use stillwater::Validation;

// Standard Result: stops at first error
let email = validate_email(input)?;  // Stops here
let age = validate_age(input)?;      // Never reached if email fails

// Stillwater: accumulates all errors
let user = Validation::all((
    validate_email(input),
    validate_age(input),
    validate_name(input),
))?;
// Returns: Err(vec![EmailError, AgeError, NameError])

use stillwater::validation::homogeneous::validate_homogeneous;
use std::mem::discriminant;

#[derive(Clone, Debug, PartialEq)]
enum Aggregate {
    Sum(f64),     // Can combine Sum + Sum
    Count(usize), // Can combine Count + Count
    // But Sum + Count is a type error!
}

// Without validation: runtime panic
let mixed = vec![Aggregate::Count(5), Aggregate::Sum(10.0)];
// items.into_iter().reduce(|a, b| a.combine(b))  // PANIC!

// With validation: type-safe error accumulation
let result = validate_homogeneous(
    mixed,
    |a| discriminant(a),
    |idx, _, _| format!("Type mismatch at index {}", idx),
);

match result {
    Validation::Success(items) => {
        // Safe to combine - all same type!
        let total = items.into_iter().reduce(|a, b| a.combine(b));
    }
    Validation::Failure(errors) => {
        // All mismatches reported: ["Type mismatch at index 1"]
    }
}

use stillwater::prelude::*;

// Pure business logic (no DB, easy to test)
fn calculate_discount(customer: &Customer, total: Money) -> Money {
    match customer.tier {
        Tier::Gold => total * 0.15,
        _ => total * 0.05,
    }
}

// Effects at boundaries (mockable) - zero-cost by default
fn process_order(id: OrderId) -> impl Effect<Output = Invoice, Error = AppError, Env = AppEnv> {
    from_fn(move |env: &AppEnv| env.db.fetch_order(id))  // I/O
        .and_then(|order| {
            let total = calculate_total(&order);  // Pure!
            from_fn(move |env: &AppEnv| env.db.fetch_customer(order.customer_id))
                .map(move |customer| (order, customer, total))
        })
        .map(|(order, customer, total)| {
            let discount = calculate_discount(&customer, total);  // Pure!
            create_invoice(order.id, total - discount)            // Pure!
        })
        .and_then(|invoice| from_fn(move |env: &AppEnv| env.db.save(invoice))) // I/O
}

// Test with mock environment
#[tokio::test]
async fn test_with_mock_db() {
    let env = MockEnv::new();
    let result = process_order(id).run(&env).await?;
    assert_eq!(result.total, expected);
}

use stillwater::prelude::*;

// Combine independent effects - neither depends on the other
fn load_user_profile(id: UserId) -> impl Effect<Output = UserProfile, Error = AppError, Env = AppEnv> {
    fetch_user(id)
        .zip(fetch_settings(id))
        .zip(fetch_preferences(id))
        .map(|((user, settings), prefs)| UserProfile { user, settings, prefs })
}

// Or use zip3 for cleaner flat tuples
fn load_user_profile_v2(id: UserId) -> impl Effect<Output = UserProfile, Error = AppError, Env = AppEnv> {
    zip3(
        fetch_user(id),
        fetch_settings(id),
        fetch_preferences(id),
    )
    .map(|(user, settings, prefs)| UserProfile { user, settings, prefs })
}

// Combine results with a function directly using zip_with
let effect = fetch_price(item_id)
    .zip_with(fetch_quantity(item_id), |price, qty| price * qty);

use stillwater::prelude::*;

fetch_user(id)
    .context("Loading user profile")
    .and_then(|user| process_data(user))
    .context("Processing user data")
    .run(&env).await?;

// Error output:
// Error: UserNotFound(12345)
//   -> Loading user profile
//   -> Processing user data

use stillwater::prelude::*;

#[derive(Clone)]
struct Config {
    timeout: u64,
    retries: u32,
}

// Functions don't need explicit config parameters
fn fetch_data() -> impl Effect<Output = String, Error = String, Env = Config> {
    // Ask for config when needed
    asks(|cfg: &Config| format!("Fetching with timeout={}", cfg.timeout))
}

fn fetch_with_extended_timeout() -> impl Effect<Output = String, Error = String, Env = Config> {
    // Temporarily modify environment for specific operations
    local(
        |cfg: &Config| Config { timeout: cfg.timeout * 2, ..*cfg },
        fetch_data()
    )
}

let config = Config { timeout: 30, retries: 3 };
let result = fetch_with_extended_timeout().run(&config).await?;
// Uses timeout=60 without changing the original config

use stillwater::effect::bracket::{bracket, bracket2, acquiring, BracketError};
use stillwater::prelude::*;

