Updating structs: Map.put vs %Foo{oldfoo | new: value} vs put_in

Qqwy

TypeCheck Core Team

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:

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.
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?

31 comments

#maps #structs #pipeline #access

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2020-07-04 06:01:18 UTC

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NobbZ

The special Form %name{} pipes pretty well I think, you need to wrap in an anonymous funtion though):

iex(1)> defmodule S do
...(1)>   defstruct f: 0
...(1)> end
{:module, S, <<...>>, %S{f: 0}}
iex(2)> s = %S{}
%S{f: 0}
iex(3)> 1 |> (&%S{s | f: &1}).()
%S{f: 1}
iex(4)> s |> (&%S{&1 | f: &1.f + 5}).()
%S{f: 5}

Post #1

hubertlepicki

Yes, I raised the issue because I did waste some time on a stupid typo. Generally when I use struct, I want to have only defined fields. Otherwise I’d go with a Map.

But if you do use Map.put/3 on a struct, it works, yet not properly since I mistyped/used wrong key.

I agree that not being able to do it in pipe is not ideal. I found myself wrapping the update syntax in a tiny function/anonymous or othwerwise that does that for the very reason. I don’t know, maybe there is better way somehow so I’m listening too.

Post #2

hubertlepicki

My eyes hurt. I do the same, but I die a bit every time I have to write it.

Post #3

NobbZ

In this post it was just for demonstrational purpose, usually I do it in a def(p)-ed function.

Post #4

michalmuskala

Erlang has :maps.update/3, which is basically the %{x | key => value} syntax as a function. I think having something similar in elixir would be beneficial. The biggest problem is the name.

Post #5

hubertlepicki

How about something obvious like:

Struct.update/3 or Struct.put/3

Post #6

wmnnd

You could also use Map.update!/3 which will pipe nicely and raise if the given key doesn’t exist, though not at compile-time.

Post #7

michalmuskala

We don’t have a Struct module - having it for just one-two functions feels wired.

Post #8

hubertlepicki

I can accept @wmnnd’s solution of using Map.update!/3 however.

Post #9

michalmuskala

Yes, Map.update! is fine - if only you didn’t have to create a useless anonymous function. It’s awesome in many cases that Map.update! accepts a function, but not for this case.

Post #10

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