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Unit Type (F#)

The unit type is a type that indicates the absence of a specific value; the unit type has only a single value, which acts as a placeholder when no other value exists or is needed.

// The value of the unit type.
()

Every F# expression must evaluate to a value. For expressions that do not generate a value that is of interest, the value of type unit is used. The unit type resembles the void type in languages such as C# and C++.

The unit type has a single value, and that value is indicated by the token ().

The value of the unit type is often used in F# programming to hold the place where a value is required by the language syntax, but when no value is needed or desired. An example might be the return value of a printf function. Because the important actions of the printf operation occur in the function, the function does not have to return an actual value. Therefore, the return value is of type unit.

Some constructs expect a unit value. For example, a do binding or any code at the top level of a module is expected to evaluate to a unit value. The compiler reports a warning when a do binding or code at the top level of a module produces a result other than the unit value that is not used, as shown in the following example.

let function1 x y = x + y
// The next line results in a compiler warning.
function1 10 20 
// Changing the code to one of the following eliminates the warning. 
// Use this when you do want the return value. 
let result = function1 10 20
// Use this if you are only calling the function for its side effects, 
// and do not want the return value.
function1 10 20 |> ignore

This warning is a characteristic of functional programming; it does not appear in other .NET programming languages. In a purely functional program, in which functions do not have any side effects, the final return value is the only result of a function call. Therefore, when the result is ignored, it is a possible programming error. Although F# is not a purely functional programming language, it is a good practice to follow functional programming style whenever possible.

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