One potential benefit to a Map.fetch/3 command that wasn't considered is
that it could help users define more specific error types. There's a
loose convention to use {:ok, val} | :error | {:error, reason} as a
"result" type in Elixir (analogous to Rust's Option and Result or
Haskell's Maybe and Either). Map.fetch/3 conforms to this by returning
{:ok, val} | :error which is very useful!
However, sometimes I want to return a more specific error from Map.fetch
failure. Consider the following case:
states = %{"AL" => "Alabama", "AK" => "Alaska", "AR" => "Arkansas", ... }
Map.fetch(states, abbreviation)
This will return either {:ok, name} or :error. However, for the caller's
convenience, I would return {:ok, name} or {:error, "State not found!"}.
Map.get/2 could be used:
case Map.get(states, abbreviation) do
nil -> {:error, "State not found!"}
name -> {:ok, name}
end
or Map.fetch/2:
case Map.fetch(states, abbreviation) do
{:ok, name} -> {:ok, name}
:error -> {:error, "State not found!"}
end
But I would truly like to have a Map.fetch/3 function where the third
argument is alternate default value (a la Map.get/3)
Map.fetch(states, abbreviation, {:error, "State not found!"})
On 1/26/21 3:53 AM, Paweł Urbanek wrote:
Thanks, I was not aware of that.
On Tuesday, January 26, 2021 at 9:48:23 AM UTC+1 [email protected]
wrote:
Hello,
I think you're looking for `.get_lazy/3` functions.
Best regards,
Jean
Le mardi 26 janvier 2021 à 00:47 -0800, Paweł Urbanek a écrit :
Looks like get does not evaluate the fallback function:
Keyword.get([{:a, 1}], :b, fn () -> 2 + 2 end) =>
#Function<45.97283095/0 in :erl_eval.expr/5>
Maybe fetch!/3 could work like that?
Keyword.fetch!([{:a, 1}], :b, fn () -> 2 + 2 end) => 4
On Tuesday, January 26, 2021 at 9:40:13 AM UTC+1
[email protected] wrote:
Hello,
There is Map.get/3 already, how would this differ?
Best
Adam
On Tue, 26 Jan 2021 at 08:38, Paweł Urbanek
<[email protected]> wrote:
I'd like to propose extending the API for Keyword and Map,
fetch! method:
Map.fetch!(%{a: 1}, :b, 2) => 2
Keyword.fetch!([a: 1], :b, 2) => 2
Third argument represents value which will be returned in case
the requested key is missing. Optionally maybe it could also
accept the function which is executed only in case the key is
missing and it's return value is used.
It would allow a simple way to fallback to default option values.
--
You received this message because you are subscribed to the Google
Groups "elixir-lang-core" group.
To unsubscribe from this group and stop receiving emails from it, send
an email to [email protected]
<mailto:[email protected]>.
To view this discussion on the web visit
https://groups.google.com/d/msgid/elixir-lang-core/eec03b82-3c23-4d56-a930-de1077913782n%40googlegroups.com
<https://groups.google.com/d/msgid/elixir-lang-core/eec03b82-3c23-4d56-a930-de1077913782n%40googlegroups.com?utm_medium=email&utm_source=footer>.
--
You received this message because you are subscribed to the Google Groups
"elixir-lang-core" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To view this discussion on the web visit
https://groups.google.com/d/msgid/elixir-lang-core/4ff59ad2-bda9-3353-494a-9de0d1c05e78%40gmail.com.
