Currying and Partial Application: Functions Returning Functions

In previous chapters, we’ve seen that functions are first-class values with types, and even operators like (+) are essentially functions.

We also previewed how applying only one argument to (+) (e.g., (+) 1) created a new function (add1).

This behavior, where providing an argument to a function that expects multiple arguments results in a new function, is a direct consequence of HOF Pattern 1 ('a -> ('b -> 'c)) discussed in Section 3.

Let’s now explore the underlying mechanism that makes this possible: Currying, and its practical outcome, Partial Application.

Revisiting Multi-Argument Functions and Type Signatures

Consider a function for multiplication. In many languages, you might define it to accept two arguments directly:

function multiply(x, y) { return x * y; }
// Expects x and y together

In F#, a similar definition let multiply x y = x * y appears to also take two arguments. However, its type signature, typically int -> int -> int, tells a deeper story.

As discussed in Section 3 regarding type signatures, this is shorthand for int -> (int -> int). This nested structure implies that the function fundamentally operates by taking arguments one at a time.

Currying: The “One Argument at a Time” Mechanism

This “one argument at a time” behavior is achieved through a process called Currying. Many functional languages, including F#, automatically transform functions that appear to take multiple arguments (like let multiply x y = ...) into a sequence of nested functions, each accepting a single argument.

The definition let multiply x y = x * y is essentially convenient syntax for:

let multiply = fun x -> (fun y -> x * y)
// Type: int -> (int -> int)
//    or int -> int -> int

This multiply function now works as follows:

1. It takes the first argument x (an int).
2. It returns a new function (fun y -> x * y). This new function has “remembered” x and has the type int -> int. This step perfectly aligns with HOF Pattern 1 ('a -> ('b -> 'c)), where 'a is the type of x, and 'b -> 'c is the type of the returned function (int -> int).
3. This new function then takes the second argument y (an int).
4. Finally, it performs the calculation x * y and returns the int result.

This transformation, where a function taking multiple arguments is expressed as a chain of functions each taking a single argument and returning the next function in the chain (until the final value is computed), is known as Currying, named after the mathematician Haskell Curry.

Partial Application: The Natural Result of Currying

With currying in place, Partial Application becomes a natural consequence.

• Definition: Partial application is simply the act of calling a function with fewer arguments than it notionally expects.
• In a Curried System: Since all functions fundamentally take one argument at a time, applying the first argument(s) to a curried function is partial application. The result is the intermediate function that’s next in the curried chain. No special syntax is needed.

So, when we wrote let double = (*) 2 in the previous chapter:

let multiplyOperatorAsFunction = (*)
// Type: int -> int -> int
// or    int -> (int -> int)
let double = multiplyOperatorAsFunction 2
// Apply first arg '2'
// 'double' is now
// 'int -> int'
let result = 10 |> double
// result is 20

multiplyOperatorAsFunction 2 (or (*) 2) is a partial application. The (*) function (type int -> (int -> int)) receives its first int argument (2) and returns the intermediate function (type int -> int), which we named double.

Analogy: The Multiplication Table

Let’s visualize this using the familiar multiplication table.

1. The Full Operation: The complete multiplication operation, represented by the binary function (*), needs two numbers (e.g., a row number and a column number) to give you a result from the table. It corresponds to the entire table:

(Requires two inputs, like (*) 3 4 )

2. Fixing One Argument (Partial Application): Now, what happens if we partially apply the multiplication function by fixing the first number, say, to 3?

In F#, we write this as (*) 3. This is like selecting just one row from the table – the “3 times” row:

(Represents the function (*) 3 )

By providing only the first argument (3) to the two-argument function (*), we’ve created a new function.

Let’s call it multiplyBy3.

This new function only needs one more argument (the number for the column) and corresponds to this specific row.

// Create a new function "multiplyBy3"
// by partially applying (*) with 3
let multiplyBy3 = (*) 3

multiplyBy3 is the “3 times table” function; it waits for one more number.

3. Applying the New Function:

Once we have our specialized function multiplyBy3, we can give it the final argument.

For example, applying it to 4 (multiplyBy3 4) is like looking up the 4th column in the 3rd row to find the result 12 :

(Represents applying the function: multiplyBy3 4)

// Now use the new function - it only needs one argument
let result = multiplyBy3 4 // result is 12 (3 * 4)
printfn "3 times 4 is: %d" result

Applying this to our previous examples:

This process of fixing one argument to create a new, simpler function is exactly what we did earlier:

// Create a 'multiply by 2' function from (*)
let double = (*) 2

// Create an 'add 1' function from (+)
let add1 = (+) 1

We created specialized unary functions (double, add1) from general binary functions ((*), (+)) using partial application.

This ability to easily create new, specialized functions from existing ones by partially applying arguments is a common and powerful technique in FP.

Connecting Partial Application to HOF Pattern 1

As demonstrated with the multiplyBy3, double, and add1 examples, partial application takes some initial input (like the number 3 for (*), or 2 for (*), or 1 for (+)) and returns a new function.

This perfectly matches **HOF Pattern 1:

1. Value |> Function = Function

// Create a 'multiply by 2' function from (*)
let double = 2 |> (*)

// Create an 'add 1' function from (+)
let add1 = 1 |> (+)

---

Value |> Function = Value

let result = 5 |> double // 10

let result' = 10 |> add1 // 11

Therefore, partial application is a prime example of Higher-Order Functions in action, specifically illustrating the pattern where functions return other functions. It showcases how treating functions as first-class values allows us to manipulate and create new functions dynamically.

Summary

• Many functional languages employ Currying, a mechanism where functions appearing to take multiple arguments are automatically treated as a sequence of functions each taking a single argument and returning the next function in the chain, until the final result is produced.
• A type signature like T1 -> T2 -> TResult reflects this, being shorthand for T1 -> (T2 -> TResult). This is an instance of HOF Pattern 1.
• Partial Application is the natural outcome of applying arguments to a curried function. Providing fewer arguments than notionally specified results in an intermediate function being returned.
• This mechanism allows for the easy creation of specialized functions (like add1 or double) from more general ones (like the operators (+) or (*)).
• This entire behavior—a function taking an argument and returning a new function—is a prime example of HOF Pattern 1 in action.

It might seem that F#‘s unary function model makes it awkward to pass multiple related pieces of data (like coordinates (x, y)) compared to JavaScript’s multi-argument functions (f(x, y)).

While currying handles functions that logically take multiple independent arguments step-by-step, what if you simply want to pass a single, grouped piece of data containing multiple components?

F# addresses this with Tuples. A tuple, written (a, b) or (a, b, c, ...), groups multiple values into a single, composite value. This is different from a list ([a; b; c]) and is a data structure not present in the same way in JavaScript.

Because a tuple like (x, y) is considered a single value, it can be passed as the one argument to a unary F# function:

// Define a function that takes ONE argument: a tuple of two integers
let addCoordinates (coords: int * int) =
    let (x, y) = coords
    // Deconstruct the tuple inside the function
    x + y

// Call the unary function,
// passing the tuple as the single argument
let result = addCoordinates (3, 4)
// result is 7

Notice that the function call addCoordinates (3, 4) looks syntactically similar to a JavaScript call addCoordinates(3, 4) which might take two separate arguments.

However, in F#, addCoordinates is still a unary function accepting a single tuple value.

This provides a convenient syntax for working with grouped data within the unary function model, offering another example of F#‘s pragmatic and expressive design.