Refining

examples/sample2.hcl

@@ -0,0 +1,13 @@
+io_mode = "async"
+
+service "http" "web_proxy" {
+  listen_addr = "127.0.0.1:8080"
+  
+  process "main" {
+    command = ["/usr/local/bin/awesome-app", "server"]
+  }
+
+  process "mgmt" {
+    command = ["/usr/local/bin/awesome-app", "mgmt"]
+  }
+}

pkg/yqlib/decoder_hcl.go

@@ -237,6 +237,36 @@ func convertHclExprToNode(expr hclsyntax.Expression, src []byte) *CandidateNode
 		// This ensures HCL roundtrips preserve quotes, and YAML properly quotes strings with ${}
 		node.Style = DoubleQuotedStyle
 		return node
+	case *hclsyntax.ScopeTraversalExpr:
+		// Simple identifier/traversal (e.g. unquoted string literal in HCL)
+		r := e.Range()
+		start := r.Start.Byte
+		end := r.End.Byte
+		if start >= 0 && end >= start && end <= len(src) {
+			text := strings.TrimSpace(string(src[start:end]))
+			return createStringScalarNode(text)
+		}
+		// Fallback to root name if source unavailable
+		if len(e.Traversal) > 0 {
+			if root, ok := e.Traversal[0].(hcl.TraverseRoot); ok {
+				return createStringScalarNode(root.Name)
+			}
+		}
+		return createStringScalarNode("")
+	case *hclsyntax.FunctionCallExpr:
+		// Preserve function calls as raw expressions for roundtrip
+		r := e.Range()
+		start := r.Start.Byte
+		end := r.End.Byte
+		if start >= 0 && end >= start && end <= len(src) {
+			text := strings.TrimSpace(string(src[start:end]))
+			node := createStringScalarNode(text)
+			node.Style = LiteralStyle
+			return node
+		}
+		node := createStringScalarNode(e.Name)
+		node.Style = LiteralStyle
+		return node
 	default:
 		// try to evaluate the expression (handles unary, binary ops, etc.)
 		val, diags := expr.Value(nil)
@@ -250,8 +280,10 @@ func convertHclExprToNode(expr hclsyntax.Expression, src []byte) *CandidateNode
 		end := r.End.Byte
 		if start >= 0 && end >= start && end <= len(src) {
 			text := string(src[start:end])
-			// Unquoted identifier - no style
-			return createStringScalarNode(text)
+			// Mark as raw expression so encoder can emit without quoting
+			node := createStringScalarNode(text)
+			node.Style = LiteralStyle
+			return node
 		}
 		return createStringScalarNode(fmt.Sprintf("%v", expr))
 	}

