try_create 0.1.2

# Try Create

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`try_create` is a small Rust utility library that provides generic traits for object creation, offering standardized ways to handle infallible, fallible, conditional, and policy-based validated construction.

## Overview

This library introduces a set of traits to streamline the creation of new type instances:

*   **`IntoInner`**: A re-exported trait from the [`into_inner`](https://crates.io/crates/into_inner) crate, used as a supertrait to define the input type for construction.
*   **`TryNew`**: For fallible construction, where creating an instance might fail (e.g., due to validation rules). It returns a `Result`.
*   **`New`**: For infallible construction. If an invariant is violated, this method is expected to panic.
*   **`ConditionallyCreate`**: A utility trait that switches creation logic based on the build profile:
    *   In **debug** mode, it uses `TryNew::try_new().expect()`, panicking if `try_new` fails.
    *   In **release** mode, it uses `New::new()`.
*   **`ValidationPolicy`**: Defines a contract for validation logic. A policy specifies how a value should be validated and what error type is returned upon failure. This allows for reusable validation strategies.
*   **`TryNewValidated`**: Extends `TryNew` by associating a specific `ValidationPolicy` with the type. The `TryNewValidated::try_new_validated` method first applies the policy and then, if successful, proceeds with the underlying `TryNew` construction logic.

These traits are designed to be general-purpose and can be used for various types, promoting a consistent API for object instantiation. The library supports `no_std` environments.

## Installation

Add `try_create` to your `Cargo.toml`:

```toml
[dependencies]
try_create = "0.1" # Replace with the latest version
into_inner = "0.1" # try_create re-exports IntoInner, but you might depend on it directly too
```

## Usage

### 1. `IntoInner` Trait

Both `TryNew` and `New` require `IntoInner` to be implemented. This trait defines the `InnerType` that your constructor will accept and a way to retrieve this inner value from an instance.

```rust
use try_create::IntoInner;

#[derive(Debug, PartialEq)]
struct MyValueContainer {
    value: i32,
}

impl IntoInner for MyValueContainer {
    type InnerType = i32;

    fn into_inner(self) -> Self::InnerType {
        self.value
    }
}
```

### 2. `TryNew` Trait (Fallible Creation)

Use `TryNew` when the creation process can fail and you want to return a `Result`.

```rust
use try_create::{TryNew, IntoInner};
#[cfg(feature = "std")]
use std::fmt;
#[cfg(feature = "std")]
use std::error::Error;

// Define a custom error type
#[derive(Debug, PartialEq)]
struct NotPositiveError;

#[cfg(feature = "std")]
impl fmt::Display for NotPositiveError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "Value must be positive")
    }
}

#[cfg(feature = "std")]
impl Error for NotPositiveError {}
// For no_std, NotPositiveError would just need to implement core::fmt::Debug,
// which it does via #[derive(Debug, PartialEq)].

// A struct that wraps an i32, ensuring it's positive.
#[derive(Debug, PartialEq)]
struct PositiveInteger {
    value: i32,
}

impl IntoInner for PositiveInteger {
    type InnerType = i32;
    fn into_inner(self) -> Self::InnerType { self.value }
}

impl TryNew for PositiveInteger {
    type Error = NotPositiveError;
    // `InnerType` is `i32`, inherited from `IntoInner`.

    fn try_new(value: Self::InnerType) -> Result<Self, Self::Error> {
        if value > 0 {
            Ok(PositiveInteger { value })
        } else {
            Err(NotPositiveError)
        }
    }
}

// Usage
assert_eq!(PositiveInteger::try_new(10), Ok(PositiveInteger { value: 10 }));
assert_eq!(PositiveInteger::try_new(0), Err(NotPositiveError));
assert_eq!(PositiveInteger::try_new(-5), Err(NotPositiveError));

let positive_num = PositiveInteger::try_new(42).unwrap();
assert_eq!(positive_num.into_inner(), 42);
```

### 3. `New` Trait (Infallible/Panicking Creation)

Use `New` when the creation process should not fail in a recoverable way. If invariants are violated, `New::new` should panic.

```rust
use try_create::{New, IntoInner, TryNew}; // TryNew often used to implement New
#[cfg(feature = "std")]
use std::fmt;
#[cfg(feature = "std")]
use std::error::Error;

// Example struct
#[derive(Debug, PartialEq)]
struct MyType(i32);

// Custom error for TryNew implementation (if used to implement New)
#[derive(Debug, PartialEq)]
struct MyTypeError(String);

#[cfg(feature = "std")]
impl fmt::Display for MyTypeError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "MyTypeError: {}", self.0)
    }
}

#[cfg(feature = "std")]
impl Error for MyTypeError {}


impl IntoInner for MyType {
    type InnerType = i32;
    fn into_inner(self) -> Self::InnerType { self.0 }
}

// Optional TryNew, can be used to implement New
impl TryNew for MyType {
    type Error = MyTypeError;
    fn try_new(value: i32) -> Result<Self, Self::Error> {
        if value < 0 { Err(MyTypeError("Value cannot be negative".to_string())) }
        else { Ok(MyType(value)) }
    }
}

impl New for MyType {
    fn new(value: i32) -> Self {
        // Example: using try_new and panicking on error
        // match Self::try_new(value) {
        //     Ok(instance) => instance,
        //     Err(e) => panic!("MyType::new failed: {:?}", e),
        // }
        // Or direct implementation:
        if value < 0 {
            panic!("MyType::new: Value cannot be negative");
        }
        MyType(value)
    }
}

// Usage
assert_eq!(MyType::new(10), MyType(10));
// The following would panic:
// MyType::new(-5);
```

### 4. `ConditionallyCreate` Trait

This trait provides a `create_conditionally` method that behaves differently in debug and release builds.

```rust
use try_create::{ConditionallyCreate, TryNew, New, IntoInner};
#[cfg(feature = "std")]
use std::fmt;
#[cfg(feature = "std")]
use std::error::Error;

// Using PositiveInteger and NotPositiveError from the TryNew example above
#[derive(Debug, PartialEq)]
struct NotPositiveError;

#[cfg(feature = "std")]
impl fmt::Display for NotPositiveError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { 
        write!(f, "Value must be positive") 
    } 
}

#[cfg(feature = "std")]
impl Error for NotPositiveError {}

#[derive(Debug, PartialEq)]
struct PositiveInteger { value: i32 }
impl IntoInner for PositiveInteger {
    type InnerType = i32; 
    
    fn into_inner(self) -> Self::InnerType { 
        self.value 
    } 
}

impl TryNew for PositiveInteger {
    type Error = NotPositiveError;

    fn try_new(value: Self::InnerType) -> Result<Self, Self::Error> {
        if value > 0 { Ok(PositiveInteger { value }) } else { Err(NotPositiveError) }
    }
}

// PositiveInteger must also implement New to use ConditionallyCreate
impl New for PositiveInteger {
    fn new(value: Self::InnerType) -> Self {
        match Self::try_new(value) {
            Ok(instance) => instance,
            Err(e) => panic!("PositiveInteger::new failed for a non-positive value. Error: {:?}", e),
        }
    }
}

// Usage of ConditionallyCreate
let p1 = PositiveInteger::create_conditionally(10);
assert_eq!(p1, PositiveInteger { value: 10 });

// If PositiveInteger::create_conditionally(-5) was called:
// - In debug mode: it would panic with "ConditionallyCreate: try_new() failed in debug mode".
// - In release mode: it would panic with "PositiveInteger::new failed...".
```

### 5. `ValidationPolicy` Trait

Defines a reusable validation strategy.

```rust
use try_create::ValidationPolicy;
#[cfg(feature = "std")]
use std::fmt;
#[cfg(feature = "std")]
use std::error::Error;

#[derive(Debug, PartialEq)]
struct MinValueError(String);

#[cfg(feature = "std")]
impl fmt::Display for MinValueError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "MinValueError: {}", self.0)
    }
}

#[cfg(feature = "std")]
impl Error for MinValueError {}

struct MinValuePolicy {
    min_value: i32,
}

impl ValidationPolicy for MinValuePolicy {
    type Value = i32;
    type Error = MinValueError;

    // `validate` (consuming) has a default implementation that calls `validate_ref`.
    // We only need to implement `validate_ref`.
    fn validate_ref(v: &Self::Value) -> Result<(), Self::Error> {
        // For this example, we'll imagine the policy has a configured minimum.
        // In a real scenario, MinValuePolicy might store `min_value` or it could be a const.
        // Let's assume a fixed minimum for simplicity in this example.
        const EXAMPLE_MIN: i32 = 0;
        if *v >= EXAMPLE_MIN {
            Ok(())
        } else {
            Err(MinValueError(format!("Value {} is less than minimum {}", v, EXAMPLE_MIN)))
        }
    }
}

// Usage
assert_eq!(MinValuePolicy::validate_ref(&5), Ok(()));
assert!(MinValuePolicy::validate_ref(&-1).is_err());
assert_eq!(MinValuePolicy::validate(10), Ok(10)); // Uses default validate
assert!(MinValuePolicy::validate(-5).is_err());   // Uses default validate
```

