Elixir has a useful control flow structure called cond
that lets you branch on arbitrary conditions. Unlike the more common switch
control structure (case
in Elixir), cond
doesn’t match against a predetermined value. Instead, it evaluates each condition, in order, looking for the first one that evaluates to a truthy value (not nil
or false
).
numbers = [1, 2, 3]
result = cond do
4 in numbers ->
:four
6 == Enum.sum(numbers) ->
:sum
true ->
:default
end
This is all probably old hat to you.
As I mentioned, cond
can be an incredibly useful control structure, and there are times when I’ve missed it while working in languages like Javascript that only have switch
expressions.
A traditional Javascript implementation of the above (with a little help from Lodash) would look something like this:
let numbers = [1, 2, 3];
if (_.includes(numbers, 4)) {
var result = "four";
} else if (6 === _.sum(numbers)) {
var result = "sum";
} else {
var result = "default";
}
However, I recently stumbled upon a trick that lets you implement a switch
statement in Javascript that behaves very similarly to a cond
expression in Elixir. The key is to switch
on the value of true
. The case expressions that evaluate to true
will match, and their corresponding statements will be evaluated in order.
let numbers = [1, 2, 3];
switch (true) {
case _.includes(numbers, 4):
var result = "four";
break;
case 6 === _.sum(numbers):
var result = "sum";
break;
default:
var result = "default";
break;
}
Whether or not this is any more useful or readable than a series of if
/else
blocks is debatable. That said, this is definitely an interesting example of perspective shifting and seeing old code in a new light. Hopefully you find it as interesting as I do.
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