Assert

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
1
    ! 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
1

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:

• Handle errors with ordinary if-then logic (perhaps using control structures). This is probably the best path for user-facing applications where displaying an error goes through the user interface.
• Write a function Message to compute the message, and call 𝕨 Message⊸!⍟(1⊸≢) 𝕩 or similar instead of !.
• If the error will be caught elsewhere in the program, use a closure for the message and evaluate it when caught. With a function Message as above, message ! 𝕩 works, and {…}˙⊸! 𝕩 is a convenient syntax for block functions.