Fancier YS Conditionals

Most languages support a case or switch construct which is a way to handle multiple conditions.

YS supports several similar but different constructs for this.

Today we'll focus on the cond and case functions.

Using cond

The cond function takes an ordered set of pairs each consisting of a predicate and an action. Since we are writing code in YAML, it uses mappings for this. Each pair has a predicate key and an action value.

# cond.ys
!YS-v0
each x [-1 2 30 400 5000 "sixty thousand"]:
  kind =:
    cond:
      x:int?:not: "not a"
      x < 0:      "a negative"
      x < 10:     "a single digit"
      x < 100:    "a double digit"
      x < 1000:   "a triple digit"
      else:       "a huge"
  say: "$x is $kind number"

Let's run it:

$ ys cond.ys
-1 is a negative number
2 is a single digit number
30 is a double digit number
400 is a triple digit number
5000 is a huge number
sixty thousand is not a number

The cond function tries each predicate in order until one of them is true. Then it applies the action of that predicate and it is finished.

Just in case

There's a similar function called case. This one takes a value and a set of things it might be equal to. The set consists of a mapping whose pairs contain a value and an action. The case function also supports an else pair, like cond.

# case.ys
!YS-v0
each n (1 .. 4):
  say:
    case n:
      1: "One for the money."
      2: "Two for the show."
      3: "Three to get ready."
      else: "Now, go cat, go!"

And then:

$ ys case.ys
One for the money.
Two for the show.
Three to get ready.
Now, go cat, go!

Your YS wish is my command.

Gotta go!

See you tomorrow.

Comments