Re: How to read large numbers?

[ I've Cc'ed Ben on this. Ben, please note the continuable signal related question at the end. ] Kai.Grossjohann(a)CS.Uni-Dortmund.DE (Kai writes: Let me see if I understand this correctly. So, for instance, you have a piece of buffer that looks like this: (foo bar 1927120979049047309282389079128346218936) Is it correct to say that you want to call (read (current-buffer)) and have it return (foo bar -1)? Your suggestion to trap the overflow and re-try seems to work, but requires modifying the buffer. If this is not allowed, you must create a temporary buffer, which is slower. You could optimize by trying to read from the buffer in-place, and if that fails, copy the expression in a temporary buffer and continue from there, replacing the overflowing constants with -1. This is not hard to do, but it's tedious. One interesting feature that XEmacs supports are continuable errors. This means that you should be able to use a condition-case-like a handler that returns from a new value, in your case -1. You can test this from the debugger: set debug-on-error to t, evaluate (read (current-buffer)) before the above sexp, and press `r -1 RET' in the debugger. The returned value will be the desired (foo bar -1). However, this feature currently only works from the debugger. The way to code it would be like this: (defun read-trapping-overflow () (call-with-condition-handler (lambda (error-info) ;; Correct code would check whether the error really is integer ;; overflow and rethrow by returning `signal' if not. ;; Otherwise, replace overflowing value with -1. -1) (lambda () (read (current-buffer))))) But for this to work, you need to enable that functionality with this patch: --- src/eval.c.orig Mon Jul 29 00:27:19 2002 +++ src/eval.c Mon Jul 29 00:41:48 2002 ＠＠ -1928,12 +1928,12 ＠＠ NNUNGCPRO; } NUNGCPRO; -#if 0 - if (!EQ (tem, Qsignal)) - return return_from_signal (tem); -#endif + /* If handler didn't throw, try another handler */ Vcondition_handlers = all_handlers; + + if (!EQ (tem, Qsignal)) + return return_from_signal (tem); } /* It's a condition-case handler */ I have no idea why someone went to all the trouble to implement the continuable errors, and then disabled their use! If the feature is disabled because it's unstable, then why is it available from the debugger? Ben, can you comment on this? Does this patch clash with your modified `signal' workspace? Unless there is a good reason to disable this, I'd like this patch to be applied.