BQN takes the position that errors exist to indicate exceptional conditions that the developer of a given program didn't expect. However, the types of errors that BQN naturally checks for, such as mismatched shapes in Couple (), aren't always enough to detect exceptional conditions. Issues like numeric values that don't make physical sense will slip right through. BQN makes it easy for a programmer to check for these sorts of problems by building in the primitive Assert, written !. This function checks whether 𝕩 matches 1: if it does, then it does nothing and returns 𝕩, and otherwise it gives an error.

    ! 2=2  # Passed
    ! 2=3  # Failed
Error: Assertion error

To pass, the right argument must be exactly the number 1; any other value causes an error. For example, an array of 1s still causes an error; use ´ to convert a boolean array to a single boolean that indicates whether all of its values are true.

    ! (∧=∨¬)⌜˜ 2
Error: 2‿2⥊1‿1‿1‿1
    ! ´ (∧=∨¬)⌜˜ 2

Assert can take a left argument, which gives a message to be associated with the error. It's typical to use a string for the left argument in order to display it to the programmer, but the left argument can be any value.

    "Message" ! 0
Error: Message
    ,"abc",˜ ! '0'
Error: ⟨∘,"abc",˜⟩

Computing the error message on demand

Because the left argument to a function is always computed before the function is called, Assert doesn't let you compute the error message only if there's an error. This might be a problem if the error message computation is slow or has side effects. There are a few ways to work around the issue: