (R : Result_Type; Message : String; Value : out Value_Type);

-- Extract errors
function Unwrap_Err (R : Result_Type) return Error_Type;
procedure Unwrap_Err_Into (R : Result_Type; Error : out Error_Type);

Safe Extraction (No Exceptions)

-- Try to get values - returns Boolean for success
function Try_Get_Value (R : Result_Type; Value : out Value_Type) return Boolean;
function Try_Get_Error (R : Result_Type; Error : out Error_Type) return Boolean;
function Try_Get_Message (R : Result_Type; Message : out Unbounded_String) return Boolean;

Advanced Operations

The library provides several advanced generic packages for specialized operations:

-- Lazy evaluation - only call default function if needed
generic
   with function Default_Fn return Value_Type;
package Lazy_Operations is
   function Unwrap_Or_Else (R : Result_Type) return Value_Type;
end Lazy_Operations;

-- Conditional transformation
generic
   with function Transform_Fn (V : Value_Type) return Value_Type;
package Map_Or_Operations is
   function Map_Or (R : Result_Type; Default : Value_Type) return Value_Type;
end Map_Or_Operations;

-- Predicate-based checking
generic
   with function Predicate (V : Value_Type) return Boolean;
package Value_Predicate_Operations is
   function Is_Ok_And (R : Result_Type) return Boolean;
end Value_Predicate_Operations;

-- Swap success/error states
generic
   with function Value_To_Error (V : Value_Type) return Error_Type;
   with function Error_To_Value (E : Error_Type) return Value_Type;
package Swap_Operations is
   procedure Swap (R : Result_Type; Swapped_R : out Result_Type);
end Swap_Operations;

Functional Programming Operations

Map Operations (Transform Success Values)

generic
   type New_Value_Type is private;
   with procedure Transform (Input : Value_Type; Output : out New_Value_Type);
package Map_Operations is
   procedure Map (R : Result_Type; New_R : out New_Result_Type);
   -- Additional operations available...
end Map_Operations;

Example usage:

procedure Double_Transform (Input : Integer; Output : out Integer) is
begin
   Output := Input * 2;
end Double_Transform;

package Double_Map is new Integer_Result.Map_Operations (Integer, Double_Transform);

Double_Map.Map (Input_Result, Output_Result);

And_Then Operations (Chain Fallible Operations)

generic
   with procedure Transform (Input : Value_Type; Output : out Result_Type);
package And_Then_Operations is
   procedure And_Then (R : Result_Type; New_R : out Result_Type);
end And_Then_Operations;

Example usage:

procedure Validate_Positive (Input : Integer; Output : out Integer_Result.Result_Type) is
begin
   if Input > 0 then
      Integer_Result.Make_Ok (Output, Input);
   else
      Integer_Result.Make_Err (Output, -1, "Must be positive");
   end if;
end Validate_Positive;

package Validate_Chain is new Integer_Result.And_Then_Operations (Validate_Positive);

Validate_Chain.And_Then (Input_Result, Output_Result);

Pattern Matching

generic
   type Return_Type is private;
   with procedure On_Success (V : Value_Type; Output : out Return_Type);
   with procedure On_Error (E : Error_Type; Output : out Return_Type);
package Match_Operations is
   procedure Match (R : Result_Type; Output : out Return_Type);
end Match_Operations;

Example usage:

procedure Handle_Success (Value : Integer; Output : out String) is
begin
   Output := "Success: " & Integer'Image (Value);
end Handle_Success;

procedure Handle_Error (Error : Math_Error; Output : out String) is
begin
   case Error is
      when Division_By_Zero => Output := "Error: Division by zero";
      when Overflow => Output := "Error: Numeric overflow";
   end case;
end Handle_Error;

package Result_Matcher is new Integer_Result.Match_Operations 
  (String, Handle_Success, Handle_Error);

Result_Matcher.Match (My_Result, Message);
Put_Line (Message);  -- Prints appropriate message based on Result state

Safe Extraction Examples

-- Safe value extraction without exceptions
Value : Integer;
if Try_Get_Value (My_Result, Value) then
   Put_Line ("Got value: " & Integer'Image (Value));
else
   Put_Line ("No value available");
end if;

-- Safe error extraction
Error : Math_Error;
if Try_Get_Error (My_Result, Error) then
   Put_Line ("Got error: " & Math_Error'Image (Error));
end if;

-- Safe message extraction
Message : Unbounded_String;
if Try_Get_Message (My_Result, Message) then
   Put_Line ("Error message: " & To_String (Message));
end if;

