Functions That Accept Other Functions as Parameters

Functions can accept other functions as parameters. The best example of a function that accepts another function as an argument can be found in the sort package. We can provide the sort.Slice() function with another function as an argument that specifies the way sorting is implemented.

Signature of sort.Slice()

The signature of sort.Slice() is func Slice(slice interface{}, less func(i, j int) bool). This means the following:

• The sort.Slice() function does not return any data—the slice variable is modified inside sort.Slice().

• The sort.Slice() function requires two arguments, a slice of type interface{} and another function.

• The function parameter of sort.Slice() is named less and should have the func(i, j int) bool signature. While there is no need for us to name the anonymous function that we pass to sort.Slice(), the name less is required because Go requires all function parameters to have a name.

• The i and j parameters of less are indexes of the slice parameter.

The sort.SliceIsSorted() function

Similarly, there is another function in the sort package named sort.SliceIsSorted() that is defined as func SliceIsSorted(slice interface{}, less func(i, j int) bool) bool. This function returns a bool value and checks whether the slice parameter is sorted according to the rules of the second parameter, which is a function.

Note: We are not obliged to use an anonymous function in either sort.Slice() or sort.SliceIsSorted(). We can define a regular function with the required signature and use that. However, using an anonymous function is more convenient.

Coding example

The use of both sort.Slice() and sort.SliceIsSorted() is illustrated in the Go program that follows—the name of the source file is sorting.go: