Szathmáry László honlapja @ DEIK

Positive case

let x = 10

assert x == 10

Negative case

let x = 10

assert x == 5

Execution:

$ ./main
/home/jabba/Dropbox/nim/Nim-2026/a12-assert/main.nim(15) main
/home/jabba/.choosenim/toolchains/nim-2.2.8/lib/std/assertions.nim(45) failedAssertImpl
/home/jabba/.choosenim/toolchains/nim-2.2.8/lib/std/assertions.nim(40) raiseAssert
/home/jabba/.choosenim/toolchains/nim-2.2.8/lib/system/fatal.nim(62) sysFatal
Error: unhandled exception: /home/jabba/Dropbox/nim/Nim-2026/a12-assert/main.nim(15, 1) `x == 5`
[AssertionDefect]

Optionally, you can add a message too:

let x = 10

assert x == 5, "x should be 5"

Execution:

$ ./main
...
Error: unhandled exception: .../nim/Nim-2026/a12-assert/main.nim(15, 1) `x == 5` x should be 5
[AssertionDefect]

Notice that the message "x should be 5" is also printed.