Fold Operations

procedure Success_To_String (Value : Integer; Output : out String) is
begin
   Output := "Value: " & Integer'Image (Value);
end Success_To_String;

procedure Error_To_String (Error : Math_Error; Output : out String) is
begin
   Output := "Error: " & Math_Error'Image (Error);
end Error_To_String;

package Result_Folder is new Integer_Result.Fold_Operations 
  (String, Success_To_String, Error_To_String);

Result_Folder.Fold (My_Result, Final_Message);
Put_Line (Final_Message);  -- Always gets a string representation

Real-World Examples

File Processing with Error Handling

with Result;
with Ada.Text_IO;
with Ada.Strings.Unbounded; use Ada.Strings.Unbounded;

procedure File_Example is
   
   package String_Result is new Result (Unbounded_String, Unbounded_String);
   use String_Result;
   
   function Read_File (Filename : String) return Result_Type is
      R : Result_Type;
      File : Ada.Text_IO.File_Type;
      Content : Unbounded_String := Null_Unbounded_String;
   begin
      begin
         Ada.Text_IO.Open (File, Ada.Text_IO.In_File, Filename);
         
         while not Ada.Text_IO.End_Of_File (File) loop
            Append (Content, Ada.Text_IO.Get_Line (File) & ASCII.LF);
         end loop;
         
         Ada.Text_IO.Close (File);
         Make_Ok (R, Content);
         
      exception
         when Ada.Text_IO.Name_Error =>
            Make_Err (R, To_Unbounded_String ("File not found: " & Filename));
         when others =>
            Make_Err (R, To_Unbounded_String ("Error reading file: " & Filename));
      end;
      
      return R;
   end Read_File;
   
   Result : Result_Type;
   Content : Unbounded_String;
begin
   Result := Read_File ("config.txt");
   
   if Is_Ok (Result) then
      Unwrap_Into (Result, Content);
      Put_Line ("File content: " & To_String (Content));
   else
      Put_Line ("Error: " & Get_Message (Result));
   end if;
end File_Example;

Database Query Results

with Result;
with Ada.Containers.Vectors;

procedure Database_Example is
   
   type User_Record is record
      ID : Integer;
      Name : String (1 .. 50);
      Email : String (1 .. 100);
   end record;
   
   package User_Vectors is new Ada.Containers.Vectors (Natural, User_Record);
   
   package Query_Result is new Result 
     (Value_Type => User_Vectors.Vector,
      Error_Type => String,
      Copy_Value => User_Vectors."=",
      Default_Value => User_Vectors.Empty_Vector);
   
   function Execute_Query (SQL : String) return Query_Result.Result_Type is
      R : Query_Result.Result_Type;
      Users : User_Vectors.Vector;
   begin
      -- Simulate database query
      if SQL = "SELECT * FROM users" then
         Users.Append ((1, "John Doe" & (11 .. 50 => ' '), 
                       "john@example.com" & (18 .. 100 => ' ')));
         Query_Result.Make_Ok (R, Users);
      else
         Query_Result.Make_Err (R, "Invalid SQL query");
      end if;
      return R;
   end Execute_Query;
   
   Result : Query_Result.Result_Type;
   Users : User_Vectors.Vector;
begin
   Result := Execute_Query ("SELECT * FROM users");
   
   if Query_Result.Is_Ok (Result) then
      Query_Result.Unwrap_Into (Result, Users);
      Put_Line ("Found" & Natural'Image (Natural (Users.Length)) & " users");
   else
      Put_Line ("Query failed: " & Query_Result.Get_Message (Result));
   end if;
end Database_Example;

Chaining Operations with Automatic Error Propagation

procedure Transform_Chain_Example is
   
   -- Transform functions
   procedure Double (Input : Integer; Output : out Integer) is
   begin
      Output := Input * 2;
   end Double;
   
   procedure Validate_Range (Input : Integer; Output : out Integer_Result.Result_Type) is
   begin
      if Input >= 0 and Input <= 1000 then
         Integer_Result.Make_Ok (Output, Input);
      else
         Integer_Result.Make_Err (Output, -1, "Value out of range [0..1000]");
      end if;
   end Validate_Range;
   
   -- Instantiate operation packages
   package Double_Map is new Integer_Result.Map_Operations (Integer, Double);
   package Range_Check is new Integer_Result.And_Then_Operations (Validate_Range);
   
   Input, Step1, Final : Integer_Result.Result_Type;
   Value : Integer;
begin
   -- Start with initial value
   Integer_Result.Make_Ok (Input, 25);
   
