Currying in Programming Languages: Beyond Laziness

Is technical prowess in currying limited to lazy languages? Does the essence of currying require a specific evaluation strategy or merely functional programming capabilities? This article explores the relationship between currying and laziness in programming languages, demonstrating that while a sophisticated lexical scoping and function passing mechanism is indeed essential, laziness is not inherently required for a language to support currying.

Understanding Currying

Currying is a powerful technique in functional programming where a function with multiple arguments is transformed into a series of functions that each take a single argument. This transformation is beneficial because it allows for more flexible and reusable code, as well as the creation of higher-order functions. For instance, a function that takes two arguments can be curried to produce a sequence of functions: one that takes the first argument and returns another function that requires the second argument.

Currying in Lazy and Eager Languages

The common misconception that currying requires laziness is often derived from the famous lazy language, Haskell. However, this notion does misrepresent the core principles of currying. Currying is fundamentally about function transformation and decomposition, not about the evaluation strategy of a language.

Haskell: A Showcase of Currying

Haskell is renowned for its syntactic support for currying, making it a popular choice for functional programming tasks. However, the technique of currying can be implemented in a wide range of languages, depending on their support for closures and higher-order functions. While Haskell’s built-in support simplifies the syntax, any language that supports passing functions as arguments and returning them as results can achieve similar results with manual implementation.

Examples of Supporting Languages

JavaScript, Python, Scala—none of these languages rely on laziness to implement currying. They have unique features or libraries that enable currying.

JavaScript

JavaScript can curried using closures. A common technique involves defining a function that returns a new function with each parameter. This nested function construction is a form of currying:

code
function add(x) {
  return function(y) {
    return x   y;
  };
}
let add5  add(5);
console.log(add5(10)); // Output: 15
/code

Python

In Python, you can curried by utilizing or by manually defining nested functions:

code
from functools import partial
def add(x, y):
  return x   y
partial_add  partial(add, 5)
print(partial_add(10))  # Output: 15
# Manually defined
def add(x):
  return lambda y: x   y
def add5  add(5)
print(add5(10))  # Output: 15
/code

Scala

Scala natively supports currying in its syntax. Consider the following example:

code
def add(x: Int, y: Int): Int  x   y
val add5  (5)
println(add5(10))  // Output: 15
/code

The Role of Laziness

While Haskell, a lazy language, provides elegant syntax for currying, it is not a requirement for support. Many eager languages can implement currying, albeit with more manual effort.

Language Considerations

The implementation of currying varies across different languages. In some languages, such as OCaml and Standard ML, the feature is syntactically enabled. In others, such as Reason, F#, Elm, and PureScript, manual implementation is used. Even languages like Curry have unique characteristics, making them somewhat esoteric in their approach to currying.

OCaml: Named Currying

OCaml, an ML-family language, provides a unique feature called named currying. This allows for more expressive code in functional programming.

Syntactic Sugar vs. Functionality

Haskell’s direct currying is more about syntactic sugar rather than requiring laziness. The language’s ability to transform a function with multiple parameters into a series of functions that accept one parameter each is a testament to its robust support for higher-order functions. Similar functionality can be achieved in other languages, though often with more manual steps.

Final Thoughts

While laziness introduces certain benefits and simplifications for functional programming concepts, it is not essential for implementing currying. Language designers need to focus on providing the necessary lexical scoping and closures to enable currying. With the right support, any language that supports functions as first-class citizens can curried, simplifying the process and making function composition more streamlined.