// Single resource with guaranteed cleanup
let result = bracket(
    open_connection(),                           // Acquire
    |conn| async move { conn.close().await },    // Release (always runs)
    |conn| fetch_user(conn, user_id),            // Use
).run(&env).await;

// Multiple resources with LIFO cleanup order
let result = bracket2(
    open_database(),
    open_file(path),
    |db| async move { db.close().await },        // Released second
    |file| async move { file.close().await },    // Released first (LIFO)
    |db, file| process(db, file),
).run(&env).await;

// Fluent builder for ergonomic multi-resource management
let result = acquiring(open_conn(), |c| async move { c.close().await })
    .and(open_file(), |f| async move { f.close().await })
    .and(acquire_lock(), |l| async move { l.release().await })
    .with_flat3(|conn, file, lock| do_work(conn, file, lock))
    .run(&env)
    .await;

// Explicit error handling with BracketError
let result = bracket_full(acquire, release, use_fn).run(&env).await;
match result {
    Ok(value) => println!("Success"),
    Err(BracketError::AcquireError(e)) => println!("Acquire failed"),
    Err(BracketError::UseError(e)) => println!("Use failed, cleanup succeeded"),
    Err(BracketError::CleanupError(e)) => println!("Use succeeded, cleanup failed"),
    Err(BracketError::Both { use_error, cleanup_error }) => println!("Both failed"),
}

use stillwater::effect::retry::{retry, retry_if, retry_with_hooks};
use stillwater::RetryPolicy;
use std::time::Duration;

// Stillwater: Policy as Data
// Define retry policies as pure, testable values
let api_policy = RetryPolicy::exponential(Duration::from_millis(100))
    .with_max_retries(5)
    .with_max_delay(Duration::from_secs(2))
    .with_jitter(0.25);

// Test the policy without any I/O
assert_eq!(api_policy.delay_for_attempt(0), Some(Duration::from_millis(100)));
assert_eq!(api_policy.delay_for_attempt(1), Some(Duration::from_millis(200)));
assert_eq!(api_policy.delay_for_attempt(2), Some(Duration::from_millis(400)));

// Reuse the same policy across different effects
retry(|| fetch_user(id), api_policy.clone());
retry(|| save_order(order), api_policy.clone());

// Conditional retry: only retry transient failures
retry_if(
    || api_call(),
    api_policy,
    |err| matches!(err, ApiError::Timeout | ApiError::ServerError(_))
);

// Observability: hook into retry events for logging/metrics
retry_with_hooks(
    || api_call(),
    policy,
    |event| log::warn!(
        "Attempt {} failed: {}, retrying in {:?}",
        event.attempt, event.error, event.next_delay
    )
);

use stillwater::effect::writer::prelude::*;
use stillwater::effect::prelude::*;

// Without Writer: manually threading state
fn process(x: i32, logs: &mut Vec<String>) -> Result<i32, Error> {
    logs.push("Starting".into());
    let y = step1(x, logs)?;
    logs.push(format!("Step 1: {}", y));
    Ok(y)
}

// With Writer Effect: automatic accumulation
fn process_with_writer(x: i32) -> impl WriterEffect<
    Output = i32, Error = String, Env = (), Writes = Vec<String>
> {
    tell_one::<_, String, ()>("Starting".to_string())
        .and_then(move |_| into_writer::<_, _, Vec<String>>(pure::<_, String, ()>(x * 2)))
        .tap_tell(|y| vec![format!("Step 1: {}", y)])
}

// Run and get both result and accumulated logs
let (result, logs) = process_with_writer(21).run_writer(&()).await;
assert_eq!(result, Ok(42));
assert_eq!(logs, vec!["Starting", "Step 1: 42"]);

// Use any Monoid for accumulation - not just Vec!
use stillwater::monoid::Sum;

// Count operations
let effect = tell::<Sum<u32>, String, ()>(Sum(1))
    .and_then(|_| tell(Sum(1)))
    .and_then(|_| tell(Sum(1)));

let (_, Sum(count)) = effect.run_writer(&()).await;
assert_eq!(count, 3);

use stillwater::effect::resource::*;

