Go Generics

5 minute read

This post was put together after reading through Type Parameters Proposal and some experimentation.

The generics feature in this guide requires go 1.18 or higher.

Generic Functions

A function I have written several times for various packages is a Has function:

func Has(items []string, target string) bool {
	for _, item := range items {
		if item == target {
			return true
		}
	}
	return false
}

Without generics, we need to rewrite this function for every type we need to search for; we may end up with functions like:

func HasStr(items []string, target string) bool 
func HasInt(items []int, target int) bool 
func HasF64(items []float64, target float64) bool 

With generics, we can define the function once:

func Has[T comparable](items []T, target T) bool {
	for _, item := range items {
		if item == target {
			return true
		}
	}
	return false
}

This can be called with any type T that enforces the constraint comparable. The comparable constraint requires that:

T == T is implemented
T != T is implemented

We can use Has for T = string because string is a comparable:

list := []string{"a", "b", "c", "d", "e"}
fmt.Printf("%v: %v\n", "a", Has(list, "a")) // a: true
fmt.Printf("%v: %v\n", "h", Has(list, "h")) // h: false

Type Inference

In the above example, we used inferrence to determine what type T should use. This was easy for the compiler to deduce because our input arguments provide the type.

Determining T implicitly isn’t always possible.

For example, I might want a convenient way to get a nil pointer to a specific type, so I write:

func NilPtr[T any]() *T {
	return nil
}

In this case, type inference doesn’t work, we need to explicitly name the type we will make nil:

value := NilPtr[int]()      // OK
value := NilPtr[struct{}]() // OK
value := NilPtr()           // Compile Error

The last example gives us the compile error: "cannot infer T"

Multiple type parameters

A type parameter list can have multiple type parameters:

func doSomething[A, B any, C ~int](a1 A, a2 A, b B, c C)

In the above example:

a1 and a2 can be any type, but must be the same type
b can be any type and can be a different from a1 and a2
c must have an underlying type of int (more on this later)

Generic Types

Generics can be applied to types. Here are two flavors we will probably see often:

type MyList[T any] []T

type MyStruct[T any] struct{
	a  T
	// ...
}

To use these generic types, we must specify the type. For example:

f32List := MyList[float32]{0.0, 1.5} // OK
f32List := MyList{0.0, 1.5}          // Compile Error

Example: Generic Set

A common source of repeated code is writing sets in go as a map[MyType]struct{}.

With generic types I can define:

type Set[T any] struct{ m map[T]struct{} }

And the set can have methods:

func NewSet[T comparable](values ...T) Set[T] {
	m := make(map[T]struct{}, len(values))
	for _, v := range values {
		m[v] = struct{}{}
	}
	return Set[T]{m: m}
}

func (s *Set[T]) Has(item T) bool {
	_, has := s.m[item]
	return has
}

// ... And more!

Which can be used like:

f64Set := NewSet(1.2, 3.5, 7.75)
fmt.Printf("%v: %v \n", 1.2, f64Set.Has(1.2)) // 1.2: true
fmt.Printf("%v: %v \n", 5.6, f64Set.Has(1.2)) // 5.6: false

One gotcha in go 1.18 (and maybe later versions? Who knows…) is that a member function cannot have its own generic types.

For example, we cannot define:

func (s *Set[T]) Length[N: ~int|~uint]() N {
	return N(len(s.m))
}

This will result in the compile error: "method must have no type parameters".

Constraints

A constraint is applied to a generic type argument to enforce what types can be used.

If we wanted to write a Has fuction, we can take several approaches, helped by constraints.

Constraint: `any`

any allows any type.

If we use the any constraint to write Has we get:

func Has[T any](items []T, v T, equal func(a,b T) bool) bool

any is very vague, so we provide an equal function to help Has check equality.

Constraint: `comparable`

comparable restricts a type to only those which provide == and != operators.

func Has[T comparable](items []T, v T) bool

Constraint: Interface

Interfaces can be used as a constraint. If we have an interface:

type Cmp[T any] interface {
	Equal(other *T) bool
}

Then we can write:

func Has[T Cmp[T]](items []T, v T) bool

This looks a a bit funky, because the interface also takes a type argument.

A working type which can be used with Has[T Cmp[T]] is:

type MyType struct{ a int }

func (t *MyType) Equal(other *MyType) bool {
	return t.a == other.a
}

Another (less complicated) example would be using the fmt.Stringer interface as a constraint:

func Format[T fmt.Stringer](item T) string

Constraint: Type Sets

Type sets define potential candidate types.

// Can only use int - kind of pointless.
func Has[T int](items []T, v T) bool

// Can only use int or uint
func Has[T int | uint](items []T, v T) bool

// Can only use int or types with string as the
// underlying type.
func Has[T int | ~string](items []T, v T) bool

On underlying types:

The tilde '~' symbol denotes underlying type
- In this case we have ~string which means any type with the underlying type string
type myString string has the underlying type string
type myString struct{string} has the underlying type struct{string} not string
- attempting to use struct{string} as ~string results in compile error: "myString does not implement ~string"

On unions:

The bar '|' symbol denotes a union
[T int | uint] means any type which meets the contraint int or uint may be used
- Only operations which both int and uint share can be used on T
Unions can be applied to any constraint
- [T io.Writer | string] accepts anything which provides io.Writer or a string
- T cannot use the io.Writer Write(p []byte) (n int, err error) function, because T is constrained by string which does not provide a Write function

Combining constraints

We can combine constraints in an interface:

type Currency interface {
	~uint64
	ISO4127Code() string
	Decimals() int
}

In this case, Currency specifies an interface which requires:

The underlying type must be uint64
ISO4127Code and Decimals must be implemented

The following type NZD meets these requirements:

type NZD uint64

func (c NZD) ISO4127Code() string { return "NZD" }
func (c NZD) Decimals() int       { return 2 }

Now, NZD can be used in a generic function like CurrencyString:

func CurrencyString[T Currency](c T) string {

We can call CurrencyString like so:

fmt.Println(CurrencyString(NZD(500))) // 5.00 NZD

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