if - We Must

Yesterday we talked about if being a special form.

It turns out that if is special in other ways. For instance, it requires both the then and else clauses to be present… but… it doesn't require the actual then and else keywords.

Today we'll finish our conversation about if and go over all the specifics.

The Lisp Point of View

In the Lisp code that YS compiles to, the form looks like this: (if <condition> <then> <else>).

Note

Lisp allows you to skip the <else> clause, in which case it defaults to nil. But it also discourages this, and encourages you to use the when function instead. We'll talk about when in a future post.

YS does not allow you to skip the <else> clause. If you don't have one, use nil or switch to when.

In YS we can write that in many ways:

x =:
  if a: b c
x =:
  if a:
    =>: b
    =>: c
x =: if(a b c)
x =: a.if(b c)

This works fine when the <then> and <else> clauses are a single form (both in Clojure and YS).

If a clause needs more than one form you need to combine them into a single form. In Clojure you do this with a do form, and you can do that in YS too. The do form is a special form that evaluates all its arguments and returns the value of the last one.

x =:
  if a:
    do:
      b: 1
      b: 2
    do:
      c: 1
      c: 2

YS makes this prettier by allowing you to use the words then and else instead of do or =>:.

$ ys -ce '
x =:
  if a:
    then:
      b: 1
      b: 2
    else:
      c: 1
      c: 2
'
(def x (if a (do (b 1) (b 2)) (do (c 1) (c 2))))

The Fine Print

In YS you can use else without using a then first, but if you use then you must use else too. You'll get an error if you don't.

YS just reads better this way.

$ ys -ce '
x =:
  if a:
    b: 1
    else:
      c: 1
      c: 2
'
(def x (if a (b 1) (do (c 1) (c 2))))
$ ys -ce '
x =:
  if a:
    then:
      b: 1
      b: 2
    c: 1
'
Compile error: Form after 'then' must be 'else'

Now you know all about if.

There are several other cool conditional functions in YS.

We'll get there!

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