Original source of discussion: This topic on the Pragmatic Programmers’ Functional Web Development with Elixir, OTP, and Phoenix forum.

In the (beta version of the) book, they update structs using Map.put/3. I’ve found myself doing the same in my own code. @hubertlepicki astutely observed that this opens up the opportunity for mistyped field names when updating your structs: This is totally allowed, the result of Map.put/3 will then be a map (instead of an instance of your struct).

Example:

iex> defmodule Foo do
iex>   defstruct bar: 1 , baz: 2
iex> end

iex> foo = %Foo{}
iex> foo |> Map.put(:qux, 4)
%{__struct__: Foo, bar: 1, baz: 2, qux: 4}  # <- note that it is printed as a map.

The other two methods known to me to update structs are:

1. The special struct update syntax, %Foo{oldfoo | bar: 4}. This will result in a compile-time error when wrong field names are used. Drawback: It cannot be piped.
2. put_in/2: newfoo = put_in(foo.bar, 4). This will also result in a compile-time error when a wrong field name is used. Drawback: It, too, cannot be piped.

put_in/3 (as in: newfoo = put_in(foo, [:bar], 4) would be able to be piped, but it is not available for structs; it only works on things that implement the Access protocol.

So what is the best way to update structs in practice? Am I missing any method in this list? Would it be a good idea to have a version of put_in that works on structs and is pipeable?