Control flow

Make decisions with if, repeat work with for, loop, while and until, and leave loops early.

Programs make decisions and repeat things. OSL does both with a small set of statements, and every block, the code a statement controls, goes in round brackets.

if, else if, else#

int temperature = 23

if temperature > 30 (
  log "hot"
) else if temperature > 15 (
  log "warm"
) else (
  log "cold"
)

The ( that opens a block goes at the end of the line that owns it. else can sit on the same line as the ) before it, as here, or start the next line.

Conditions are combined with and and or, and flipped with !:

int age = 20
boolean member = false

if age >= 18 and !member (
  log "adult, not a member"
)
if age < 13 or member (
  log "discount"
)

Choosing a value#

When you want a value rather than to run statements, use if as an expression, with then and else:

int count = 3
string noun = if count == 1 then "item" else "items"
log `${count} ${noun}`

int score = 72
string grade = if score >= 90 then "A"
  else if score >= 70 then "B"
  else "C"
log grade

Guards#

A condition followed by return, break or continue can sit on one line. These guards keep the main path of a function flat:

def describe(int n) string (
  if n < 0 return "negative"
  if n == 0 return "zero"
  return "positive"
)

log describe(-4)
log describe(0)
log describe(9)

Anything else in an if needs a block.

Repeating a number of times#

loop repeats a block. for does the same and also counts, from 1 up to the number:

loop 3 (
  log "hip hip"
)
log "hooray"

for i 4 (
  log i * i
)

Looping over things#

for ... in goes through the items of an array, the characters of a string or the entries of an object. Give it two names to get the position as well:

string[] tasks = ["write", "test", "ship"]

for task in tasks (
  log "- " ++ task
)

for n, task in tasks (
  log `${n}. ${task}`
)

To count between two numbers, loop over a range:

for year in 2020 to 2023 (
  log year
)

while and until#

while repeats as long as its condition is true. until repeats until its condition becomes true:

int n = 27
int steps = 0
while n != 1 (
  n = if n % 2 == 0 then n / 2 else (n * 3) + 1
  steps++
)
log steps

int tries = 0
until tries == 3 (
  tries++
)
log tries

Leaving early#

break leaves the loop it's in. continue skips to the next time round:

for n in 1 to 10 (
  if n % 2 == 0 continue
  if n > 7 break
  log n
)

Blocks and names#

A variable declared inside a block belongs to that block, and disappears when it ends:

boolean ready = true
if ready (
  string message = "inside"
)
log message

Declare it before the block when you need it afterwards:

boolean ready = true
string message = "before"
if ready (
  message = "changed inside"
)
log message

for n in 1 to 15 (
  log n
)

for n in 1 to 15 (
  log n
)

1
2
Fizz
4
Buzz
Fizz
7
8
Fizz
Buzz
11
Fizz
13
14
FizzBuzz