In Scala, a partially applied function is a function that has some, but not all, of its arguments applied. A partially applied function creates a new function that takes the remaining arguments as input, and returns the result of applying the original function with all of its arguments.
Partially applied functions are useful when you want to reuse a function with some common arguments across multiple calls, but vary the remaining arguments. They can also be used for dependency injection and currying.
In Scala, you can create a partially applied function by fixing some of the arguments of a function using the `_` placeholder syntax. For example, consider the following function that takes two arguments and returns their sum:
scala def add(x: Int, y: Int): Int = x + y
To create a partially applied function that fixes the first argument of `add()`, you can use the `_` placeholder syntax as follows:
scala val add2 = add(2, _: Int)
In this example, `add2` is a partially applied function that takes a single input parameter of type `Int`, and returns the result of adding 2 to that parameter. The placeholder syntax `_` represents the second argument of the `add()` function, which has not yet been specified.
You can now use the partially applied function `add2` to add 2 to any integer value as follows:
scala val result = add2(3) // returns 5
Inthis example, the `add2` function is called with a single input parameter of value `3`, which is passed as the second argument to the `add()` function. The result is `5`, which is the sum of `2` and `3`.
Partially applied functions can also be created using currying, which involves splitting a function with multiple arguments into a chain of functions, each taking a single argument. For example, the `add()` function can be curried as follows:
scala def add(x: Int)(y: Int): Int = x + y
In this version of the `add()` function, the first argument `x` is passed in a separate parameter list from the second argument `y`. This allows you to create a partially applied function by calling the first parameter list with some fixed value, and the second parameter list with a placeholder. For example:
scala val add2 = add(2) _
In this example, `add2` is a partially applied function that takes a single input parameter of type `Int`, and returns the result of adding 2 to that parameter. The underscore `_` is used to represent the second parameter list, which has not yet been specified.
Overall, partially applied functions are a powerful feature of Scala that allow for flexible and reusable code, particularly when combined with higher-order functions and currying.