Error: replacement string item contains newlines

[singlexyz]

I have a configuration that can easily add custom surround before and after a motion/text object.

However, when I change the join character from a space to a \n, it throws an error in the case of multiple lines.

local function surround_cword(keymap, left, right)
  local function surround(lines)
    local str = vim.iter(lines):join(" ")
    return { left .. str .. right }
  end
  op_map("v", keymap, surround)
  vim.keymap.set("n", keymap, function()
    return require("yop").operate(surround) .. "iw"
  end, { expr = true, remap = true })
end
surround_cword("8d", "--", "")
surround_cword("8v", "var(--", ")")
surround_cword("8c", "calc(", ")")
surround_cword("8x", "${", "}")
surround_cword("8s", "{ condition ? ", " : null }")

local str = vim.iter(lines):join("\n")  -- error on multiple lines

Record: