Two consecutive identical calls to echo have different output

Question

Question reposted on StackOverflow, where it's receiving more attention.

Given the following function in the .vimrc file,

fu! MyFun(count)
  echo a:count
  echo a:count
  if a:count > 0
    normal ,
    call MyFun(a:count - 1)
  endif
endf

calling :call MyFun(3) generates the following output.

3
3
2
2
1
1
0
0

However, if I define the mapping nn , :<C-U>execute "call MyFun(" . v:count . ")"<CR>, then the call to :call MyFun(3) generates

3
0
2
0
1
0
0
0

I do understand that the mapping of , makes the MyFun function call itself twice (if a:count > 0), however I cannot understand how this can cause a different result of the two successive calls to echo a:count.

Answer 1

The problem is all about screen redraw (see :h echo-redraw) in Vim.

Changing echo to echom still produces the same (broken) screen output (3 0 2 0 1 0 0 0), but :mess reveals what is hidden: 3 3 0 0 2 2 0 0 1 1 0 0 0 0.

Answer 2

I don't know why it happens, but if you add a \n after the echo it all works and the result is what you would expect:

fu! MyFun(count)
  echo a:count . "\n"
  echo a:count . "\n"
  if a:count > 0
    normal ,
    call MyFun(a:count - 1)
  endif
endf

But maybe you already knew that and was just curious to know why (as I confess I am).