• Python

f = lambda x, y: f'{x}, {y}'
print(reduce(f, a))

• Python

print(', '.join(map(str, a)))

• Python

a = [1, 12, 42]
print(*a, sep=', ')

• Python

print(a[0], end='')
for x in a[1:]:
    print(',', x, end='')

• Pascal

var
  a: array of integer;
  i: Integer;
begin
  a := [1,12,42];
  for i := Low(a) to High(a) do
  begin
    write(a[i]);
    if i <> High(a) then write(', ');
  end;
end.

• Dart

print(a.join(", "));

• Ada

declare
   A : array (1 .. 3) of Integer := (1, 12, 42);
begin
   for I in A'Range loop
      Ada.Text_IO.Put (A (I)'Image);
      if I /= A'Last then
         Ada.Text_IO.Put (", ");
      end if;
   end loop;
end;

• C++

std::array a{1, 12, 42};

{
  std::stringstream _ss;
  auto iter = a.begin();
  _ss << *iter++;
  for (; iter != a.cend(); _ss << std::string_view(", ") << *iter++);
  std::cout << _ss.view();
}

• C++

::std::cout << (a
  | ::std::views::transform([](int const i){return ::std::to_string(i);})
  | ::std::views::join_with(::std::string(", "))
  | ::std::ranges::to<::std::string>());

• C#

Console.Write(string.Join(", ", a));

• Fortran

  integer, dimension(:), allocatable :: a
  a = [1,12,42]
  write (*,'(*(I0:", "))') a

• Go

a := []int{1, 12, 42}

for i, j := range a {
	if i > 0 {
		fmt.Print(", ")
	}
	fmt.Print(j)
}

• Haskell

let a = [1, 12, 42] in
  putStrLn $ intercalate ", " (show <$> a)

• JS

let s = a.join(', ')
console.debug(s)

• JS

let s = a.reduce((x, y) => `${x}, ${y}`)
console.debug(s)

• Java

String s = of(a)
    .mapToObj(String::valueOf)
    .reduce((x, y) -> x + ", " + y)
    .get();
out.println(s);

• Java

String s = of(a)
    .mapToObj(String::valueOf)
    .collect(joining(", "));
out.println(s);

• Java

out.print(a[0]);
int i = 1, n = a.length;
while (i < n) out.print(", " + a[i++]);

• Perl

@a = qw (1 12 42);
print join(", ",@a),"\n";

• Ruby

puts a.join(", ")

• Rust

let a = [1, 12, 42];
println!("{}", a.map(|i| i.to_string()).join(", "))