Add `basename` and `dirname` functions

Adds `basename` and `dirname` interpolation. I want to add a `stack` tag to our infrastructure, the value of which is set to `${basename(path.cwd)}`. We currently use `${replace(path.cwd, "/^.+\\//", "")}` instead, but this is extremeley unreadable. The existance of a `basename` function would be very useful for this use case.

I don't have an immediate use case for a `dirname` function, but it seemed reasonable to add it as well.

config/interpolate_funcs.go

@@ -11,6 +11,7 @@ import (
 	"io/ioutil"
 	"math"
 	"net"
+	"path/filepath"
 	"regexp"
 	"sort"
 	"strconv"
@@ -52,6 +53,7 @@ func listVariableValueToStringSlice(values []ast.Variable) ([]string, error) {
 // Funcs is the mapping of built-in functions for configuration.
 func Funcs() map[string]ast.Function {
 	return map[string]ast.Function{
+		"basename":     interpolationFuncBasename(),
 		"base64decode": interpolationFuncBase64Decode(),
 		"base64encode": interpolationFuncBase64Encode(),
 		"base64sha256": interpolationFuncBase64Sha256(),
@@ -62,6 +64,7 @@ func Funcs() map[string]ast.Function {
 		"coalesce":     interpolationFuncCoalesce(),
 		"compact":      interpolationFuncCompact(),
 		"concat":       interpolationFuncConcat(),
+		"dirname":      interpolationFuncDirname(),
 		"distinct":     interpolationFuncDistinct(),
 		"element":      interpolationFuncElement(),
 		"file":         interpolationFuncFile(),
@@ -600,6 +603,17 @@ func interpolationFuncIndex() ast.Function {
 	}
 }
 
+// interpolationFuncBasename implements the "dirname" function.
+func interpolationFuncDirname() ast.Function {
+	return ast.Function{
+		ArgTypes:   []ast.Type{ast.TypeString},
+		ReturnType: ast.TypeString,
+		Callback: func(args []interface{}) (interface{}, error) {
+			return filepath.Dir(args[0].(string)), nil
+		},
+	}
+}
+
 // interpolationFuncDistinct implements the "distinct" function that
 // removes duplicate elements from a list.
 func interpolationFuncDistinct() ast.Function {
@@ -1006,6 +1020,17 @@ func interpolationFuncValues(vs map[string]ast.Variable) ast.Function {
 	}
 }
 
+// interpolationFuncBasename implements the "basename" function.
+func interpolationFuncBasename() ast.Function {
+	return ast.Function{
+		ArgTypes:   []ast.Type{ast.TypeString},
+		ReturnType: ast.TypeString,
+		Callback: func(args []interface{}) (interface{}, error) {
+			return filepath.Base(args[0].(string)), nil
+		},
+	}
+}
+
 // interpolationFuncBase64Encode implements the "base64encode" function that
 // allows Base64 encoding.
 func interpolationFuncBase64Encode() ast.Function {

config/interpolate_funcs_test.go

@@ -852,6 +852,18 @@ func TestInterpolateFuncMerge(t *testing.T) {
 
 }
 
+func TestInterpolateFuncDirname(t *testing.T) {
+	testFunction(t, testFunctionConfig{
+		Cases: []testFunctionCase{
+			{
+				`${dirname("/foo/bar/baz")}`,
+				"/foo/bar",
+				false,
+			},
+		},
+	})
+}
+
 func TestInterpolateFuncDistinct(t *testing.T) {
 	testFunction(t, testFunctionConfig{
 		Cases: []testFunctionCase{
@@ -1777,6 +1789,18 @@ func TestInterpolateFuncElement(t *testing.T) {
 	})
 }
 
+func TestInterpolateFuncBasename(t *testing.T) {
+	testFunction(t, testFunctionConfig{
+		Cases: []testFunctionCase{
+			{
+				`${basename("/foo/bar/baz")}`,
+				"baz",
+				false,
+			},
+		},
+	})
+}
+
 func TestInterpolateFuncBase64Encode(t *testing.T) {
 	testFunction(t, testFunctionConfig{
 		Cases: []testFunctionCase{

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

@@ -140,6 +140,8 @@ syntax `name(arg, arg2, ...)`. For example, to read a file:
 
 The supported built-in functions are:
 
+  * `basename(path)` - Returns the last element of a path.
+
   * `base64decode(string)` - Given a base64-encoded string, decodes it and
     returns the original string.
 
@@ -183,6 +185,8 @@ The supported built-in functions are:
   * `concat(list1, list2, ...)` - Combines two or more lists into a single list.
      Example: `concat(aws_instance.db.*.tags.Name, aws_instance.web.*.tags.Name)`
 
+  * `dirname(path)` - Returns all but the last element of path, typically the path's directory.
+
   * `distinct(list)` - Removes duplicate items from a list. Keeps the first
      occurrence of each element, and removes subsequent occurrences. This
      function is only valid for flat lists. Example: `distinct(var.usernames)`