Vim script/function to pipe selection through a command (or run it as Vim script)

ExecRange.vim

function! ExecRange(cmd) range
    let l:old_a = @a
    exec a:firstline . ',' . a:lastline . 'yank a'
    if a:cmd == ''
        exec @a
    else
        echo system(a:cmd, @a)
    endif
    let @a = l:old_a
endfunction

command! -nargs=* -range=% ExecRange <line1>,<line2>call ExecRange(<q-args>)