// Mark effects with resource acquisition at the TYPE level
fn open_file(path: &str) -> impl ResourceEffect<Acquires = Has<FileRes>> {
    pure(FileHandle::new(path)).acquires::<FileRes>()
}

fn close_file(handle: FileHandle) -> impl ResourceEffect<Releases = Has<FileRes>> {
    pure(()).releases::<FileRes>()
}

// The bracket pattern guarantees resource neutrality
// Use the builder for ergonomic syntax (single type parameter)
fn read_file_safe(path: &str) -> impl ResourceEffect<Acquires = Empty, Releases = Empty> {
    bracket::<FileRes>()
        .acquire(open_file(path))
        .release(|h| async move { close_file(h).run(&()).await })
        .use_fn(|h| read_contents(h))
}

// Transaction protocols enforced at compile time
fn begin_tx() -> impl ResourceEffect<Acquires = Has<TxRes>> { /* ... */ }
fn commit(tx: Tx) -> impl ResourceEffect<Releases = Has<TxRes>> { /* ... */ }

// This function MUST be resource-neutral or it won't compile
fn transfer_funds() -> impl ResourceEffect<Acquires = Empty, Releases = Empty> {
    bracket::<TxRes>()
        .acquire(begin_tx())
        .release(|tx| async move { commit(tx).run(&()).await })
        .use_fn(|tx| execute_queries(tx))
}
// Zero runtime overhead - all tracking is compile-time only!

use stillwater::prelude::*;

// 1. Validation with error accumulation
fn validate_user(input: UserInput) -> Validation<User, Vec<Error>> {
    Validation::all((
        validate_email(&input.email),
        validate_age(input.age),
        validate_name(&input.name),
    ))
    .map(|(email, age, name)| User { email, age, name })
}

// 2. Effect composition (zero-cost by default)
fn create_user(input: UserInput) -> impl Effect<Output = User, Error = AppError, Env = AppEnv> {
    // Validate (pure, accumulates errors)
    from_validation(validate_user(input).map_err(AppError::Validation))
        // Check if exists (I/O)
        .and_then(|user| {
            from_fn(move |env: &AppEnv| env.db.find_by_email(&user.email))
                .and_then(move |existing| {
                    if existing.is_some() {
                        fail(AppError::EmailExists)
                    } else {
                        pure(user)
                    }
                })
        })
        // Save user (I/O)
        .and_then(|user| {
            from_fn(move |env: &AppEnv| env.db.insert_user(&user))
                .map(move |_| user)
        })
        .context("Creating new user")
}

// 3. Run at application boundary
let env = AppEnv { db, cache, logger };
let result = create_user(input).run(&env).await?;

// Free-standing constructors (not methods)
let effect = pure(42);                    // Not Effect::pure(42)
let effect = fail("error");               // Not Effect::fail("error")
let effect = from_fn(|env| Ok(env.value)); // Not Effect::from_fn(...)

// Chain combinators - each returns a concrete type, zero allocations
let result = pure(1)
    .map(|x| x + 1)
    .and_then(|x| pure(x * 2))
    .map(|x| x.to_string());

// Use .boxed() when you need type erasure
fn dynamic_effect(flag: bool) -> BoxedEffect<i32, String, ()> {
    if flag {
        pure(1).boxed()
    } else {
        pure(2).boxed()
    }
}

// Collections of effects require boxing
let effects: Vec<BoxedEffect<i32, String, Env>> = vec![
    effect1.boxed(),
    effect2.boxed(),
];
let results = par_all(effects, &env).await;

// Before (0.10.x)
Effect::pure(x)
Effect::fail(e)
Effect::from_fn(f)

// After (current API)
pure(x)
fail(e)
from_fn(f)

// Return types changed
fn old() -> Effect<T, E, Env> { ... }      // Boxed by default
fn new() -> impl Effect<...> { ... }        // Zero-cost by default
fn boxed() -> BoxedEffect<T, E, Env> { ... } // Explicit boxing

// Your existing code works as-is
fn validate_email(email: &str) -> Result<Email, Error> {
    // ...
}

// Upgrade to accumulation when you need it
fn validate_form(input: FormInput) -> Validation<Form, Vec<Error>> {
    Validation::all((
        Validation::from_result(validate_email(&input.email)),
        Validation::from_result(validate_age(input.age)),
    ))
}

// Convert back to Result when needed
let result: Result<Form, Vec<Error>> = validation.into_result();