RiscLua 7

Yes, BASIC’s ON ERROR LOCAL is very nice.

Lua, being written to be embedded in an application, pushes all the responsibility for error-handling back to the application. For RiscLua the application is the stand-alone Lua interpreter, whose main loop simply prints the error-message to stdout and continues showing the prompt and responding to input. To handle errors inside Lua code you need to use the function pcall which calls its first argument in protected mode, so that it catches any errors raised during the call. If there are no errors it returns true followed by the values the call would ordinarily return, otherwise it returns false followed by the error-object. An error-object can be raised (thrown) by using the function error. If you want to know what was on the stack before the error occurred there is a function xpcall which takes as a second argument a message-handler function, which, using the debug library can be used to construct more complex error-inspection tools according to taste.

Page 168 of the BBC BASIC GUIDE gives an example of how to use ON ERROR LOCAL. A sort of Lua equivalent could be

local calculate = \ (a)
    for i = -4,4 do
      local ok, err = pcall (\ ( )
           print (a // i) end )
      if not ok then print (err) end -- if
    end -- for
end -- function
calculate (24)

-6
-8
-12
-24
RAM::RamDisc0.$.Test:3: attempt to divide by zero
24
12
8
6

It is possible to achieve a similar effect in C using setjmp and longjmp. Create an array of type jmp_buf and some suitable functions and you can have nested error trapping so that exceptions can be thrown to the nearest enclosing handler. Much nicer than goto. It takes a bit of work, but very nice to use once you’ve written the code.

It is possible to achieve a similar effect in C …

And in BASIC, but it looks very messy. Simpler is Basalt’s:

ERROR LOCAL
 <try>
WHEN <error1>
 <catch erro1>
WHEN <error2>
 <catch error2>
ERROR RESTORE :REM throw uncaught errors to enclosing handler
<continue>