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Printing and Assertions

NumFu offers simple tools for debugging and verification. Use print, println, assert, or sugar syntax to check results and trace execution.

Printing Output​

Use print and println from the io module to display values. All values are automatically converted to strings before printing.

import print, println from "io"

print("I like cheese")
print(" and tomatoes.\n")

println("They are indeed very delicious.")

Output:

I like cheese and tomatoes.
They are indeed very delicious.

println(x) is simply shorthand for:

print(x + "\n")

Reading Input​

The built-in function input lets you read text entered by the user. You can optionally pass a prompt string that will be displayed before the user types.

import input, println from "io"

let name = input("What is your name? ")
println("Hello, " + name + "!")

Example Output:

What is your name? John
Hello, John!

Assertions​

Use assertions to validate that values match expectations. They throw an error if the condition fails.

Basic Form​

// Assert boolean condition
assert(2 + 2 == 4)

// Optional return value for use in expressions
let greet = {name -> "Hello " + assert(length(name) == 5, name)}

greet("James")
greet("Mary")

Output:

true
Hello James
...
AssertionError

Syntactic Sugar​

Top-level expressions can use the ---> operator for readable assertions.

2 + 2 ---> $ == 4             // same as assert(2 + 2 == 4)
length("hello") ---> $ == 5
  • $ refers to the expression on the left
  • This form is only valid at top-level, not inside functions

Errors​

Use error(message) to stop execution with a custom message.

let divide = {a, b ->
  if b != 0 then a / b
  else error("Cannot divide by zero!")
} in
  divide(42, 0)

// RuntimeError: Cannot divide by zero!

You can also specify a custom error name:

error("Cannot divide by zero!", "MathError")

// MathError: Cannot divide by zero!