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Advanced Features

This guide covers advanced Epos features including generics, higher-order programming, and working with complex data flows.

Generic Programming

Generic Functions with Multiple Parameters

Create functions that work with multiple generic types:

fn zip(first: list(a), second: list(b)): list(Pair(a, b))
  # Implementation would recursively build pairs
  {}  # Placeholder
end

fn fold(items: list(t), initial: a, combiner: fn(a, t) -> a): a
  match len(items) then
    0 => initial
    _ => tail(items).fold(initial.combiner(head(items)), combiner)
  end
end

Generic Type Constraints

While Epos doesn’t have explicit constraints, patterns emerge for type-safe generic code:

# Generic container with operations
record Container(t)
  items: list(t)
  size: int
end

fn add-to-container(container: Container(t), item: t): Container(t)
  @{
    items => append(container.items, item),
    size => container.size + 1
  }
end

Advanced Pattern Matching

Matching with Complex Conditions

record Result(t, e)
  value?: t
  error?: e
  is-success: bool
end

fn handle-result(result: Result(string, string)): string
  match result.is-success then
    true => match result.value then
      some-value => "Success: #{some-value}"
      _ => "Success but no value"
    end
    false => match result.error then
      some-error => "Error: #{some-error}"
      _ => "Unknown error"
    end
  end
end

Pattern Matching with Deconstruction

# Match and extract from complex structures
fn process-request(request: HttpRequest): HttpResponse
  match request.method then
    "GET" => handle-get(request.path, request.params)
    "POST" => handle-post(request.path, request.body)
    "PUT" => handle-put(request.path, request.body)
    _ => @{
      status => 405,
      body => "Method not allowed"
    }
  end
end

Higher-Order Programming Patterns

Currying and Partial Application

# Curried functions
fn curried-add(a: int): fn(int) -> int
  fn(b: int): int => a + b
end

add-five := curried-add(5)
result := add-five(10)  # 15

# Partial application pattern
fn make-validator(min-length: int): fn(string) -> bool
  fn(input: string): int => len(input) >= min-length
end

validate-password := make-validator(8)
is-valid := validate-password("mypassword")

Working with Lists - Advanced Operations

Custom List Operations

# Take first n elements
fn take(items: list(t), count: int): list(t)
  match count <= 0 or len(items) == 0 then
    true => {}
    false => {head(items), ..tail(items).take(count - 1)}
  end
end

# Drop first n elements
fn drop(items: list(t), count: int): list(t)
  match count <= 0 then
    true => items
    false => match len(items) then
      0 => {}
      _ => tail(items).drop(count - 1)
    end
  end
end

# Group consecutive elements
fn group-by(items: list(t), key-fn: fn(t) -> k): list(list(t))
  # Implementation would group items by key function result
  {}  # Placeholder
end

List Processing Pipelines

# Complex data processing pipeline
fn process-data(raw-data: list(string)): list(ProcessedRecord)
  raw-data
    .filter(fn(line: string) => len(line) > 0)     # Remove empty lines
    .map(fn(line: string) => parse-csv-line(line))  # Parse each line
    .filter(fn(record: RawRecord) => validate-record(record))  # Validate
    .map(fn(record: RawRecord) => transform-record(record))    # Transform
end

Error Handling Patterns

Option Type Pattern

record Option(t)
  value?: t
  has-value: bool
end

fn some(value: t): Option(t)
  @{
    value => value,
    has-value => true
  }
end

fn none(): Option(t)
  @{
    has-value => false
  }
end

fn map-option(opt: Option(a), transform: fn(a) -> b): Option(b)
  match opt.has-value then
    true => some(transform(opt.value))
    false => none()
  end
end

Result Type Pattern

record Result(t, e)
  value?: t
  error?: e
  is-success: bool
end

fn ok(value: t): Result(t, e)
  @{
    value => value,
    is-success => true
  }
end

fn err(error: e): Result(t, e)
  @{
    error => error,
    is-success => false
  }
end

Module-like Organization

Namespace Patterns

# Group related functions using record-like syntax
math-utils := @{
  add => fn(a: int, b: int): int => a + b,
  multiply => fn(a: int, b: int): int => a * b,
  power => fn(base: int, exp: int): int => 
    match exp then
      0 => 1
      _ => base * math-utils.power(base, exp - 1)
    end
}

# Usage
result := math-utils.add(5, 3)
squared := math-utils.power(4, 2)

Performance Patterns

Tail Recursion

# Tail-recursive factorial
fn factorial-tail(n: int, acc: int = 1): int
  match n <= 1 then
    true => acc
    false => factorial-tail(n - 1, n * acc)
  end
end

# Tail-recursive list processing
fn reverse-tail(items: list(t), acc: list(t) = {}): list(t)
  match len(items) then
    0 => acc
    _ => reverse-tail(tail(items), {head(items), ..acc})
  end
end

Lazy Evaluation Simulation

# Lazy computation using functions
record Lazy(t)
  compute: fn() -> t
  computed?: t
  is-computed: bool
end

fn lazy(computation: fn() -> t): Lazy(t)
  @{
    compute => computation,
    is-computed => false
  }
end

fn force(lazy-val: Lazy(t)): t
  match lazy-val.is-computed then
    true => lazy-val.computed
    false => lazy-val.compute()
  end
end

These advanced features enable sophisticated functional programming patterns while maintaining Epos’s clean syntax and type safety.