User function unexpectedly returns the same output every call

Question

UPDATE

As @Matt pointed out g:jsonTemplate is a list that needs to be copied. I originally thought that multiline strings were strings and overlooked the varible assignment detail after debugging. The doc for the mutiline string is :h let-heredoc.

There was another issue, unrelated to this question, that I solved thanks to @D. Ben Knoble's answer. I misunderstood the docs when trying to make the search pattern case sensitive and used a non-existant 'C' flag for substitute(), instead of prepending the search pattern arg with '\C' (and using single quotes). After reading 27.1 from :h usr_27, I realised that the cause was a forgotten setting in my .vimrc (set ignorecase).

Working solution:

function InsertJsonObj()
  let date = trim(system('date +"%T"'))
  let uuid = GetUUID()
  let copy = copy(g:jsonTemplate) " now using copy()
  "next 2 lines: substituted double quotes for single quotes, removed erroneous flag and prepended pat with \C to make it case sensitive.
  let copy[1] = substitute(copy[1], '\CUUID', uuid, '')
  let copy[2] = substitute(copy[2], '\CDATE', date, '')
  call append(line('.'), copy)
endfunction

---

ORIGINAL POST

The goal:

I want to insert a dynamically generated json string with 1) a unique UUID and 2) the date

The issue:

The function I made appends the same string every time I ':call InsertJsonObj()'

My understanding:

Well ... I don't understand. I have other functions that insert different json strings and each function produces a distinct UUID but every call of a function returns the same UUID (i.e. InsertA() will always return id = A1 and InsertB() will always return id = B1).

After replacing the date format to show the time I noticed that the string did change but only each time the vimrc is sourced (InsertA() now returns id = A2 and InsertB() return id = B2).

I don't understand why a function is only made once when the config is sourced but using :call calls the function when the command is executed. I tried reading ':h function' but nothing really jumped at me.

My questions:

What is the cause of the problem?

What doc page or paragraph should I be looking into here?

The functions:

function GetUUID()
  return trim(system('uuidgen')) "a script
endfunction

function InsertJsonObj()
  let date = trim(system('date +"%T"'))
  let uuid = GetUUID()
  "call append(line('.'), GetUUID()) "this line appends a different value every time
  let copy = g:jsonTemplate
  let copy[1] = substitute(copy[1], "UUID", uuid, "C")
  let copy[2] = substitute(copy[2], "DDDD", date, "C")
  call append(line('.'), copy)
endfunction

let g:jsonTemplate =<< END
{
"id": "UUID",
"date": ["DDDD"],
},
END

Bellow are some of the calls to one of the functions:

# some calls to :call InsertJsonObj() after editing and resourcing .vimrc
{
"id": "91fe667f-7caf-47e5-b650-da1bc7532d8c",
"date": ["15:21:59"],
},
{
"id": "950e3fef-f4f5-4b47-a684-c28a1b70b6e5",
"date": ["15:21:01"],
},
{
"id": "950e3fef-f4f5-4b47-a684-c28a1b70b6e5",
"date": ["15:21:01"],
},
{
"id": "bda591b6-c9cd-4093-8659-697738b191f4",
"date": ["Dec 09, 2022"],
},

# bellow GetUUID appended twice with ':call append(line('.'), GetUUID())'
c79011d8-2f01-41ef-adca-6e12b39e02df
6d265289-8e16-4848-a719-f5ff6897c4d3


# adding 'call append(line('.'), GetUUID())' to the function body produces a different UUID but the same template string
{
"id": "11eb069b-ed95-4e03-af70-aae2711f10a0",
"date": ["15:28:31"],
},
def19400-d01f-4b40-8594-52a6fdc668aa

{
"id": "11eb069b-ed95-4e03-af70-aae2711f10a0",
"date": ["15:28:31"],
},
8f1ab0c4-8d8a-404b-a62c-b8379094f2f5

Answer 1

let copy = g:jsonTemplate

Obviously, this looks very wrong, as "jsonTemplate" has a type of List. And simple assignment of a list doesn't make a new copy in VimScript (and also in many or even all(?) other languages). Use copy() function or similar stuff.