diff --git a/WDL/StdLib.py b/WDL/StdLib.py
index ef79a789..d437b1da 100644
--- a/WDL/StdLib.py
+++ b/WDL/StdLib.py
@@ -77,6 +77,10 @@ def sub(input: Value.String, pattern: Value.String, replace: Value.String) -> Va
         def defined(v: Value.Base):
             return Value.Boolean(not isinstance(v, Value.Null))
 
+        @static([Type.String(), Type.Array(Type.String())], Type.String())
+        def sep(sep: Value.String, iterable: Value.Array) -> Value.String:
+            return Value.String(sep.value.join(v.value for v in iterable.value))
+
         # write_*
         static([Type.Array(Type.String())], Type.File(), "write_lines")(
             self._write(_serialize_lines)
diff --git a/WDL/_grammar.py b/WDL/_grammar.py
index caf63589..d04fe024 100644
--- a/WDL/_grammar.py
+++ b/WDL/_grammar.py
@@ -321,12 +321,7 @@
 output_decls: "output" "{" bound_decl* "}"
 
 // WDL task commands: with {} and <<< >>> command and ${} and ~{} placeholder styles
-!?placeholder_key: "default" | "false" | "true" | "sep"
-?placeholder_value: string_literal
-                  | INT -> int
-                  | FLOAT -> float
-placeholder_option: placeholder_key "=" placeholder_value
-placeholder: placeholder_option* expr
+placeholder: expr
 
 ?command: command1 | command2
 
diff --git a/tests/test_5stdlib.py b/tests/test_5stdlib.py
index 4a6b86cf..4ffd74a9 100644
--- a/tests/test_5stdlib.py
+++ b/tests/test_5stdlib.py
@@ -713,6 +713,50 @@ def test_zip_cross(self):
         }
         """, expected_exception=WDL.Error.EvalError)
 
+    def test_sep(self):
+        outputs = self._test_task(R"""
+        version development
+        task SepTest {
+            input {
+                Array[String] inp = ["value1", "value2", "value3"]
+            }
+            command {}
+            output {
+                String out = sep(",", inp)
+            }
+        }
+        """)
+        self.assertEqual("value1,value2,value3", outputs["out"])
+
+        outputs = self._test_task(R"""
+        version development
+        task SepTest {
+            input {
+                Array[String] inp = ["value1", "value2", "value3"]
+            }
+            command <<<
+                echo ~{sep(",", inp)}
+            >>>
+
+            output {
+                String out = read_string(stdout())
+            }
+        }
+        """)
+        self.assertEqual("value1,value2,value3", outputs["out"])
+
+        self._test_task(R"""
+        version development
+        task SepTest {
+            input {
+                Array[String] inp = ["value1", "value2", "value3"]
+            }
+            command <<<
+                echo ~{sep="," inp}
+            >>>
+        }
+        """, expected_exception=WDL.Error.SyntaxError)
+
 
     def test_suffix(self):
         outputs = self._test_task(R"""