Testing

Keep checks next to your code with test blocks, run them with osl test, and write test files for bigger cases.

OSL builds testing into the language. A test block next to a function lists things that should be true, and osl test checks them. No framework to install, and no assertion library to learn.

Test blocks#

Write test ( ... ) at the top level of a file. Every line inside that's a boolean is checked:

def clamp(number value, number low, number high) number -> min(high, max(low, value))

test (
  clamp(-1, 0, 10) == 0
  clamp(5, 0, 10) == 5
  clamp(11, 0, 10) == 10
)

Run the checks for a file, a folder, or the whole project:

osl test clamp.osl
osl test src/
osl test

PASS clamp.osl:3
1 passed

When a check fails, you see which line, and the values on both sides of the comparison:

FAIL clamp.osl:3
  clamp.osl:5: clamp(5, 0, 10) == 5
    left:  0
    right: 5

osl run and osl compile leave test blocks out completely, so they cost nothing in the program you ship.

Loops and setup in tests#

A test block is ordinary code. Declare data, loop over cases, call functions; every boolean line, even inside a loop or an if, is a check:

def isLeapYear(int year) boolean (
  if year % 400 == 0 return true
  if year % 100 == 0 return false
  return year % 4 == 0
)

test (
  for year in [1996, 2000, 2024] (
    isLeapYear(year)
  )
  for year in [1900, 2023, 2100] (
    !isLeapYear(year)
  )
)

Each block has its own variables. Blocks in one file share the file's globals and run in order.

Testing failures#

A test that expects an error can check the result:

def parsePort(string text) result<int, error> -> text.tryAs<int>

test (
  parsePort("8080").unwrap() == 8080
  parsePort("eighty").isErr()
)

A throw inside a test block fails that block, and the next block still runs.

Assertions with messages#

When a plain comparison doesn't say enough, std:testing has assertions that take a message:

import "std:testing"

def average(number[] values) number -> values.sum() / values.len

test (
  testing.equal(average([2, 4]), 3, "average of two values")
  testing.near(average([1, 2, 2]), 1.667, 0.001)
)

Test files#

For bigger tests, put them in a file whose name ends in .test.osl. It's an ordinary program, found by osl test, which fails if the program throws:

src/
  pricing.osl
  pricing.test.osl

import "std:testing"

testing.equal(1 + 1, 2)
log "all good"

What to test#

• Pure functions first: anything that takes values and returns a value is easy to test with a block right underneath it.
• Edge cases: empty arrays, zero, negative numbers, text with spaces, the last day of the month.
• Bugs you've fixed: add a check that would have caught it, so it can't come back.

def slug(string title) string -> title.trim().toLower().replace(" ", "-")

log slug("Hello World") == "hello-world"
log slug("  OSL  ") == "osl"
log slug("a b c") == "a-b-c"

def slug(string title) string -> title.trim().toLower().replace(" ", "-")

log slug("Hello World") == "hello-world"
log slug("  OSL  ") == "osl"
log slug("a b c") == "a-b-c"

true
true
true