Elixir: Using variables in case statement - case

Question

I've got this case statement which is giving an error 'variable constant1 is unused'. It seems to be ignoring the variables and returning the top line, so the variables obviously haven't got scope. If I replace the constant with a number 1 then it works. What is the best way of doing this in Elixir?
defmodule Main
do
def constant1, do: 1
def constant2, do: 1
def constant3, do: 1
x = 1
y = 0
z = 0
{a, b, c, d} =
case {x, y, z} do
{constant1, constant2, constant3} -> {1, 2, 3, 4}
{constant1, constant2, _} -> {5, 6, 7, 8}
{constant1, _, _} -> {9, 10, 11, 12}
{_, _, _} -> {13, 14, 15, 16}
end
IO.inspect {a, b, c, d}
end
Here is the output:
warning: variable constant1 is unused
Untitled 9:15
{1, 2, 3, 4}
Changing the constants to variables also doesn't work.

Answer 1

You have defined constant1 being a function. When you try to use it in pattern matching, Elixir expects the variable to be there and you’ve got an error. One can’t pattern match to functions.
What you’ve wanted is likely:
defmodule Main do
constant1 = 1
constant2 = 1
constant3 = 1
x = 1
y = 0
z = 0
{a, b, c, d} =
case {x, y, z} do
{^constant1, ^constant2, ^constant3} -> {1, 2, 3, 4}
{^constant1, ^constant2, _} -> {5, 6, 7, 8}
{^constant1, _, _} -> {9, 10, 11, 12}
{_, _, _} -> {13, 14, 15, 16}
end
IO.inspect {a, b, c, d}
end
#⇒ { 9, 10, 11, 12 }
Also, please remember that to pattern match to already defined value, one should use the pin operator ^ in front of matcher, otherwise the code
a = 42
case x do
a -> ...
end
will overwrite the value of a, setting it to the value of x (in the scope of case block, outside of case a will remain 42.) With ^, the code below will match if and only x == 42:
a = 42
case x do
^a -> ...
end

Answer 2

Answering the subsequent questions about “how to,” “can I use globals,” etc.
Elixir (as all known functional languages) has no notion of “global,” since everything is immutable from the outside point of view. True constants are being implemented as macros. Macros are compiled during the compilation stage to AST and therefore might be used as constants inside match:
defmodule Constants do
defmacro constant1 do
quote do: 1
end
defmacro constant2 do
quote do: 1
end
defmacro constant3 do
quote do: 1
end
end
defmodule Main do
require Constants
x = 1
y = 0
z = 0
{a, b, c, d} =·
case {x, y, z} do
{Constants.constant1, Constants.constant2, Constants.constant3} -> {1, 2, 3, 4}
{Constants.constant1, Constants.constant2, _} -> {5, 6, 7, 8}
{Constants.constant1, _, _} -> {9, 10, 11, 12}
{_, _, _} -> {13, 14, 15, 16}
end
IO.inspect {a, b, c, d}
end
The above works, because after compilation there is no Constants.constant1 anymore in Main code: there are real values: 1s and the code being run is:
case {x, y, z} do
{1, 1, 1} -> {1, 2, 3, 4}
{1, 1, _} -> {5, 6, 7, 8}
{1, _, _} -> {9, 10, 11, 12}
{_, _, _} -> {13, 14, 15, 16}
end
Hope it helps.