• Lisp

(defvar y (format nil "~{~A~^, ~}" x))

• Lisp

(setf y (format nil "~{~a~^,~}" x))

• Lisp

(setf y (reduce (lambda (a b)
                   (concatenate 'string a ", " b))
                 x))

• Ada

declare
   Last : Cursor := X.Last;
   Y : Unbounded_String;
         
begin
   for C in X.Iterate loop
      Y := Y & Element (C) & (if C = Last then "" else ", ");
   end loop;
end;

• C

#define DELIM ", "
#define L 64

char y[L] = {'\0'};

for (int i = 0; i < N; ++i)
{
    if (i && x[i][0])
        strcat(y, DELIM);
    
    strcat(y, x[i]);
}

• Clojure

(clojure.string/join "," '("abc" "def" "ghi") )

• C++

y = x | views::join_with(',') | to<std::string>();

• C++

std::vector<std::string> x;
std::string y;
const char* const delim = ", ";

switch (x.size())
{
	case 0: y = "";   break;
	case 1: y = x[0]; break;
	default:
		std::ostringstream os;
		std::copy(x.begin(), x.end() - 1,
			std::ostream_iterator<std::string>(os, delim));
		os << *x.rbegin();
		y = os.str();
}

• C#

string y = string.Join(", ", x);

• D

string y = x.join(", ");

• Dart

y = x.join(', ');

• Elixir

y = Enum.join(x, ", ")

• Erlang

Y = string:join(X, ",").

• Fortran

  write (unit=y,fmt='(*(A,:,", "))') x

• Go

y := strings.Join(x, ", ")

• Groovy

String y = x.join(', ')

• Haskell

y = intercalate ", " x

• Haskell

{-# LANGUAGE OverloadedStrings #-}
y :: T.Text
y = T.intercalate ", " x

• JS

y = x.join(", ");

• Java

String y = x.get(0);
int i = 1, n = x.size();
while (i < n)
    y = y + ", " + x.get(i++);

• Java

String y = String.join(", ", x);

• Java

String y = x.stream()
    .reduce((a, b) -> a + ", " + b)
    .get();

• Kotlin

val y = listOf(x).joinToString(", ")

• Lua

y = table.concat(x, ", ")

• Obj-C

NSString y=[x componentsJoinedByString:@", "];

• PHP

$y = implode(', ', $x);

• Pascal

var
  _x: Array of string;
  _y: String;
  i: Integer;
begin
  _y := ''; //initialize result to an empy string
  // assume _x is initialized to contain some values
  for i := Low(_x) to High(_x) do
  begin
    _y := _y + _x[i];
    if i < High(_x) then _y := _y + ';';
  end;
end;

• Perl

$y = join(", ", @x)

• Python

y = ', '.join(map(str, x))

• Python

y = ', '.join(x)

• Python

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

• Ruby

y = x.join(", ")

• Rust

let y = x.join(", ");

• Scala

val y = x.mkString(",")

• Scheme

(define y
  (foldr (lambda (a b)
           (if (string=? b "")
               a
               (string-append a ", " b)))
         ""
         x))

• Smalltalk

y := x joinSeparatedBy: ', '.

• Smalltalk

', ' join: #('abc' 'def' 'ghi')

• VB

Dim x = {"a", "b", "c", "d"}.ToList
Dim y = String.Join(",", x)