Question

I try to capture lines with calculations in my text document
and execute them.

I use this in my function:

for i in range(startline,endline) 
let calculation = getline(i)  
...  
let out = eval(calculation)  
...  
endfor  

sometimes something goes wrong and I receive this message:

Error detected while processing function....  
Line ...  
E488: Trailing Characters   

Line .. is the line-nr in my function.
I would like to know also which calculation it concerns (which line in my text doc):

If Error detected = echo calculation

How can I check if there is an error message and echo the variable “calculation”?

Answer 1

There are two ways to handle script errors inside a function:

The first is suppressing the error via :silent!. Two downsides: You have to manually check for success, and any normal output from the evaluated script is suppressed, too (unless you do contortions with :unsilent).

let v:errmsg = ''
silent! let out = eval(calculation)
if v:errmsg != ''
    " error
endif

I would recommend the second way via try…catch, which avoids the issues with the output and having to explicitly check for an error:

try
    let out = eval(calculation)
catch /^Vim\%((\a\+)\)\=:E/
    " v:exception contains what is normally in v:errmsg, but with extra
    " exception source info prepended, which we cut away.
    let v:errmsg = printf("Line: %d\nCalculation: %s\nError: %s", i, calculation, substitute(v:exception, '^Vim\%((\a\+)\)\=:', '', ''))
    echohl ErrorMsg
    echomsg v:errmsg
    echohl None
endtry