If structs are just maps underneath then how does pattern matching to ensure struct type work? For example: defmodule Attendee do defstruct name: "", paid: false, over_18: true def may_attend_after_party(attendee = %Attendee{}) do attendee.paid && attendee.over_18 end def…

Structs have a magic hidden key __struct__ which gets implicitly matched on when you prepend the struct module in between % and {. The default key/values are not filled in for matches, just the struct key.

Your example code omits the definition of Attendee.may_attend_after_party_without_struct/1; I tried def may_attend_after_party_without_struct(attendee = %{}) do and it worked, so you’ll need to provide more information about what behavior you’re expecting.

[image] d4mr: %Attendee{} evaluates to %Attendee{name: "", over_18: true, paid: false}, and if structs are just maps underneath, then shouldn’t the behaviour remain similar? In fact they are! :smiling_imp: iex> Map.put(%{}, :__struct__, Attendee) %{__struct__: Attendee} iex> %Attende…

[image] d4mr: then shouldn’t the behaviour remain similar I’m not certain, but i think the way you pattern match in your function “without struct” is not what you intend to do. What you have is: def may_attend_after_party_without_struct(attendee = %{name: "", paid: false, over_18: true}) do …

I understand your pattern matching snippet, and in fact that is what I would expect to work. However, because structs are matched in this manner: def may_attend_after_party(attendee = %Attendee{}) do attendee.paid && attendee.over_18 end and because %Attendee{} evaluates to %Attendee{name: ""…

[image] d4mr: and because %Attendee{} evaluates to %Attendee{name: "", over_18: true, paid: false} Do you mean those are the default values, if you create an empty Attendee struct? Because as with maps - at least i think - the pattern matching for “apparently empty stuff” is maybe not instant…

[image] d4mr: and because %Attendee{} evaluates to %Attendee{name: "", over_18: true, paid: false} I expected the other behaviour to work. “Evaluates to” is the gotcha here, the arguments of a function are always a “match context” which has special properties - this: def some_fun(whole_arg =…

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How does pattern matching with structs work?

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d4mr December 27, 2020, 4:43pm 3

My bad, updated the example