It can also be done as a case if you are more comfortable with that.
def new(attrs) do
case struct(__MODULE__, attrs) do
%{bytes: nil} = struct ->
%{struct | bytes: random_bytes()}
struct ->
struct
end
end
tomkonidas
2
msimonborg
Another option would be to just build the default opts with Keyword.put_new_lazy/3. Then you create the struct once without needing to update it, and you still only run the random_bytes() generator when necessary. I don’t know how this would compare performance-wise with the with statement plus the update.
def new(attrs) when is_list(attrs) do
attrs = Keyword.put_new_lazy(attrs, :bytes, &random_bytes/0)
struct(__MODULE__, attrs)
end
# optionally if you want to accept both list and map arguments
def new(attrs) when is_map(attrs), do: new(Enum.to_list(attrs))
I always use __MODULE__ just in case I want to change the module name down the line, I don’t have to update anything else in that file.
2
tomkonidas
The thing is attrs is an Enumerable.t(), so it can be a map or a keyword. So that is why i do it after i create the struct.
But yea very valid if you want to have the two functions as you showed
3
tomkonidas
We can even mix and match both solutions:
def new(attrs) do
__MODULE__
|> struct(attrs)
|> maybe_with_random_bytes()
end
defp maybe_with_random_bytes(%{bytes: nil} = struct) do
Map.put(struct, :bytes, random_bytes())
end
defp maybe_with_random_bytes(struct), do: struct
3
cjbottaro
I felt this way too until I divorced the word new from “object instantiation”.
I think the official Elixir documentation even says…
Just use data directly:
%__MODULE__{email: email, bytes: random_bytes()}
If that’s not good enough, make a function:
def new(email) do
%__MODULE__{email: email, bytes: random_bytes()}
end
And finally… metaprogramming (no example… 
).
See the “In other words:” part of this section:
2
al2o3cr
Quick note: 99% of the time when you write struct you really mean struct! because
struct!will raise if required keys from@enforce_keysare omittedstruct!will raise if given keys that aren’t allowed in the struct
Another pattern that can catch bad keys at compile-time: use a struct literal combined with a function to apply defaults. Something like:
defmodule SessionRequest do
defstruct [:email, :random_bytes]
def setup(%__MODULE__{} = session) do
Map.put_new_lazy(session, :bytes, fn -> random_bytes() end)
end
end
# at the callsite:
session_request =
%SessionRequest{
email: some_email
}
|> SessionRequest.setup()
This is a little over-abstracted for the case where SessionRequest only has one field supplied by the user, but would make more sense if there were a dozen:
- the compiler will catch invalid keys in the literal
- if you’ve declared types for the specific keys, Dialyzer may catch errors like assigning a literal string to a key declared to be
integer(). It will NOT catch misconfigured use ofstruct/struct!
1
cjbottaro
I think the post is really about “struct default values that are determined at runtime”…
defmodule Foo
defstruct [:email, bytes: &random_bytes/0]
defp random_bytes do
:erlang.rand_bytes(16)
end
end
Which admittedly would be kinda nice… 
2
ycherniavskyi
Your example does not compile 
(I have fixed some compile errors already):
Erlang/OTP 25 [erts-13.0.2] [source] [64-bit] [smp:10:10] [ds:10:10:10] [async-threads:1] [jit] [dtrace]
Interactive Elixir (1.13.4) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> defmodule Foo do
...(1)> defstruct [:email, bytes: &random_bytes/0]
...(1)>
...(1)> defp random_bytes do
...(1)> :rand.uniform(100)
...(1)> end
...(1)> end
** (CompileError) iex:2: undefined function random_bytes/0 (there is no such import)
(elixir 1.13.4) expanding macro: Kernel.defstruct/1
iex(1)>
0
cjbottaro
I was trying to say “this is an example of what OP was asking for”. It doesn’t work, but it would be nice if it did.
0
sodapopcan
I personally don’t think this would be nice as it would mean coming across a wild %Foo{} means there could be side-effects. I’m perfectly happy with status quo where %Foo{} means “static thing I’m allow to manipulate” and Foo.new() means “I’m special, please let me construct myself!”.
2