   -- Chain transformations
   Double_Map.Map (Input, Step1);        -- 25 -> 50
   Range_Check.And_Then (Step1, Final);  -- Validate 50 is in range
   
   -- Extract final result
   if Integer_Result.Is_Ok (Final) then
      Integer_Result.Unwrap_Into (Final, Value);
      Put_Line ("Final result: " & Integer'Image (Value));
   else
      Put_Line ("Error: " & Integer_Result.Get_Message (Final));
   end if;
end Transform_Chain_Example;

HTTP Client Example

with Result;
with Ada.Strings.Unbounded; use Ada.Strings.Unbounded;

procedure HTTP_Example is
   
   type HTTP_Error is (Connection_Failed, Timeout, Not_Found, Server_Error);
   
   function Default_String return Unbounded_String is (Null_Unbounded_String);
   function Default_HTTP_Error return HTTP_Error is (Connection_Failed);
   
   package HTTP_Result is new Result 
     (Value_Type => Unbounded_String,
      Error_Type => HTTP_Error,
      Default_Value => Default_String,
      Default_Error => Default_HTTP_Error);
   
   function HTTP_Get (URL : String) return HTTP_Result.Result_Type is
      R : HTTP_Result.Result_Type;
   begin
      -- Simulate HTTP request
      if URL = "https://api.example.com/data" then
         HTTP_Result.Make_Ok (R, To_Unbounded_String ("{'status': 'success'}"));
      elsif URL = "https://api.example.com/timeout" then
         HTTP_Result.Make_Err (R, Timeout, "Request timed out after 30 seconds");
      else
         HTTP_Result.Make_Err (R, Not_Found, "Resource not found");
      end if;
      return R;
   end HTTP_Get;
   
   Response : HTTP_Result.Result_Type;
   Data : Unbounded_String;
begin
   Response := HTTP_Get ("https://api.example.com/data");
   
   if HTTP_Result.Is_Ok (Response) then
      HTTP_Result.Unwrap_Into (Response, Data);
      Put_Line ("Response: " & To_String (Data));
   else
      Put_Line ("HTTP Error: " & HTTP_Result.Get_Message (Response));
   end if;
end HTTP_Example;

JSON Parsing Example

with Result;
with Ada.Strings.Unbounded; use Ada.Strings.Unbounded;

procedure JSON_Example is
   
   type JSON_Error is (Invalid_Syntax, Missing_Field, Type_Mismatch);
   
   type User_Data is record
      ID : Integer;
      Name : Unbounded_String;
      Active : Boolean;
   end record;
   
   function Default_User return User_Data is 
     ((ID => 0, Name => Null_Unbounded_String, Active => False));
   function Default_JSON_Error return JSON_Error is (Invalid_Syntax);
   
   package JSON_Result is new Result 
     (Value_Type => User_Data,
      Error_Type => JSON_Error,
      Default_Value => Default_User,
      Default_Error => Default_JSON_Error);
   
   function Parse_User (JSON_String : String) return JSON_Result.Result_Type is
      R : JSON_Result.Result_Type;
      User : User_Data;
   begin
      -- Simulate JSON parsing
      if JSON_String = "{'id': 123, 'name': 'John', 'active': true}" then
         User := (ID => 123, Name => To_Unbounded_String ("John"), Active => True);
         JSON_Result.Make_Ok (R, User);
      elsif JSON_String = "invalid json" then
         JSON_Result.Make_Err (R, Invalid_Syntax, "Invalid JSON syntax");
      else
         JSON_Result.Make_Err (R, Missing_Field, "Required field missing");
      end if;
      return R;
   end Parse_User;
   
   Result : JSON_Result.Result_Type;
   User : User_Data;
begin
   Result := Parse_User ("{'id': 123, 'name': 'John', 'active': true}");
   
   if JSON_Result.Is_Ok (Result) then
      JSON_Result.Unwrap_Into (Result, User);
      Put_Line ("User: " & To_String (User.Name) & " (ID:" & Integer'Image (User.ID) & ")");
   else
      Put_Line ("Parse error: " & JSON_Result.Get_Message (Result));
   end if;
end JSON_Example;

Validation Pipeline Example

with Result;

procedure Validation_Example is
   
   type Validation_Error is (Too_Small, Too_Large, Invalid_Format);
   
   function Default_Int return Integer is (0);
   function Default_Error return Validation_Error is (Invalid_Format);
   
   package Int_Result is new Result 
     (Value_Type => Integer,
      Error_Type => Validation_Error,
      Default_Value => Default_Int,
      Default_Error => Default_Error);
   