pkg/yqlib/encoder_hcl.go

@@ -88,6 +88,56 @@ func isValidHCLIdentifier(s string) bool {
 	return true
 }
 
+// tokensForRawHCLExpr produces a minimal token stream for a simple HCL expression so we can
+// write it without introducing quotes (e.g. function calls like upper(message)).
+func tokensForRawHCLExpr(expr string) (hclwrite.Tokens, error) {
+	var tokens hclwrite.Tokens
+	for i := 0; i < len(expr); {
+		ch := expr[i]
+		switch {
+		case ch == ' ' || ch == '\t':
+			i++
+			continue
+		case isHCLIdentifierStart(rune(ch)):
+			start := i
+			i++
+			for i < len(expr) && isHCLIdentifierPart(rune(expr[i])) {
+				i++
+			}
+			tokens = append(tokens, &hclwrite.Token{Type: hclsyntax.TokenIdent, Bytes: []byte(expr[start:i])})
+			continue
+		case ch >= '0' && ch <= '9':
+			start := i
+			i++
+			for i < len(expr) && ((expr[i] >= '0' && expr[i] <= '9') || expr[i] == '.') {
+				i++
+			}
+			tokens = append(tokens, &hclwrite.Token{Type: hclsyntax.TokenNumberLit, Bytes: []byte(expr[start:i])})
+			continue
+		case ch == '(':
+			tokens = append(tokens, &hclwrite.Token{Type: hclsyntax.TokenOParen, Bytes: []byte{'('}})
+		case ch == ')':
+			tokens = append(tokens, &hclwrite.Token{Type: hclsyntax.TokenCParen, Bytes: []byte{')'}})
+		case ch == ',':
+			tokens = append(tokens, &hclwrite.Token{Type: hclsyntax.TokenComma, Bytes: []byte{','}})
+		case ch == '.':
+			tokens = append(tokens, &hclwrite.Token{Type: hclsyntax.TokenDot, Bytes: []byte{'.'}})
+		case ch == '+':
+			tokens = append(tokens, &hclwrite.Token{Type: hclsyntax.TokenPlus, Bytes: []byte{'+'}})
+		case ch == '-':
+			tokens = append(tokens, &hclwrite.Token{Type: hclsyntax.TokenMinus, Bytes: []byte{'-'}})
+		case ch == '*':
+			tokens = append(tokens, &hclwrite.Token{Type: hclsyntax.TokenStar, Bytes: []byte{'*'}})
+		case ch == '/':
+			tokens = append(tokens, &hclwrite.Token{Type: hclsyntax.TokenSlash, Bytes: []byte{'/'}})
+		default:
+			return nil, fmt.Errorf("unsupported character %q in raw HCL expression", ch)
+		}
+		i++
+	}
+	return tokens, nil
+}
+
 // encodeNode encodes a CandidateNode directly to HCL, preserving style information
 func (he *hclEncoder) encodeNode(body *hclwrite.Body, node *CandidateNode) error {
 	if node.Kind != MappingNode {
@@ -110,6 +160,14 @@ func (he *hclEncoder) encodeNode(body *hclwrite.Body, node *CandidateNode) error
 			// Render as attribute: key = value
 			// Check the style to determine how to encode strings
 			if valueNode.Kind == ScalarNode && valueNode.Tag == "!!str" {
+				if valueNode.Style&LiteralStyle != 0 {
+					tokens, err := tokensForRawHCLExpr(valueNode.Value)
+					if err != nil {
+						return err
+					}
+					body.SetAttributeRaw(key, tokens)
+					continue
+				}
 				// Check style: DoubleQuotedStyle means template, no style could be unquoted or regular
 				// To distinguish unquoted from regular, we check if the value is a valid identifier
 				if valueNode.Style&DoubleQuotedStyle != 0 && strings.Contains(valueNode.Value, "${") {
@@ -199,6 +257,14 @@ func (he *hclEncoder) encodeNodeAttributes(body *hclwrite.Body, node *CandidateN
 
 		// Check if this is an unquoted identifier (no DoubleQuotedStyle)
 		if valueNode.Kind == ScalarNode && valueNode.Tag == "!!str" && valueNode.Style&DoubleQuotedStyle == 0 {
+			if valueNode.Style&LiteralStyle != 0 {
+				tokens, err := tokensForRawHCLExpr(valueNode.Value)
+				if err != nil {
+					return err
+				}
+				body.SetAttributeRaw(key, tokens)
+				continue
+			}
 			// Unquoted identifier - use traversal
 			traversal := hcl.Traversal{
 				hcl.TraverseRoot{Name: valueNode.Value},

pkg/yqlib/hcl_test.go

@@ -37,6 +37,18 @@ var hclFormatScenarios = []formatScenario{
 		expected:     "message = \"Hello, ${name}!\"\n",
 		scenarioType: "roundtrip",
 	},
+	{
+		description:  "Function roundtrip",
+		input:        `shouty_message = upper(message)`,
+		expected:     "shouty_message = upper(message)\n",
+		scenarioType: "roundtrip",
+	},
+	{
+		description:  "Arithmetic roundtrip",
+		input:        `sum = 1 + addend`,
+		expected:     "sum = 1 + addend\n",
+		scenarioType: "roundtrip",
+	},
 	{
 		description:  "number attribute",
 		input:        `port = 8080`,