I have implemented like this
defmodule KeyFoundError do
defexception [:key, :term]
def message(%__MODULE__{key: key, term: term}) do
"key #{inspect(key)} found in: #{inspect(term)}"
end
end
defmodule MyMap do
def put_new!(map, key, value) do
if Map.has_key?(map, key) do
raise KeyFoundError, key: key, term: map
else
Map.put(map, key, value)
end
end
end
and use it to ensure that an algorithm building a map with a non-predefined
set of keys fulfills the requirement to write each key only once, which if
it is not the case may be a signal that something is wrong with the
algorithm.
I agree that this is perhaps only a need in more complex, and thus, less
common algorithms, like in my use case, where the keys come from an
external data source, and I want to be sure that this is not sending the
same key duplicated.
Mário
segunda-feira, 29 de Julho de 2019 às 11:27:07 UTC+1, José Valim escreveu:
>
> > 1. put_new and put_new! pair behaves differently from !-paired
> functions, I’m not sure if it’s good thing to break expectations in that
> case.
>
> I believe the concern here is that while all functions raise when the key
> does not exist, the proposed put_new! raises when the key exists. It is how
> put_new! should behave... but it is a different behaviour than the one that
> already exists, highlighted by the fact a new exception would have to be
> added.
>
> Ben commented on the second point already.
>
> Also, your proposal does not include any code examples or snippets on how
> this pattern would be used. I personally can't recall a need for put_new!,
> especially because *the best use of "static maps" is by defining all keys
> upfront and then updating/replacing them*.
>
> *José Valim*
> www.plataformatec.com.br
> Skype: jv.ptec
> Founder and Director of R&D
>
>
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