### 6. `TryNewValidated` Trait

Combines a `ValidationPolicy` with `TryNew` for two-phase construction.

```rust
use try_create::{TryNewValidated, ValidationPolicy, TryNew, IntoInner};
#[cfg(feature = "std")]
use std::fmt;
#[cfg(feature = "std")]
use std::error::Error;

// --- Reusing MinValuePolicy and MinValueError from previous example ---
#[derive(Debug, PartialEq)]
struct MinValueError(String);

#[cfg(feature = "std")]
impl fmt::Display for MinValueError { 
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { 
        write!(f, "MinValueError: {}", self.0) 
    } 
}

#[cfg(feature = "std")]
impl Error for MinValueError {}
struct MinValuePolicy; // Simplified for example, assumes fixed min of 0
impl ValidationPolicy for MinValuePolicy {
    type Value = i32; type Error = MinValueError;

    fn validate_ref(v: &Self::Value) -> Result<(), Self::Error> {
        if *v >= 0 { Ok(()) } else { Err(MinValueError("Value must be non-negative".to_string())) }
    }
}

// --- Define the type to be created and its specific errors ---
#[derive(Debug, PartialEq)]
enum ValidatedNumberError {
    Policy(MinValueError), // Error from the validation policy
    Construction(String),  // Error from the TryNew part
}

// Implement From for error conversion (Policy::Error -> Self::Error)
impl From<MinValueError> for ValidatedNumberError {
    fn from(e: MinValueError) -> Self {
        ValidatedNumberError::Policy(e)
    }
}

#[cfg(feature = "std")]
impl fmt::Display for ValidatedNumberError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            ValidatedNumberError::Policy(e) => write!(f, "Policy validation failed: {}", e),
            ValidatedNumberError::Construction(s) => write!(f, "Construction failed: {}", s),
        }
    }
}

#[cfg(feature = "std")]
impl Error for ValidatedNumberError {
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        match self {
            ValidatedNumberError::Policy(e) => Some(e),
            ValidatedNumberError::Construction(_) => None,
        }
    }
}


#[derive(Debug, PartialEq)]
struct ValidatedNumber(i32);

impl IntoInner for ValidatedNumber {
    type InnerType = i32;
    fn into_inner(self) -> Self::InnerType { self.0 }
}

// 1. Implement TryNew for the core construction logic (post-validation)
impl TryNew for ValidatedNumber {
    type Error = ValidatedNumberError; // Uses the combined error type

    fn try_new(value: Self::InnerType) -> Result<Self, Self::Error> {
        // Assume after policy validation, we have another rule: value must be even.
        if value % 2 == 0 {
            Ok(ValidatedNumber(value))
        } else {
            Err(ValidatedNumberError::Construction(format!("Value {} is not even", value)))
        }
    }
}

// 2. Implement TryNewValidated, associating the policy
impl TryNewValidated for ValidatedNumber {
    type Policy = MinValuePolicy; // Specify which policy to use
    // `InnerType` is inherited from `IntoInner`.
    // `Error` is inherited from `TryNew`.
}

// Usage
// Policy: value >= 0. Construction: value must be even.

// Valid: 10 >= 0 (policy pass), 10 is even (construction pass)
assert_eq!(ValidatedNumber::try_new_validated(10), Ok(ValidatedNumber(10)));

// Invalid: Policy fails (-5 < 0)
let err_policy = ValidatedNumber::try_new_validated(-5).unwrap_err();
match err_policy {
    ValidatedNumberError::Policy(MinValueError(msg)) => assert!(msg.contains("Value must be non-negative")),
    _ => panic!("Expected policy error"),
}

// Invalid: Policy passes (5 >= 0), but construction fails (5 is not even)
let err_constr = ValidatedNumber::try_new_validated(5).unwrap_err();
match err_constr {
    ValidatedNumberError::Construction(msg)) => assert!(msg.contains("Value 5 is not even")),
    _ => panic!("Expected construction error"),
}
```

## `no_std` Support

The library is `no_std` compatible.
When the `std` feature is not enabled (default for `no_std` environments):
*   `TryNew::Error` and `ValidationPolicy::Error` only require `core::fmt::Debug`.
*   You are responsible for defining error types that conform to this.

## Contributing

Contributions, issues, and feature requests are welcome!

## License

This project is licensed under the terms of the MIT license and the Apache License (Version 2.0). See [LICENSE-MIT](LICENSE-MIT) and [LICENSE-APACHE](LICENSE-APACHE) for details. (You'll need to add these license files to your project).