When to when

Yesterday we mentioned the when function as an alternative to if.

It might seem like a weaker form, so why would you use it?

It turns out that when is is really useful for a few reasons. You might end up using it more than if!

Argument Validation🔗

One particularly good place to use when is for argument validation. Typically when an argument is not valid, you'll want to throw an error.

defn double-sqrt(x):
  when not(number?(x)):
    die: "x must be a number"
  when x < 0:
    die: "x must be positive"
  =>: sqrt(x) * 2

An if would be inappropriate here, because when the condition is false, there's nothing to do.

Remember that YS is a functional language. Therefore the when function always has to return a value.

If the condition is false, it returns nil.

What if you need an if call that returns a number or a nil?

You can just use when to do that.

x =:
  if a > b:
    then: a + b
    else: nil
# Some thing as above:
x =:
  when a > b: a + b

Remember how we used then and else in if, when we needed to do more than one thing? The then and else clauses compiled to a do form.

Well when always lets you do that.

when a < 0:
  say: 'a'
  say: 'is'
  say: 'negative'

There's a function called when-not that is the opposite of when.

Remember this from above?

defn double-sqrt(x):
  when not(number?(x)):
    die: "x must be a number"

We could also write that as:

defn double-sqrt(x):
  when-not number?(x):
    die: "x must be a number"

Probably tomorrow.