36 Go interview questions on generics, with detailed answers and code.
interface{}/any parameters did not?Before Go 1.18 you had two choices for reusable code: write it once per type (or with go generate ), or accept interface{} and give up static typing. With any the…
func F(s fmt.Stringer) and func F[T fmt.Stringer](s T)? When does it matter?For a single argument they behave almost the same. The differences show up in three places: Same type across parameters and results: func Pick[T fmt.Stringer](a, b T) T…
[P *C] parsing gotcha.A constraint is an interface that limits which type arguments are allowed. Since Go 1.18 an interface defines a type set . It can list methods, type terms ( int | string…
~ (tilde) token mean in a constraint? What happens without it?~T means "every type whose underlying type is T". Without the tilde, only the exact type T itself matches. Named types such as type Celsius float64 or time.Duration…
Every interface denotes a type set : A method element gives the set of all types that have that method. A union A | B gives the union of the two sets. Elements on…
comparable? What changed in Go 1.20, and what does this program print?comparable is a predeclared constraint for types that support == and != . It covers booleans, numbers, strings, pointers, channels, arrays and structs of comparable…
An operation is allowed on a value of type T only if it is valid for every type in T's type set, and the spec explicitly permits it: Operators ( + , < , == ) work if…
Go infers type arguments in two ways: Function argument inference unifies parameter types with argument types. Constraint type inference derives a parameter from another…
No. A generic function is not a value until it is instantiated . There are no "generic function values", so f := Map fails with cannot use generic function Map without…
T?You cannot write return nil , and you cannot write T{} for an arbitrary T . nil is not valid for int, and composite literals need a core type. Use one of these idioms:…
func IsNil[T any](v T) bool { return any(v) == nil } func main() { var p *int var err error fmt.Println(IsNil(p), IsNil[any](nil), IsNil(err), IsNil[error](nil)) }…
type MyInt int func Describe[T any](v T) string { switch any(v).(type) { case int: return "int" case fmt.Stringer: return "Stringer" default: return "other" } } func…
Methods can use their receiver type's parameters, but a method cannot declare new type parameters of its own. func (l List[T]) Map[U any](f func(T) U) List[U] is…
func (s Stack[int]) Sum() int?The receiver must list the type's parameters by name, and you may rename them. You cannot repeat or tighten the constraints in the receiver, and you cannot specialize a…
The main restrictions, several of which exist to keep method sets statically known: No embedding a type parameter: type Wrapper[T any] struct{ T } fails with "embedded…
PT interface{ *T; M() }). Why is it needed?Problem: you want to create values of type T inside a generic function and call a method that has a pointer receiver , such as UnmarshalText or Set . T itself does not…
An interface can itself have type parameters. That lets a constraint refer to the type being constrained, similar to F-bounded polymorphism or Java's Comparable<T> . It…
slices.Clone have the signature func Clone[S ~[]E, E any](s S) S instead of func Clone[E any](s []E) []E?The goal is to preserve the caller's named slice type . With []E in and []E out, a named type like type IDs []int comes back as a plain []int , and its methods are lost.…
Go uses a hybrid of the two classic strategies. Full monomorphization (C++, Rust) makes one copy per type. Full boxing (Java) makes one copy for everything. Go instead…
Interviewers want judgment, not enthusiasm. Guidance, broadly following Ian Lance Taylor's "When To Use Generics": When you only call methods: if the function just calls…
type Stack[T any] struct{ items []T } func (s *Stack[T]) Push(v T) { s.items = append(s.items, v) } func (s *Stack[T]) Pop() (T, bool) { var zero T if len(s.items) == 0…
cmp.Ordered with an in-order iterator.type Tree[K cmp.Ordered, V any] struct { root *node[K, V] size int } type node[K cmp.Ordered, V any] struct { key K val V left, right *node[K, V] } func (t *Tree[K, V])…
Set[T] with union, intersection and iteration.type Set[T comparable] map[T]struct{} func SetOf[T comparable](vs ...T) Set[T] { s := make(Set[T], len(vs)) for _, v := range vs { s[v] = struct{}{} } return s } func (s…
GetOrLoad method. What concurrency pitfalls exist?type Cache[K comparable, V any] struct { mu sync.Mutex items map[K]*entry[V] } type entry[V any] struct { once sync.Once val V err error } func NewCache[K comparable, V…
Map, Filter and Reduce for slices. Why aren't they in the standard library's slices package?func Map[T, U any](xs []T, f func(T) U) []U { out := make([]U, len(xs)) for i, x := range xs { out[i] = f(x) } return out } func Filter[S ~[]E, E any](xs S, keep func(E)…
golang.org/x/exp/constraints, how does it relate to cmp.Ordered, and how do you define your own numeric constraint?x/exp/constraints was the experimental home for common constraints: Signed , Unsigned , Integer , Float , Complex and Ordered . Only Ordered moved into the standard…
cmp package: Compare, Less, Or. How do you do a multi-key sort, and how is NaN handled?cmp (Go 1.21) provides Ordered , Compare(a, b) int (returns -1, 0 or +1) and Less(a, b) bool . Go 1.22 added Or(vals ...T) T , which returns the first non-zero argument…
slices and maps should a senior Go developer know, and what are their gotchas?slices : Sort , SortFunc , BinarySearch , Index , Contains , Insert , Delete , DeleteFunc , Compact , Equal , Clone , Grow , Reverse , Max , Min . Iterator helpers (Go…
iter.Seq)? Write lazy Filter and Map adapters and explain the yield contract.iter.Seq[V] is a generic function type, func(yield func(V) bool) , and iter.Seq2[K, V] is the two-value form. for v := range seq calls seq with a compiler-generated…
iter.Pull, and when do you need it? Implement a generic Zip.Range-over-func iterators are push iterators: the sequence drives the loop. That makes it impossible to step through two sequences in lockstep with a single for .…
A type definition , type Set[T comparable] map[T]struct{} , creates a new, distinct named type that can have methods. A type alias , type A = B , is just another name…
Box[Dog] be used as a Box[Animal], or a Box[int] converted to a Box[MyInt]?No. Go generics are invariant . Each instantiation is a distinct named type, and there is no subtyping between instantiations, even when the type arguments are related…
func Nest[T any](n int) { if n > 0 { Nest[[]T](n - 1) } } func main() { Nest[int](3) } Result: compile error instantiation cycle: T instantiated as []T . Go instantiates…
%T and reflection on generic types)type Pair[K comparable, V any] struct { Key K Val V } type Box[T any] struct{ v T } type MyInt int func main() { fmt.Printf("%T\n", Pair[string, int]{})…
Merge for channels. What are the concurrency pitfalls?func Merge[T any](ctx context.Context, chans ...<-chan T) <-chan T { out := make(chan T) var wg sync.WaitGroup for _, c := range chans { wg.Go(func() { // Go 1.25;…
Ptr[T] helper with Go 1.26 new(expr).Optional fields in API structs (JSON, protobuf, AWS SDK) are often pointers, and &42 or &"x" is not valid Go. Before generics every library shipped aws.String ,…