Functional Programming in hica

The article "Functional Programming in hica" offers a comprehensive yet approachable guide to functional programming (FP) concepts, utilizing the hica language as a practical demonstration. It highlights hica's foundational design around FP principles, emphasizing immutability, the omnipresence of expressions, and the treatment of functions as first-class values. The guide is structured to be accessible, assuming only a basic understanding of functions and match expressions.

• Expressions vs. Statements: In hica, nearly everything, including control flow constructs like if and code blocks, is an expression that yields a value, enabling seamless composition.
• Immutability by Default: The language prioritizes immutable data via let bindings, with mutable var declarations explicitly opt-in and locally scoped, enhancing code predictability and testability.
• Pure Functions: Functions are pure by design, guaranteeing consistent outputs for identical inputs and freedom from side effects. The type system, inherited from Koka, tracks and makes side effects visible when they do occur.
• First-Class Functions & Closures: hica treats functions as regular values, allowing them to be assigned, passed as arguments, or returned from other functions. Closures are also explained, demonstrating how functions can capture and retain variables from their creation context.
• Higher-Order List Operations: Essential functions like map, filter, and fold are detailed for transforming, selecting, and reducing lists, alongside flatten and flat_map for handling nested list structures.
• Composition with Pipe Operator (|): The |> operator facilitates chaining function calls, promoting a left-to-right data flow that improves readability and emphasizes transformation sequences. The article also touches on "point-free" style.
• Recursion: Presented as the functional alternative to loops, recursion is illustrated with pattern matching for data structures like lists, alongside an example of mutual recursion.
• Algebraic Data Types (ADTs): hica uses struct for product types (grouping fields) and enum for sum types (representing choices), with the compiler enforcing exhaustiveness checking for enum variants to prevent runtime errors.
• Safe Error Handling (Maybe and Result): The article introduces Maybe (Some/None) for optional values and Result (Ok/Err) for explicit error propagation. It explains combinators like and_then and the ? operator for chaining fallible operations while avoiding nested error handling logic.

The article concludes by underscoring how hica's design inherently encourages functional programming best practices. This leads to code that is not only more testable and reusable but also more naturally composable, reinforcing the practical benefits of the paradigm through clear, well-structured examples.