In Go, why isn't append() implemented as a method, but as a function?

Question

In Python, lists offer an append() method which can be called using standard Python method syntax; for example:

>>> my_list = []
>>> my_list.append('a', 'b', 'c')
>>> my_list
['a', 'b', 'c']

This syntax is intuitive to me, and this pattern is common in list implementations across many OO languages.

Lately, I've been working more closely with Go, and I'm trying to get a better understanding of some of the design choices made in the language. Most notably, why is append() defined as a regular function (taking the slice to append to as the first argument, and returning the newly-constructed slice), rather than a method on the slice type?

I'm aware that Go doesn't have a self/this construct, but the standard library takes some liberties with how its functions are defined with respect to the rest of the language; for example, make() takes a type as a first argument, and this is, as far as I know, not possible when defining a new function.

Is it just a nod to C's pattern of passing references to variables, rather than returning new variables?

Answer 1

The Go FAQ has a related question:

Why is len a function and not a method?

We debated this issue but decided implementing len and friends as functions was fine in practice and didn't complicate questions about the interface (in the Go type sense) of basic types.

len would seem to be in the same boat as append. If they were methods, they should affect whether or not the builtin types fit various interfaces. This made adding them as methods somewhat more complicated then just having special builtins for them. They decided it didn't matter in practice, so the extra complexity wasn't worth it.