Joel:
> The short answer is "you can't". One feature of REBOL this
> differs from most other programming languages is that the
> distinction between "code" and "data" is not relevant. REBOL
> can evaluate anything, including blocks that were constructed
> as the result of evaluating other expressions. So, in general,
> the concept of "source line" simply isn't meaningful.
>
> For example:
>
> a: [b:]
> c: [2 + 2]
> d: append a c
> insert d first [print]
> do d
<snip>
> Evaluating the above expressions constructs a new block and then
> evaluates that block. If an error had occurred during evaluating
> the last expression (the last value of D), how would we have any
> concept of "which line" was involved?
But a longer answer should be "you can't, but Rebol should". Rebol gives far
too little away when there is an error -- maybe Carl S doesn't make stupid
coding errors like the rest of us.
Let me modify your code to have a bug in it (it *was* correctly typed)
a: [b:]
c: [2 / 0] ;; bad code: no bananas
d: append a c
insert d first [print]
do d
And run it. We get:
** Math Error: Attempt to divide by zero
** Near: print b: 2 / 0
Try even finding where that is in a multi-programmer, cross-machine system of
a few thousand lines of code. That sort of error context information puts an
automatic brake on the size of any Rebol-based development.
Why won't (not can't) Rebol at least tell me the error line is:
"do d"
near
" d: append a c
insert d first [print]"
Better context information *is* in there somewhere -- try this: the same
error in a VID action facet:
unview/all
view layout [
button "click me"
[
a: [b:]
c: [2 / 0]
d: append a c
insert d first [print]
do d
]
]
This fails in an easier to find way -- better context information:
** Math Error: Attempt to divide by zero
** Where: func [face value][
a: [print b: 2 / 0]
c: [2 / 0]
d: append a c
insert d first [print]
do d
]
** Near: print b: 2 / 0
Sunanda.
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