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Control Flow

YS (iow Clojure) is a functional programming language. That means that everything is a function. Running a program is just a calling a function that calls other functions.

Even though YS tries hard to look like an imperative language, you must keep in mind that it's functional and get familiar with how the flow works.

This document will cover:

  • Starting a program
  • Variable scope
  • Grouping expressions
  • Looping functions and recursion
  • Conditional expressions

Note

Some of the things that are called "functions" in in this document are actually "macros" or "special forms" in Clojure. The distinction is not particularly important here, but worth mentioning.

Starting a Program🔗

A YS file generally is just a bunch of defn calls to define functions.

Sometimes there will be assignment expressions at the top level. Top level variables are global and can be used anywhere in the program. They belong to the file's namespace which is main by default.

If a program defines a function called main, it will be called when the program is run. Any command line arguments will be passed to the main function after being cast to numbers if they look like numbers.

program.ys:

!YS-v0

defn main(word='Hello!' times=3):
  each i (1 .. times):
    say: "$i) $word"

$ ys program.ys
1) Hello!
2) Hello!
3) Hello!

$ ys program.ys YS 2
1) YS
2) YS

If a program does not define a main function, then nothing will happen unless you've defined a top level function call yourself. YS files that are meant to be used as libraries will not have a main function or a top level function call.

Variable Scope🔗

One cool thing about YS (and Clojure) is that you can use any word as a variable name. Even things like if and for which are reserved words in many languages.

For example you might do this:

defn foo(list):
  count =: count(list)
  if count > 0:
    say: 'The list is not empty'
  else:
    say: 'The list is empty'

Once we bind count to the result of the count function, we can't use the count function again in that scope. Often this is just fine. And it feels nice that you don't have to think up a synonym or alternative mangling for count.

Grouping Expressions🔗

Some expression contexts allow multiple expressions to be grouped together and some only allow a single expression.

You can group multiple expressions together with a do function call when you need to to do multiple things in a context that only allows a single expression.

if a > b:
  do:
    say: 'a is greater than b'
    say: 'a must be HUGE!'
  say: 'Nothing to see here'

Note

The if function actually supports the better named then and else words for grouping, but do can also be used.

Looping Functions and Recursion🔗

YS has a few looping functions: loop, reduce, for and each.

These will be documented in more detail in the future, but for now you can see the Clojure documentation for them:

Conditional Expressions🔗

YS has a few common conditional expressions: if, when, cond and case.

The if Function🔗

In YS, an if expression is a function that takes 3 arguments: a condition, a then expression and an else expression.

if a: b c  # If a is true, return b, else return c

The b and c expressions can also be mapping pairs:

if a:
  say: 'yes'
  say: 'no'

Sometimes you want to do more than one thing in the then or else expression:

if a:
  then:
    say: 'yes'
    say: 'yes'
  else:
    say: 'no'
    say: 'no'

If you use then you must also use else, but else can be used without a then:

if a:
  say: 'yes'
  else:
    say: 'no'
    say: 'no'

Since if is a function, it has a return value.

say:
  if a: -'yes' 'no'

Any variable assigned in the then or else expression will only apply to that expression and not to the surrounding scope.

x =: 1
if a > b:
  x =: 2
  x =: 3
=>: x    # => 1

What you want to do here is capture the result of the if expression:

x =:
  if a > b:
    then: 2
    else: 3
=>: x    # => 2 or 3

Note that say returns nil, so all the if expressions above would also return nil.

The when Function🔗

YS also has a when function that is like if but without an else expression.

when a:
  say: 'yes'

The if function should only be used wen you have a then and an else expression. Otherwise, use when.

One thing about when is that its body can have multiple expressions.

when a:
  say: 'yes'
  say: 'yes'

The cond Function🔗

The cond function is like a series of if expressions but with multiple conditions.

size =:
  cond:
    a < 20:  'small'
    a < 50:  'medium'
    a < 100: 'large'
    else:    'huge'

The case Function🔗

The case function is like a cond but with a single expression to compare against.

count =:
  case a:
    1: 'one'
    2: 'two'
    3: 'three'
    else: 'many'