Errors¶

An error ends the form that caused it. Exceptions is how to catch one; this page is the message you get, and where it appears.

An uncaught error prints where the value would have been, and the prompt comes back, so the session carries on:

user=> (/ 1 0)
Execution error: Divide by zero
user=> (+ 1 2)
3.0

Syntax errors¶

A form that does not parse is never run, so nothing in it takes effect, and the message says Syntax error instead:

user=> (def)
Syntax error: Too few arguments to def
user=> (loop [] (try (recur)))
Syntax error: Can only recur from tail position

Futures¶

A control loop started with control/start runs in a future. If a round fails, the connection that started it prints Future failed. and the error as soon as the thread ends, even if nobody calls deref:

user=> (def bad (future (/ 1 0)))
#'user/bad

A moment later, the same connection prints:

Future failed. Execution error: Divide by zero

@bad raises the same error again. The line can arrive between other output, because it comes from another thread. Waiting for deref would let a control loop that died go unnoticed. Futures covers the rest of what a future does.

Stopping¶

STOP interrupts every running form and every time/sleep, which is why a loop run at the prompt ends with Execution error: Interrupted. A loop started with control/start is cancelled rather than broken, so it prints no Future failed. line, and dereferencing that future raises Execution error: Future was cancelled.

Control Loops has what each kind of stop ends.