How to replace entire buffer with the contents of a variable?

Question

I am trying to replace the entire buffer with the output from an external command. Similar to :% !myprog, but only if the command succeeds:

let output = system('myprog')
if v:shell_error == 0
    " What to do here?
    " How to replace entire buffer with the contents of output?
    " How to achieve exactly the same effect as ':% !myprog'?
else
    echoerr 'Command failed! Error message: ' . out
endif

To replace the entire buffer with the contents of the output variable, I tried :1,$d | put =output, but this is subtly different from the effect of % !myprog. Among other things, :1,$d | put =output inserts an unwanted newline, and the cursor position is different.

I am looking for a way to replace the entire buffer with the contents of a variable in exactly the same way that :% !myprog replaces the entire buffer with the output of an external command.

Update: I managed to achieve the desired effect using:

1,$d          " Delete contents of entire buffer.
0put =output  " Insert contents of variable without extra newline at the beginning.
$d            " Remove the extra newline at the end of the file.
call setpos('.', [0, 1, 1, 0])  " Move cursor to first line, first column.

This looks like a hack. Is there a better way?

Answer 1

Is there a better way?

In Vim there are always too many ways. I'd do it like this:

" get result as List
let output = systemlist('myprog')
if v:shell_error
    echoerr 'Command failed! Error message:' output[0]
else
    " add output on top
    call append(0, output)
    " delete the rest
    call deletebufline("%", len(output) + 1, "$")
    " go to first line, first column
    1
endif