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40 lines
1.2 KiB
Markdown
40 lines
1.2 KiB
Markdown
# Requiring Keys For Structs
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When defining a
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[`struct`](http://elixir-lang.org/getting-started/structs.html) in Elixir,
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we may want to ensure that certain values are provided. We can require that
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certain keys are used in the creation of a struct with the
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[`@enforce_keys`](https://hexdocs.pm/elixir/Kernel.html#defstruct/1)
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attribute. Here is an example of a `Name` struct.
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```elixir
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defmodule Name do
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@enforce_keys [:first, :last]
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defstruct [:first, :middle, :last]
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end
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```
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With this defined, we can create a struct that uses all of the keys.
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```elixir
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> jack = %Name{first: "Jack", middle: "Francis", last: "Donaghy"}
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%Name{first: "Jack", last: "Donaghy", middle: "Francis"}
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```
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We can also ignore `:middle` and just provide the required keys.
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```elixir
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> liz = %Name{first: "Liz", last: "Lemon"}
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%Name{first: "Liz", last: "Lemon", middle: nil}
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```
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We cannot, however, omit any of the keys specified in `@enforce_keys`. If we
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do omit any of them, Elixir will raise an error.
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``` elixir
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> tracy = %Name{first: "Tracy"}
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** (ArgumentError) the following keys must also be given when building struct Name: [:last]
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expanding struct: Name.__struct__/1
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iex:6: (file)
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```
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