Blog / Go Language Spec

Built-in functions

August 28, 2024

We’ve just finished up the rather long section on different types of statements. There’s only a few more sections in the spec, before we finish this series.

And today we start looking at Built-in functions!

Built-in functions

Built-in functions are predeclared. They are called like any other function but some of them accept a type instead of an expression as the first argument.

The built-in functions do not have standard Go types, so they can only appear in call expressions; they cannot be used as function values.

If you’ve ever wondered why there are some seemingly arbitrary limitations on how built-in functions can be used, this answers it.

myAppend := append

will fail to compile, with the following error:

./prog.go:6:14: append (built-in) must be called

The reason this isn’t allowed is that built-in functions simply don’t have a defined type, therefore myAppend, in the example, has an undefined type. And there’s simply no way to handle that.

If you want to reference a built-in as a variable like this, you must wrap it in your own function:

myAppend := func(a []string, b ...string) []string {
  return append(a, b...)
}

Quotes from The Go Programming Language Specification Language version go1.23 (June 13, 2024)

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