feat(filters): Add a join filter

CHANGELOG.md

@@ -18,6 +18,12 @@
     Hello {{ name|default:"World" }}
     ```
 
+- There is now a `join` filter.
+
+    ```html+django
+    {{ value|join:", " }}
+    ```
+
 ### Bug Fixes
 
 - Variables (`{{ variable.5 }}`) that reference an array index at an unknown

Sources/Filters.swift

@@ -35,3 +35,19 @@ func lowercase(_ value: Any?) -> Any? {
 func defaultFilter(value: Any?, arguments: [Any?]) -> Any? {
   return value ?? arguments.first as Any
 }
+
+func joinFilter(value: Any?, arguments: [Any?]) throws -> Any? {
+  guard arguments.count == 1 else {
+    throw TemplateSyntaxError("'join' filter takes a single argument")
+  }
+
+  guard let separator = arguments.first as? String else {
+    throw TemplateSyntaxError("'join' filter takes a separator as string")
+  }
+
+  if let value = value as? [String] {
+    return value.joined(separator: separator)
+  }
+
+  return nil
+}

Sources/Namespace.swift

@@ -49,6 +49,7 @@ public class Namespace {
     registerFilter("capitalize", filter: capitalise)
     registerFilter("uppercase", filter: uppercase)
     registerFilter("lowercase", filter: lowercase)
+    registerFilter("join", filter: joinFilter)
   }
 
   /// Registers a new template tag

Tests/StencilTests/FilterSpec.swift

@@ -103,4 +103,13 @@ func testFilter() {
       try expect(result) == "Hello World"
     }
   }
+
+  describe("join filter") {
+    let template = Template(templateString: "{{ value|join:\", \" }}")
+
+    $0.it("transforms a string to be lowercase") {
+      let result = try template.render(Context(dictionary: ["value": ["One", "Two"]]))
+      try expect(result) == "One, Two"
+    }
+  }
 }

docs/builtins.rst

@@ -130,3 +130,14 @@ value of the variable. For example:
 .. code-block:: html+django
 
     Hello {{ name|default:"World" }}
+
+``join``
+~~~~~~~~
+
+Join an array with a string.
+
+.. code-block:: html+django
+
+    {{ value|join:", " }}
+
+.. note:: The value MUST be an array of Strngs and the separator must be a string.