implement dirname function

lang/funcs/filesystem.go

@@ -77,6 +77,20 @@ var BasenameFunc = function.New(&function.Spec{
 	},
 })
 
+// DirnameFunc constructs a function that takes a string containing a filesystem path and removes the last portion from it.
+var DirnameFunc = function.New(&function.Spec{
+	Params: []function.Parameter{
+		{
+			Name: "path",
+			Type: cty.String,
+		},
+	},
+	Type: function.StaticReturnType(cty.String),
+	Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
+		return cty.StringVal(filepath.Dir(args[0].AsString())), nil
+	},
+})
+
 // File reads the contents of the file at the given path.
 //
 // The file must contain valid UTF-8 bytes, or this function will return an error.
@@ -110,3 +124,13 @@ func FileBase64(baseDir string, path cty.Value) (cty.Value, error) {
 func Basename(path cty.Value) (cty.Value, error) {
 	return BasenameFunc.Call([]cty.Value{path})
 }
+
+// Dirname takes a string containing a filesystem path and removes the last portion from it.
+//
+// The underlying function implementation works only with the path string and does not access the filesystem itself.
+// It is therefore unable to take into account filesystem features such as symlinks.
+//
+// If the path is empty then the result is ".", representing the current working directory.
+func Dirname(path cty.Value) (cty.Value, error) {
+	return DirnameFunc.Call([]cty.Value{path})
+}

lang/funcs/filesystem_test.go

@@ -141,3 +141,53 @@ func TestBasename(t *testing.T) {
 		})
 	}
 }
+
+func TestDirname(t *testing.T) {
+	tests := []struct {
+		Path cty.Value
+		Want cty.Value
+		Err  bool
+	}{
+		{
+			cty.StringVal("testdata/hello.txt"),
+			cty.StringVal("testdata"),
+			false,
+		},
+		{
+			cty.StringVal("testdata/foo/hello.txt"),
+			cty.StringVal("testdata/foo"),
+			false,
+		},
+		{
+			cty.StringVal("hello.txt"),
+			cty.StringVal("."),
+			false,
+		},
+		{
+			cty.StringVal(""),
+			cty.StringVal("."),
+			false,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(fmt.Sprintf("Dirname(%#v)", test.Path), func(t *testing.T) {
+			got, err := Dirname(test.Path)
+
+			if test.Err {
+				if err == nil {
+					t.Fatal("succeeded; want error")
+				}
+				return
+			} else {
+				if err != nil {
+					t.Fatalf("unexpected error: %s", err)
+				}
+			}
+
+			if !got.RawEquals(test.Want) {
+				t.Errorf("wrong result\ngot:  %#v\nwant: %#v", got, test.Want)
+			}
+		})
+	}
+}

lang/functions.go

@@ -47,7 +47,7 @@ func (s *Scope) Functions() map[string]function.Function {
 			"concat":       stdlib.ConcatFunc,
 			"contains":     unimplFunc, // TODO
 			"csvdecode":    stdlib.CSVDecodeFunc,
-			"dirname":      unimplFunc, // TODO
+			"dirname":      funcs.DirnameFunc,
 			"distinct":     unimplFunc, // TODO
 			"element":      funcs.ElementFunc,
 			"chunklist":    unimplFunc, // TODO