Add pow function

config/interpolate_funcs.go

@@ -89,6 +89,7 @@ func Funcs() map[string]ast.Function {
 		"merge":        interpolationFuncMerge(),
 		"min":          interpolationFuncMin(),
 		"pathexpand":   interpolationFuncPathExpand(),
+		"pow":          interpolationFuncPow(),
 		"uuid":         interpolationFuncUUID(),
 		"replace":      interpolationFuncReplace(),
 		"sha1":         interpolationFuncSha1(),
@@ -394,6 +395,17 @@ func interpolationFuncConcat() ast.Function {
 	}
 }
 
+// interpolationFuncPow returns base x exponential of y.
+func interpolationFuncPow() ast.Function {
+	return ast.Function{
+		ArgTypes:   []ast.Type{ast.TypeFloat, ast.TypeFloat},
+		ReturnType: ast.TypeFloat,
+		Callback: func(args []interface{}) (interface{}, error) {
+			return math.Pow(args[0].(float64), args[1].(float64)), nil
+		},
+	}
+}
+
 // interpolationFuncFile implements the "file" function that allows
 // loading contents from a file.
 func interpolationFuncFile() ast.Function {

config/interpolate_funcs_test.go

@@ -310,6 +310,54 @@ func TestInterpolateFuncMin(t *testing.T) {
 	})
 }
 
+func TestInterpolateFuncPow(t *testing.T) {
+	testFunction(t, testFunctionConfig{
+		Cases: []testFunctionCase{
+			{
+				`${pow(1, 0)}`,
+				"1",
+				false,
+			},
+			{
+				`${pow(1, 1)}`,
+				"1",
+				false,
+			},
+
+			{
+				`${pow(2, 0)}`,
+				"1",
+				false,
+			},
+			{
+				`${pow(2, 1)}`,
+				"2",
+				false,
+			},
+			{
+				`${pow(3, 2)}`,
+				"9",
+				false,
+			},
+			{
+				`${pow(-3, 2)}`,
+				"9",
+				false,
+			},
+			{
+				`${pow(2, -2)}`,
+				"0.25",
+				false,
+			},
+			{
+				`${pow(0, 2)}`,
+				"0",
+				false,
+			},
+		},
+	})
+}
+
 func TestInterpolateFuncFloor(t *testing.T) {
 	testFunction(t, testFunctionConfig{
 		Cases: []testFunctionCase{

website/source/docs/configuration/interpolation.html.md

@@ -312,6 +312,12 @@ The supported built-in functions are:
   * `pathexpand(string)` - Returns a filepath string with `~` expanded to the home directory. Note:
     This will create a plan diff between two different hosts, unless the filepaths are the same.
 
+  * `pow(x, y)` - Returns the base `x` of exponential `y`.
+
+    Example:
+    * `${pow(3,2)}` = 9
+    * `${pow(4,0)}` = 1
+
   * `replace(string, search, replace)` - Does a search and replace on the
       given string. All instances of `search` are replaced with the value
       of `replace`. If `search` is wrapped in forward slashes, it is treated