WestKeys
When to use a variable vs simple value returning function
Why would we use
def table do
[{?a..?m, :"foo@computer-name"}, {?n..?z, :"bar@computer-name"}]
end
Rather than
table = [{?a..?m, :"foo@computer-name"}, {?n..?z, :"bar@computer-name"}]
In this example
defmodule KV.Router do
@doc """
Dispatch the given `mod`, `fun`, `args` request
to the appropriate node based on the `bucket`.
"""
def route(bucket, mod, fun, args) do
# Get the first byte of the binary
first = :binary.first(bucket)
# Try to find an entry in the table() or raise
entry =
Enum.find(table(), fn {enum, _node} ->
first in enum
end) || no_entry_error(bucket)
# If the entry node is the current node
if elem(entry, 1) == node() do
apply(mod, fun, args)
else
{KV.RouterTasks, elem(entry, 1)}
|> Task.Supervisor.async(KV.Router, :route, [bucket, mod, fun, args])
|> Task.await()
end
end
defp no_entry_error(bucket) do
raise "could not find entry for #{inspect bucket} in table #{inspect table()}"
end
@doc """
The routing table.
"""
def table do
# Replace computer-name with your local machine name
[{?a..?m, :"foo@computer-name"}, {?n..?z, :"bar@computer-name"}]
end
end
Taken from docs
Marked As Solved
dimitarvp
I use functions in these situations to allow myself the liberty to replace a hard-coded constant with a value fetched from a configuration provider – be it plain normal OS environment variables, various secret managers like those found in the Kubernetes ecosystem, AWS-specific secret key-value stores etc.
But, if you are very sure you won’t need to fetch the value from the outside world then you might be better off served by using a module attribute because that makes the code a bit more readable and explicit:
defmodule KV.Router do
@routing_table [{?a..?m, :"foo@computer-name"}, {?n..?z, :"bar@computer-name"}]
def route(bucket, mod, fun, args) do
# ...
@routing_table |> use_it_here()
# ...
end
end
Putting constant values inside your code is a bad idea no matter the programming language. More often than not, you’ll find yourself needing to modify it down the line and then you’ll have trouble finding where did you put it exactly (trust me, we forget where we put stuff all the time). In Elixir, using module attributes clearly expresses your intent as “this is a constant value that I will use one or more times in the code of the module it’s defined in”.
Also Liked
LostKobrakai
There’s :math.pi() in erlang.
dimitarvp
Well, look at your route function above. The table() call blends in perfectly with everything else. Now turn that into an inline variable. Imagine you work actively on something else for 3 months and then come back to this code with the requirement to make that table configurable. Would you immediately remember where you put the hard-coded variable?
Yeah, that’s a bit better, and closer to what I showed you. It’s a good compromise.
davearonson
To clarify what I think you mean, inline inside the code with no name to them, such as c = 6.28318530718 * r.
As opposed to putting up top @PI 3.14159265359 (dunno offhand if Elixir provides this but anyway, it could be considered “putting a constant value inside your code”, as it’s in the module) and in the code, c = 2 * @PI * r, or better yet of course circumference = 2 * @PI * radius. That would be perfectly cromulent, in fact generally the recommended way to go.
Would that be a fair interpretation of what you’re trying to say?
dimitarvp
Yes, that’s what I am saying. Lack of description of what the inline value is supposed to be is a very common problem in most codebases I ever reviewed and participated in.
davearonson
Ah, yup. “Magic Numbers”, some folks call it.
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