docs: Remove TODO from ranger section

docs/syntax.md

@@ -35,6 +35,7 @@
     - [Slices / Arrays](#slices--arrays)
     - [Maps](#maps)
     - [Channels](#channels)
+    - [Custom](#custom-ranger)
     - [else](#else)
   - [try](#try)
   - [try / catch](#try--catch)
@@ -366,7 +367,7 @@ Use `range` to iterate over data, just like you would in Go, or how you would us
         {{.}}
     {{ end }}
 
-Jet provides built-in rangers for Go slices, arrays, maps, and channels. You can add your own by implementing the Ranger interface. TODO
+Jet provides built-in rangers for Go slices, arrays, maps, and channels. You can add your own by implementing the Ranger interface.
 
 #### Slices / Arrays
 
@@ -411,6 +412,12 @@ When iterating over a channel, you can can have Jet assign the iteration value t
 
 It's an error to use channels together with the two-variable syntax.
 
+#### Custom Ranger
+
+Any value that implements the
+[Ranger](https://pkg.go.dev/github.com/CloudyKit/jet/v6#Ranger) interface can be
+used for ranging over values. Look in the package docs for an example.
+
 #### else
 
 `range` statements can have an `else` block which is executed if there are non values to range over (as signalled by the Ranger). For example, it will run when iterating an empty slice, array or map or a closed channel:
@@ -687,4 +694,4 @@ Executing `index.jet` will produce:
 
 `import` makes all the blocks from the imported template available in the importing template. There is no way to only import (a) specific block(s).
 
-Since the imported template isn't actually executed, the blocks defined in it don't run until you `yield` them explicitely.
+Since the imported template isn't actually executed, the blocks defined in it don't run until you `yield` them explicitely.

ranger_test.go

@@ -0,0 +1,52 @@
+package jet
+
+import (
+	"fmt"
+	"os"
+	"reflect"
+)
+
+// exampleCustomBenchRanger satisfies the Ranger interface, generating fixed
+// data.
+type exampleCustomRanger struct {
+	i int
+}
+
+// Type assertion to verify exampleCustomRanger satisfies the Ranger interface.
+var _ Ranger = (*exampleCustomRanger)(nil)
+
+func (ecr *exampleCustomRanger) ProvidesIndex() bool {
+	// Return false if 'k' can't be filled in Range().
+	return true
+}
+
+func (ecr *exampleCustomRanger) Range() (k reflect.Value, v reflect.Value, done bool) {
+	if ecr.i >= 3 {
+		done = true
+		return
+	}
+
+	k = reflect.ValueOf(ecr.i)
+	v = reflect.ValueOf(fmt.Sprintf("custom ranger %d", ecr.i))
+	ecr.i += 1
+	return
+}
+
+// ExampleRanger demonstrates how to write a custom template ranger.
+func ExampleRanger() {
+	// Error handling ignored for brevity.
+	//
+	// Setup template and rendering.
+	loader := NewInMemLoader()
+	loader.Set("template", "{{range k := ecr }}{{k}}:{{.}}; {{end}}")
+	set := NewSet(loader, WithSafeWriter(nil))
+	t, _ := set.GetTemplate("template")
+
+	// Pass a custom ranger instance as the 'ecr' var.
+	vars := VarMap{"ecr": reflect.ValueOf(&exampleCustomRanger{})}
+
+	// Execute template.
+	_ = t.Execute(os.Stdout, vars, nil)
+
+	// Output: 0:custom ranger 0; 1:custom ranger 1; 2:custom ranger 2;
+}