I would like to have a struct in a module without default values for the fields, but I would like each field to be limited to a specific type. Is there a way to define and enforce this in Elixir?

You can define a typespec for your struct but you cannot enforce it. What you can do in your module is to use a constructor-like function: def new(props) do ... validate the values struct(__MODULE__, props) end (new is not a special name)

I hesitated because if you validate the values beforehand you should not have to use it, but yes it may be safer. @stevensonmt the difference is that struct!/2 will raise if the given props have extra properties or if keys declared with @enforce_keys are not defined.

Thank you! If using struct!/2 it may raise an exception at runtime and crash, but if using the suggested new/1 constructor you can handle the exceptions gracefully, is that right?

No because as I’ve said, new is not special, it is just a function that you have to define (you could call it create or anything else), and in that function (see my snippet) you call either struct/2 or struct!/2. So it is up to you if you allow to raise.

Oh, I see, struct vs struct! in the constructor function. Thanks!

AFAIK using either struct or struct! doesn’t let Dialyzer “see” what’s happening, and that’s what’s doing the type-enforcement when you use something like typed_struct. The only difference between the two is that struct silently ignores keys it doesn’t understand, while struct! raises. Writing a li…

You can always reduce the opts into your struct however you want. You can validate, rename, transform, do whatever. A simple example where opts of the wrong type are simply rejected def new(opts \\ []) do Enum.reduce(opts, %__MODULE__{}, &put_opt/2) end defp put_opt(opt, acc) defp pu…

That’s a great tip. Can I ask what the first defp put_opt(opt, acc) without further definition is for?

Oh sorry it’s a personal preference when I create a function with multiple clauses to have that. In my opinion it makes it clearer what the args should be, often helps with docs (for public functions), and gives me somewhere to put @doc, @spec, etc. It’s only really required when you have multiple c…

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Defining types for struct fields

Questions & Help Questions

hauleth October 14, 2020, 3:49pm 3

I think that in this case struct!/2 (with bang) will make more sense.