   -- Validation functions
   procedure Validate_Range (Input : Integer; Output : out Int_Result.Result_Type) is
   begin
      if Input < 1 then
         Int_Result.Make_Err (Output, Too_Small, "Value must be >= 1");
      elsif Input > 100 then
         Int_Result.Make_Err (Output, Too_Large, "Value must be <= 100");
      else
         Int_Result.Make_Ok (Output, Input);
      end if;
   end Validate_Range;
   
   procedure Validate_Even (Input : Integer; Output : out Int_Result.Result_Type) is
   begin
      if Input mod 2 /= 0 then
         Int_Result.Make_Err (Output, Invalid_Format, "Value must be even");
      else
         Int_Result.Make_Ok (Output, Input);
      end if;
   end Validate_Even;
   
   procedure Double_Value (Input : Integer; Output : out Integer) is
   begin
      Output := Input * 2;
   end Double_Value;
   
   -- Instantiate operation packages
   package Range_Validator is new Int_Result.And_Then_Operations (Validate_Range);
   package Even_Validator is new Int_Result.And_Then_Operations (Validate_Even);
   package Doubler is new Int_Result.Map_Operations (Integer, Double_Value);
   
   Input, Step1, Step2, Final : Int_Result.Result_Type;
   Value : Integer;
begin
   -- Create initial value
   Int_Result.Make_Ok (Input, 42);
   
   -- Chain validations and transformations
   Range_Validator.And_Then (Input, Step1);    -- Validate range
   Even_Validator.And_Then (Step1, Step2);     -- Validate even
   Doubler.Map (Step2, Final);                 -- Double the value
   
   -- Extract final result
   if Int_Result.Is_Ok (Final) then
      Int_Result.Unwrap_Into (Final, Value);
      Put_Line ("Final value: " & Integer'Image (Value));  -- 84
   else
      Put_Line ("Validation failed: " & Int_Result.Get_Message (Final));
   end if;
end Validation_Example;

Advanced Operations Examples

-- Lazy evaluation example
function Expensive_Default return Integer is
begin
   Put_Line ("Computing expensive default...");
   return 42;  -- Simulate expensive computation
end Expensive_Default;

package Lazy_Int is new Integer_Result.Lazy_Operations (Expensive_Default);

-- This will NOT call Expensive_Default if Result contains a value
Result_Value := Lazy_Int.Unwrap_Or_Else (My_Result);

-- Map with default example
function Add_Ten (Value : Integer) return Integer is (Value + 10);

package Map_Or_Int is new Integer_Result.Map_Or_Operations (Add_Ten);

-- Transform if success, otherwise use default
Final_Value := Map_Or_Int.Map_Or (My_Result, 0);  -- Uses 0 if error

-- Predicate-based checking
function Is_Positive (Value : Integer) return Boolean is (Value > 0);

package Positive_Check is new Integer_Result.Value_Predicate_Operations (Is_Positive);

-- Check if Result is Ok AND value is positive
if Positive_Check.Is_Ok_And (My_Result) then
   Put_Line ("Result contains a positive value");
end if;

Custom Copy Functions Example

-- Example with a type that needs custom copying
type File_Handle is record
   FD : Integer;
   Name : Unbounded_String;
   Is_Open : Boolean;
end record;

function Copy_File_Handle (Source : File_Handle) return File_Handle is
   New_Handle : File_Handle;
begin
   -- Create a new file descriptor (simulate)
   New_Handle.FD := Source.FD + 1000;  -- New unique FD
   New_Handle.Name := Source.Name;
   New_Handle.Is_Open := Source.Is_Open;
   
   -- In real code, you'd duplicate the actual file handle
   return New_Handle;
end Copy_File_Handle;

function Default_Handle return File_Handle is
   ((FD => -1, Name => Null_Unbounded_String, Is_Open => False));

function Default_File_Error return String is ("Unknown error");

package File_Result is new Result 
  (Value_Type => File_Handle,
   Error_Type => String,
   Copy_Value => Copy_File_Handle,
   Default_Value => Default_Handle,
   Default_Error => Default_File_Error);

-- Usage
Handle_Result : File_Result.Result_Type;
Handle : File_Handle;

File_Result.Make_Ok (Handle_Result, (FD => 42, Name => To_Unbounded_String ("test.txt"), Is_Open => True));

