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Go 1.18+ has generics: type parameters on functions and types.

The [T int | float64 | string] is a type constraint — T must be one of these.

Predefined constraints (golang.org/x/exp/constraints):

• Ordered — int, float, string (have <)
• Integer — all int kinds
• Float — float kinds
• Signed / Unsigned — the signed and unsigned int kinds

any is the new alias for interface{} — generic without restriction:

Generic types:

Limits:

• No method type parameters (only the receiver type's params are in scope)
• Constraints can include underlying types: ~int matches int and any type whose underlying is int
• The compiler shares one instantiation per GC shape (all pointer types share one), so it is neither full C++-style monomorphization nor free — binary size and indirection both grow a little

Use sparingly. Most Go code is concrete; reach for generics when:

• You'd otherwise duplicate the same algorithm for 5+ types
• You're writing a generic data structure
• You're writing utility functions like Map/Filter/Reduce

A note on this course's grader

Everything above describes Go 1.18 and later. The sandbox that grades the exercises in this course runs Go 1.13, which predates type parameters: func Filter[T any](...) there is a syntax error, not a missing import. So the exercise below asks for the concrete, pre-generics version of Filter — which is exactly the code that generics exist to stop you writing once per element type. Write the []int one, feel the duplication you would need for []string and []float64, and you will know precisely what a type parameter buys.