02-syntax.md

Syntax

The basic syntax for writing generic code can be explained by taking a look at an example from the previous page:

// Sum returns the sum of the provided arguments.
func Sum(args ...int) int {
	var sum int
	for i := 0; i < len(args); i++ {
		sum += args[i]
	}
	return sum
}

The above function:

• takes zero to many int arguments
• returns an int value

In fact, there is no type a part of the function signature or body that is not int. So one way to rewrite the above function generically would be to let T represent int (Go playground):

// Sum returns the sum of the provided arguments.
func Sum[T int](args ...T) T {
	var sum T
	for i := 0; i < len(args); i++ {
		sum += args[i]
	}
	return sum
}

The above function's name is suceeded by [T int], the basic form of the syntax for expressing generics in Go:

• the [] brackets are always used to define the generics
• the most basic pattern is [<ID> <CONSTRAINT>] where:
  • <ID>: is the symbol used to represent the generic type
  • <CONSTRAINT>: is the constraint that indicates which concrete types can be used

In the above example the generic type T is constrainted to int, which means it is not possible to call Sum[T int](...T) T with a list of int64 values (Golang playground):

func main() {
	fmt.Println(Sum([]int64{1, 2, 3}...))
}

Trying to run the above example will produce the following compiler error:

./prog.go:17:17: int64 does not implement int

The next page will review how to rewrite the above function so that more than one type can satisfy its constraints.

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Next: Constraints