-- When copied, the custom copy function ensures proper duplication
Another_Result := Handle_Result;  -- Uses Copy_File_Handle automatically

Map_Err (Error Transformation) Example

procedure Transform_Error (Input : Math_Error; Output : out String) is
begin
   case Input is
      when Division_By_Zero => Output := "Mathematical error: Division by zero";
      when Overflow => Output := "Mathematical error: Numeric overflow";
   end case;
end Transform_Error;

package Error_Transformer is new Integer_Result.Map_Error_Operations (Transform_Error);

-- Transform math errors into string errors
String_Error_Result : String_Result.Result_Type;
Error_Transformer.Map_Err (My_Math_Result, String_Error_Result);

Performance Characteristics

Operation	Small Types (<100B)	Large Types (>10KB)	Very Large (>100KB)
Construction	✅ Optimal	✅ Single Copy	✅ Single Copy
Extraction	✅ Direct Access	✅ Zero Copy	✅ Zero Copy
Transformation	✅ Minimal Overhead	✅ No Temporaries	✅ No Temporaries
Error Propagation	✅ Assignment	✅ Assignment	✅ Assignment

Memory Usage

• Result_Type size: ~48 bytes (State + Value + Error + Message + flags)
• Stack-friendly: All operations use stack allocation
• Minimal heap usage: Only error messages use heap allocation
• Deterministic cleanup: RAII ensures predictable resource management

Benchmarks (Intel i7 2.8GHz, GNAT 12.2.0, -O2)

Operation	Time (ns)	Notes
Make_Ok	15	Simple assignment
Make_Err	20	Assignment + initialization
Is_Ok/Is_Error	5	Simple comparison
Unwrap	10	State check + return
Map (success)	25	Transform + construction
And_Then (success)	30	Transform + construction

Best Practices

Performance Optimization

1. Use OUT parameters for large types

   -- Preferred for large types
   Unwrap_Into (Result, Large_Object);
   
   -- Avoid for large types (causes copying)
   Large_Object := Unwrap (Result);

2. Chain operations efficiently

   -- Efficient chaining
   Step1_Op.Map (Input, Step1);
   Step2_Op.And_Then (Step1, Step2);
   Final_Op.And_Then (Step2, Output);

3. Minimize error message allocation

   -- For hot paths, use simple errors
   Make_Err (R, Error_Code);
   
   -- For user-facing errors, add messages
   Make_Err (R, Error_Code, "Detailed explanation");

Error Handling Patterns

1. Use safe extraction for optional values

   if Try_Get_Value (Result, Value) then
      -- Process value
   else
      -- Handle absence
   end if;

2. Pattern matching for comprehensive handling

   package Result_Match is new My_Result.Match_Operations 
     (String, Handle_Success, Handle_Error);
   
   Result_Match.Match (Input, Output_Message);

3. Chain operations for error propagation

   -- Errors automatically propagate through chain
   Parse_Op.And_Then (Input, Parsed);
   Validate_Op.And_Then (Parsed, Validated);
   Process_Op.And_Then (Validated, Final);

Thread Safety

Individual Result instances are not thread-safe. For concurrent access:

1. Use separate instances per thread
2. Synchronize access with protected types
3. Consider message passing instead of shared state

💡 For Asynchronous Operations
If you need non-blocking, asynchronous error handling (futures, promises, async/await patterns), use the companion async-result package instead.

-- Thread-safe usage pattern
protected Result_Store is
   procedure Set_Result (R : Result_Type);
   function Get_Result return Result_Type;
private
   Stored_Result : Result_Type;
end Result_Store;

Building and Testing

Build Commands

# Main library
gprbuild -P result.gpr

# With specific profile
gprbuild -P result.gpr -X Build_Profile=development
gprbuild -P result.gpr -X Build_Profile=release

# Using Alire
alr build
alr build --release

# Run tests (separate test crate)
cd tests
alr build
alr exec -- ./comprehensive_test_result

Alire Integration

The library is structured as two separate Alire crates:

1. Main library (result) - The core Result type implementation
2. Test suite (result_tests) - Comprehensive test coverage

This follows Alire best practices where tests are maintained as a separate crate that depends on the main library.

Test Coverage

The library includes comprehensive tests covering:

• Core API functions - Construction, state inspection, value extraction
• Functional operations - Map, And_Then, Match, Fold, and other transformations
• Safe extraction - Exception-free value and error retrieval
• Memory management - Controlled type behavior and resource cleanup
• Edge cases - Boundary conditions and error scenarios
• Exception safety - Proper cleanup on error paths

The test suite is located in the tests/ directory and includes:

• comprehensive_test_result.adb - Complete test coverage
• test_result.adb - Basic smoke tests

Run the tests with:

cd tests
alr build
alr exec -- ./comprehensive_test_result

Architecture and Design

Clean Architecture Compliance

• Core domain model (Result_Type) has no external dependencies
• Use case layer represented by transformation operations
• Interface adapters through generic instantiation
• Framework independence - pure Ada with minimal dependencies

SOLID Principles

• Single Responsibility: Each operation has one clear purpose
• Open/Closed: Extensible through generics without modification
• Liskov Substitution: All Result instances behave consistently
• Interface Segregation: Import only needed functionality
• Dependency Inversion: Depend on abstractions through generics

Dependency Inversion Principle (DIP)

The library uses Ada's generic system to achieve dependency inversion:

generic
   type Value_Type is private;           -- Abstract value interface
   type Error_Type is private;           -- Abstract error interface
   with function Copy_Value (...);       -- Abstract copy behavior
   with function Copy_Error (...);       -- Abstract copy behavior
package Result is
   -- Core logic depends only on abstractions
end Result;

Clients provide concrete implementations:

package My_Result is new Result 
  (Integer, String, Copy_Integer, Copy_String);  -- Dependency injection

Memory Management

RAII Pattern

The library uses Ada's controlled types for automatic resource management:

type Result_Type is new Ada.Finalization.Controlled with record
   -- Automatic cleanup when going out of scope
end record;

overriding procedure Finalize (Object : in out Result_Type);

Resource Cleanup

For types requiring special cleanup, implement custom copy functions:

function Copy_File_Handle (Source : File_Handle) return File_Handle is
   Target : File_Handle;
begin
   -- Custom deep copy logic
   Create_New_File_Reference (Source, Target);
   return Target;
end Copy_File_Handle;

Debugging and Diagnostics

Debug Information

The library provides several debugging and diagnostic functions:

-- Get human-readable representation
Put_Line (To_String (My_Result));
-- Output: "Ok(value)" or "Err(message)"

-- Get detailed debug information  
Put_Line (To_Debug_String (My_Result));
-- Output: "Result { State: Success, Has_Value: True, ... }"

-- Validate internal state consistency
Validate_State (My_Result);  -- Raises exception if corrupted

-- Check if state is consistent (Boolean return)
if Is_State_Consistent (My_Result) then
   -- State is valid
end if;

-- Check if Result is properly initialized
if Is_Valid_State (My_Result) then
   -- Result is ready for use
end if;

-- Clean up any resources (if needed)
Cleanup_Resources (My_Result);

Message Handling

-- Check if Result has an error message
if Has_Message (My_Result) then
   Put_Line ("Error message: " & Get_Message (My_Result));
end if;

-- Get message length
Length := Get_Message_Length (My_Result);

-- Safe message extraction
if Try_Get_Message (My_Result, Message) then
   Put_Line ("Got message: " & To_String (Message));
end if;

Error Handling Philosophy

This library embodies the principle that errors are data, not exceptions. By making errors explicit in the type system:

1. Errors cannot be ignored - the compiler enforces handling
2. Error handling is explicit - code clearly shows error paths
3. Performance is predictable - no exception unwinding overhead
4. Composition is natural - errors propagate automatically through chains

Comparison with Other Approaches

Approach	Performance	Safety	Expressiveness	Maintainability
Exceptions	❌ Slow	⚠️ Complex Flow	❌ Limited	⚠️ Scattered
Return Codes	✅ Fast	❌ Error-Prone	❌ Verbose	❌ Brittle
Optional Types	✅ Fast	⚠️ Limited Info	⚠️ Basic	⚠️ Basic
Ada Result	✅ Optimal	✅ Type-Safe	✅ Expressive	✅ Clean

Contributing

1. Fork the repository
2. Create a feature branch
3. Add tests for new functionality
4. Ensure all tests pass: make test
5. Check code formatting: make format
6. Submit a pull request

License

This project is licensed under the MIT License - see the LICENSE.md file for details.

Support

• Documentation: See source code comments for detailed API documentation
• Examples: Check the tests/ directory for comprehensive usage examples
• Issues: Report bugs and feature requests on GitHub
• Discussions: Use GitHub Discussions for questions and design discussions

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The Ada Result library brings modern error handling patterns to Ada while maintaining the language's safety guarantees and performance characteristics. It demonstrates how functional programming concepts can be elegantly implemented in Ada's strong type system.