Implement computed range expansion for loops (gt-8tmz.27)

Add support for for-each expansion over computed ranges:

  loop:
    range: "1..2^{disks}-1"  # Evaluated at cook time
    var: move_num
    body: [...]

Features:
- Range field in LoopSpec for computed iterations
- Var field to expose iteration value to body steps
- Expression evaluator supporting + - * / ^ and parentheses
- Variable substitution in range expressions using {name} syntax
- Title/description variable substitution in expanded steps

Example use case: Towers of Hanoi formula where step count is 2^n-1

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/formula/controlflow.go

@@ -59,12 +59,23 @@ func validateLoopSpec(loop *LoopSpec, stepID string) error {
 		return fmt.Errorf("loop %q: body is required", stepID)
 	}
 
-	if loop.Count > 0 && loop.Until != "" {
-		return fmt.Errorf("loop %q: cannot have both count and until", stepID)
+	// Count the number of loop types specified
+	loopTypes := 0
+	if loop.Count > 0 {
+		loopTypes++
+	}
+	if loop.Until != "" {
+		loopTypes++
+	}
+	if loop.Range != "" {
+		loopTypes++
 	}
 
-	if loop.Count == 0 && loop.Until == "" {
-		return fmt.Errorf("loop %q: either count or until is required", stepID)
+	if loopTypes == 0 {
+		return fmt.Errorf("loop %q: one of count, until, or range is required", stepID)
+	}
+	if loopTypes > 1 {
+		return fmt.Errorf("loop %q: only one of count, until, or range can be specified", stepID)
 	}
 
 	if loop.Until != "" && loop.Max == 0 {
@@ -86,17 +97,30 @@ func validateLoopSpec(loop *LoopSpec, stepID string) error {
 		}
 	}
 
+	// Validate range syntax if present (gt-8tmz.27)
+	if loop.Range != "" {
+		if err := ValidateRange(loop.Range); err != nil {
+			return fmt.Errorf("loop %q: invalid range %q: %w", stepID, loop.Range, err)
+		}
+	}
+
 	return nil
 }
 
 // expandLoop expands a loop step into its constituent steps.
 func expandLoop(step *Step) ([]*Step, error) {
+	return expandLoopWithVars(step, nil)
+}
+
+// expandLoopWithVars expands a loop step using the given variable context.
+// The vars map is used to resolve range expressions with variables.
+func expandLoopWithVars(step *Step, vars map[string]string) ([]*Step, error) {
 	var result []*Step
 
 	if step.Loop.Count > 0 {
 		// Fixed-count loop: expand body N times
 		for i := 1; i <= step.Loop.Count; i++ {
-			iterSteps, err := expandLoopIteration(step, i)
+			iterSteps, err := expandLoopIteration(step, i, nil)
 			if err != nil {
 				return nil, err
 			}
@@ -115,10 +139,50 @@ func expandLoop(step *Step) ([]*Step, error) {
 		if step.Loop.Count > 1 {
 			result = chainExpandedIterations(result, step.ID, step.Loop.Count)
 		}
+	} else if step.Loop.Range != "" {
+		// Range loop (gt-8tmz.27): expand body for each value in the computed range
+		rangeSpec, err := ParseRange(step.Loop.Range, vars)
+		if err != nil {
+			return nil, fmt.Errorf("loop %q: %w", step.ID, err)
+		}
+
+		// Validate range
+		if rangeSpec.End < rangeSpec.Start {
+			return nil, fmt.Errorf("loop %q: range end (%d) is less than start (%d)",
+				step.ID, rangeSpec.End, rangeSpec.Start)
+		}
+
+		// Expand body for each value in range
+		count := rangeSpec.End - rangeSpec.Start + 1
+		iterNum := 0
+		for val := rangeSpec.Start; val <= rangeSpec.End; val++ {
+			iterNum++
+			// Build iteration vars: include the loop variable if specified
+			iterVars := make(map[string]string)
+			if step.Loop.Var != "" {
+				iterVars[step.Loop.Var] = fmt.Sprintf("%d", val)
+			}
+			iterSteps, err := expandLoopIteration(step, iterNum, iterVars)
+			if err != nil {
+				return nil, err
+			}
+			result = append(result, iterSteps...)
+		}
+
+		// Recursively expand any nested loops FIRST (gt-zn35j)
+		result, err = ApplyLoops(result)
+		if err != nil {
+			return nil, err
+		}
+
+		// THEN chain iterations on the expanded result
+		if count > 1 {
+			result = chainExpandedIterations(result, step.ID, count)
+		}
 	} else {
 		// Conditional loop: expand once with loop metadata
 		// The runtime executor will re-run until condition is met or max reached
-		iterSteps, err := expandLoopIteration(step, 1)
+		iterSteps, err := expandLoopIteration(step, 1, nil)
 		if err != nil {
 			return nil, err
 		}
@@ -147,7 +211,8 @@ func expandLoop(step *Step) ([]*Step, error) {
 
 // expandLoopIteration expands a single iteration of a loop.
 // The iteration index is used to generate unique step IDs.
-func expandLoopIteration(step *Step, iteration int) ([]*Step, error) {
+// The iterVars map contains loop variable bindings for this iteration (gt-8tmz.27).
+func expandLoopIteration(step *Step, iteration int, iterVars map[string]string) ([]*Step, error) {
 	result := make([]*Step, 0, len(step.Loop.Body))
 
 	// Build set of step IDs within the loop body (for dependency rewriting)
@@ -157,10 +222,14 @@ func expandLoopIteration(step *Step, iteration int) ([]*Step, error) {
 		// Create unique ID for this iteration
 		iterID := fmt.Sprintf("%s.iter%d.%s", step.ID, iteration, bodyStep.ID)
 
+		// Substitute loop variables in title and description (gt-8tmz.27)
+		title := substituteLoopVars(bodyStep.Title, iterVars)
+		description := substituteLoopVars(bodyStep.Description, iterVars)
+
 		clone := &Step{
 			ID:             iterID,
-			Title:          bodyStep.Title,
-			Description:    bodyStep.Description,
+			Title:          title,
+			Description:    description,
 			Type:           bodyStep.Type,
 			Priority:       bodyStep.Priority,
 			Assignee:       bodyStep.Assignee,
@@ -170,16 +239,20 @@ func expandLoopIteration(step *Step, iteration int) ([]*Step, error) {
 			Gate:           bodyStep.Gate,
 			Loop:           cloneLoopSpec(bodyStep.Loop), // Support nested loops (gt-zn35j)
 			OnComplete:     cloneOnComplete(bodyStep.OnComplete),
-			SourceFormula:  bodyStep.SourceFormula,                                                     // Preserve source (gt-8tmz.18)
+			SourceFormula:  bodyStep.SourceFormula,                                       // Preserve source (gt-8tmz.18)
 			SourceLocation: fmt.Sprintf("%s.iter%d", bodyStep.SourceLocation, iteration), // Track iteration
 		}
 
-		// Clone ExpandVars if present
-		if len(bodyStep.ExpandVars) > 0 {
-			clone.ExpandVars = make(map[string]string, len(bodyStep.ExpandVars))
+		// Clone ExpandVars if present, adding loop vars (gt-8tmz.27)
+		if len(bodyStep.ExpandVars) > 0 || len(iterVars) > 0 {
+			clone.ExpandVars = make(map[string]string)
 			for k, v := range bodyStep.ExpandVars {
 				clone.ExpandVars[k] = v
 			}
+			// Add loop variables to ExpandVars for nested expansion
+			for k, v := range iterVars {
+				clone.ExpandVars[k] = v
+			}
 		}
 
 		// Clone labels
@@ -203,6 +276,17 @@ func expandLoopIteration(step *Step, iteration int) ([]*Step, error) {
 	return result, nil
 }
 
+// substituteLoopVars replaces {varname} placeholders with values from vars map.
+func substituteLoopVars(s string, vars map[string]string) string {
+	if vars == nil || s == "" {
+		return s
+	}
+	for k, v := range vars {
+		s = strings.ReplaceAll(s, "{"+k+"}", v)
+	}
+	return s
+}
+
 // collectBodyStepIDs collects all step IDs within a loop body (including nested children).
 func collectBodyStepIDs(body []*Step) map[string]bool {
 	ids := make(map[string]bool)
@@ -521,6 +605,8 @@ func cloneLoopSpec(loop *LoopSpec) *LoopSpec {
 		Count: loop.Count,
 		Until: loop.Until,
 		Max:   loop.Max,
+		Range: loop.Range, // gt-8tmz.27
+		Var:   loop.Var,   // gt-8tmz.27
 	}
 	if len(loop.Body) > 0 {
 		clone.Body = make([]*Step, len(loop.Body))

internal/formula/controlflow_test.go

@@ -822,3 +822,159 @@ func TestApplyGates_Immutability(t *testing.T) {
 		t.Errorf("Result deploy step should have gate label, got %v", deployResult.Labels)
 	}
 }
+
+// TestApplyLoops_Range tests computed range expansion (gt-8tmz.27).
+func TestApplyLoops_Range(t *testing.T) {
+	// Create a step with a range loop
+	steps := []*Step{
+		{
+			ID:    "moves",
+			Title: "Tower moves",
+			Loop: &LoopSpec{
+				Range: "1..3",
+				Var:   "move_num",
+				Body: []*Step{
+					{ID: "move", Title: "Move {move_num}"},
+				},
+			},
+		},
+	}
+
+	result, err := ApplyLoops(steps)
+	if err != nil {
+		t.Fatalf("ApplyLoops failed: %v", err)
+	}
+
+	// Should have 3 steps (range 1..3 = 3 iterations)
+	if len(result) != 3 {
+		t.Errorf("Expected 3 steps, got %d", len(result))
+	}
+
+	// Check step IDs and titles
+	expectedIDs := []string{
+		"moves.iter1.move",
+		"moves.iter2.move",
+		"moves.iter3.move",
+	}
+	expectedTitles := []string{
+		"Move 1",
+		"Move 2",
+		"Move 3",
+	}
+
+	for i, expected := range expectedIDs {
+		if i >= len(result) {
+			t.Errorf("Missing step %d: %s", i, expected)
+			continue
+		}
+		if result[i].ID != expected {
+			t.Errorf("Step %d: expected ID %s, got %s", i, expected, result[i].ID)
+		}
+		if result[i].Title != expectedTitles[i] {
+			t.Errorf("Step %d: expected Title %q, got %q", i, expectedTitles[i], result[i].Title)
+		}
+	}
+}
+
+// TestApplyLoops_RangeComputed tests computed range with expressions.
+func TestApplyLoops_RangeComputed(t *testing.T) {
+	// Create a step with a computed range loop (like Towers of Hanoi)
+	steps := []*Step{
+		{
+			ID:    "hanoi",
+			Title: "Hanoi moves",
+			Loop: &LoopSpec{
+				Range: "1..2^3-1", // 1..7 (2^3-1 moves for 3 disks)
+				Var:   "step_num",
+				Body: []*Step{
+					{ID: "step", Title: "Step {step_num}"},
+				},
+			},
+		},
+	}
+
+	result, err := ApplyLoops(steps)
+	if err != nil {
+		t.Fatalf("ApplyLoops failed: %v", err)
+	}
+
+	// Should have 7 steps (2^3-1 = 7)
+	if len(result) != 7 {
+		t.Errorf("Expected 7 steps, got %d", len(result))
+	}
+
+	// Check first and last step
+	if len(result) >= 1 {
+		if result[0].Title != "Step 1" {
+			t.Errorf("First step title: expected 'Step 1', got %q", result[0].Title)
+		}
+	}
+	if len(result) >= 7 {
+		if result[6].Title != "Step 7" {
+			t.Errorf("Last step title: expected 'Step 7', got %q", result[6].Title)
+		}
+	}
+}
+
+// TestValidateLoopSpec_Range tests validation of range loops.
+func TestValidateLoopSpec_Range(t *testing.T) {
+	tests := []struct {
+		name    string
+		loop    *LoopSpec
+		wantErr bool
+	}{
+		{
+			name: "valid range",
+			loop: &LoopSpec{
+				Range: "1..10",
+				Body:  []*Step{{ID: "step"}},
+			},
+			wantErr: false,
+		},
+		{
+			name: "valid computed range",
+			loop: &LoopSpec{
+				Range: "1..2^3",
+				Var:   "n",
+				Body:  []*Step{{ID: "step"}},
+			},
+			wantErr: false,
+		},
+		{
+			name: "invalid - both count and range",
+			loop: &LoopSpec{
+				Count: 5,
+				Range: "1..10",
+				Body:  []*Step{{ID: "step"}},
+			},
+			wantErr: true,
+		},
+		{
+			name: "invalid - both until and range",
+			loop: &LoopSpec{
+				Until: "step.status == 'complete'",
+				Max:   10,
+				Range: "1..10",
+				Body:  []*Step{{ID: "step"}},
+			},
+			wantErr: true,
+		},
+		{
+			name: "invalid range syntax",
+			loop: &LoopSpec{
+				Range: "invalid",
+				Body:  []*Step{{ID: "step"}},
+			},
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := validateLoopSpec(tt.loop, "test")
+			if (err != nil) != tt.wantErr {
+				t.Errorf("validateLoopSpec() error = %v, wantErr %v", err, tt.wantErr)
+			}
+		})
+	}
+}

internal/formula/range.go

@@ -0,0 +1,381 @@
+// Package formula provides range expression evaluation for computed loops.
+//
+// Range expressions enable loops with computed bounds (gt-8tmz.27):
+//
+//	range: "1..10"           // Simple integer range
+//	range: "1..2^{disks}"    // Expression with variable
+//	range: "{start}..{end}"  // Variable bounds
+//
+// Supports: + - * / ^ (power) and parentheses.
+// Variables use {name} syntax and are substituted from the vars map.
+package formula
+
+import (
+	"fmt"
+	"math"
+	"regexp"
+	"strconv"
+	"strings"
+	"unicode"
+)
+
+// RangeSpec represents a parsed range expression.
+type RangeSpec struct {
+	Start int // Evaluated start value (inclusive)
+	End   int // Evaluated end value (inclusive)
+}
+
+// rangePattern matches "start..end" format.
+var rangePattern = regexp.MustCompile(`^(.+)\.\.(.+)$`)
+
+// rangeVarPattern matches {varname} placeholders in range expressions.
+var rangeVarPattern = regexp.MustCompile(`\{(\w+)\}`)
+
+// ParseRange parses a range expression and evaluates it using the given variables.
+// Returns the start and end values of the range.
+//
+// Examples:
+//
+//	ParseRange("1..10", nil)           -> {1, 10}
+//	ParseRange("1..2^3", nil)          -> {1, 8}
+//	ParseRange("1..2^{n}", {"n":"3"})  -> {1, 8}
+func ParseRange(expr string, vars map[string]string) (*RangeSpec, error) {
+	expr = strings.TrimSpace(expr)
+	if expr == "" {
+		return nil, fmt.Errorf("empty range expression")
+	}
+
+	// Parse start..end format
+	m := rangePattern.FindStringSubmatch(expr)
+	if m == nil {
+		return nil, fmt.Errorf("invalid range format %q: expected start..end", expr)
+	}
+
+	startExpr := strings.TrimSpace(m[1])
+	endExpr := strings.TrimSpace(m[2])
+
+	// Evaluate start expression
+	start, err := EvaluateExpr(startExpr, vars)
+	if err != nil {
+		return nil, fmt.Errorf("evaluating range start %q: %w", startExpr, err)
+	}
+
+	// Evaluate end expression
+	end, err := EvaluateExpr(endExpr, vars)
+	if err != nil {
+		return nil, fmt.Errorf("evaluating range end %q: %w", endExpr, err)
+	}
+
+	return &RangeSpec{Start: start, End: end}, nil
+}
+
+// EvaluateExpr evaluates a mathematical expression with variable substitution.
+// Supports: + - * / ^ (power) and parentheses.
+// Variables use {name} syntax.
+func EvaluateExpr(expr string, vars map[string]string) (int, error) {
+	// Substitute variables first
+	expr = substituteVars(expr, vars)
+
+	// Tokenize and parse
+	tokens, err := tokenize(expr)
+	if err != nil {
+		return 0, err
+	}
+
+	result, err := parseExpr(tokens)
+	if err != nil {
+		return 0, err
+	}
+
+	return int(result), nil
+}
+
+// substituteVars replaces {varname} with values from vars map.
+func substituteVars(expr string, vars map[string]string) string {
+	if vars == nil {
+		return expr
+	}
+	return rangeVarPattern.ReplaceAllStringFunc(expr, func(match string) string {
+		name := match[1 : len(match)-1] // Remove { and }
+		if val, ok := vars[name]; ok {
+			return val
+		}
+		return match // Leave unresolved
+	})
+}
+
+// Token types for expression parsing.
+type tokenType int
+
+const (
+	tokNumber tokenType = iota
+	tokPlus
+	tokMinus
+	tokMul
+	tokDiv
+	tokPow
+	tokLParen
+	tokRParen
+	tokEOF
+)
+
+type token struct {
+	typ tokenType
+	val float64
+}
+
+// tokenize converts expression string to tokens.
+func tokenize(expr string) ([]token, error) {
+	var tokens []token
+	i := 0
+
+	for i < len(expr) {
+		ch := expr[i]
+
+		// Skip whitespace
+		if unicode.IsSpace(rune(ch)) {
+			i++
+			continue
+		}
+
+		// Number
+		if unicode.IsDigit(rune(ch)) {
+			j := i
+			for j < len(expr) && (unicode.IsDigit(rune(expr[j])) || expr[j] == '.') {
+				j++
+			}
+			val, err := strconv.ParseFloat(expr[i:j], 64)
+			if err != nil {
+				return nil, fmt.Errorf("invalid number %q", expr[i:j])
+			}
+			tokens = append(tokens, token{tokNumber, val})
+			i = j
+			continue
+		}
+
+		// Operators
+		switch ch {
+		case '+':
+			tokens = append(tokens, token{tokPlus, 0})
+		case '-':
+			// Could be unary minus or subtraction
+			// If previous token is not a number or right paren, it's unary
+			if len(tokens) == 0 || (tokens[len(tokens)-1].typ != tokNumber && tokens[len(tokens)-1].typ != tokRParen) {
+				// Unary minus: parse the number with the minus
+				j := i + 1
+				for j < len(expr) && (unicode.IsDigit(rune(expr[j])) || expr[j] == '.') {
+					j++
+				}
+				if j > i+1 {
+					val, err := strconv.ParseFloat(expr[i:j], 64)
+					if err != nil {
+						return nil, fmt.Errorf("invalid number %q", expr[i:j])
+					}
+					tokens = append(tokens, token{tokNumber, val})
+					i = j
+					continue
+				}
+			}
+			tokens = append(tokens, token{tokMinus, 0})
+		case '*':
+			tokens = append(tokens, token{tokMul, 0})
+		case '/':
+			tokens = append(tokens, token{tokDiv, 0})
+		case '^':
+			tokens = append(tokens, token{tokPow, 0})
+		case '(':
+			tokens = append(tokens, token{tokLParen, 0})
+		case ')':
+			tokens = append(tokens, token{tokRParen, 0})
+		default:
+			return nil, fmt.Errorf("unexpected character %q in expression", ch)
+		}
+		i++
+	}
+
+	tokens = append(tokens, token{tokEOF, 0})
+	return tokens, nil
+}
+
+// Parser state
+type exprParser struct {
+	tokens []token
+	pos    int
+}
+
+func (p *exprParser) current() token {
+	if p.pos >= len(p.tokens) {
+		return token{tokEOF, 0}
+	}
+	return p.tokens[p.pos]
+}
+
+func (p *exprParser) advance() {
+	p.pos++
+}
+
+// parseExpr parses an expression using recursive descent.
+// Handles operator precedence: + - < * / < ^
+func parseExpr(tokens []token) (float64, error) {
+	p := &exprParser{tokens: tokens}
+	result, err := p.parseAddSub()
+	if err != nil {
+		return 0, err
+	}
+	if p.current().typ != tokEOF {
+		return 0, fmt.Errorf("unexpected token after expression")
+	}
+	return result, nil
+}
+
+// parseAddSub handles + and - (lowest precedence)
+func (p *exprParser) parseAddSub() (float64, error) {
+	left, err := p.parseMulDiv()
+	if err != nil {
+		return 0, err
+	}
+
+	for {
+		switch p.current().typ {
+		case tokPlus:
+			p.advance()
+			right, err := p.parseMulDiv()
+			if err != nil {
+				return 0, err
+			}
+			left += right
+		case tokMinus:
+			p.advance()
+			right, err := p.parseMulDiv()
+			if err != nil {
+				return 0, err
+			}
+			left -= right
+		default:
+			return left, nil
+		}
+	}
+}
+
+// parseMulDiv handles * and /
+func (p *exprParser) parseMulDiv() (float64, error) {
+	left, err := p.parsePow()
+	if err != nil {
+		return 0, err
+	}
+
+	for {
+		switch p.current().typ {
+		case tokMul:
+			p.advance()
+			right, err := p.parsePow()
+			if err != nil {
+				return 0, err
+			}
+			left *= right
+		case tokDiv:
+			p.advance()
+			right, err := p.parsePow()
+			if err != nil {
+				return 0, err
+			}
+			if right == 0 {
+				return 0, fmt.Errorf("division by zero")
+			}
+			left /= right
+		default:
+			return left, nil
+		}
+	}
+}
+
+// parsePow handles ^ (power, highest binary precedence, right-associative)
+func (p *exprParser) parsePow() (float64, error) {
+	base, err := p.parseUnary()
+	if err != nil {
+		return 0, err
+	}
+
+	if p.current().typ == tokPow {
+		p.advance()
+		exp, err := p.parsePow() // Right-associative
+		if err != nil {
+			return 0, err
+		}
+		return math.Pow(base, exp), nil
+	}
+
+	return base, nil
+}
+
+// parseUnary handles unary minus
+func (p *exprParser) parseUnary() (float64, error) {
+	if p.current().typ == tokMinus {
+		p.advance()
+		val, err := p.parseUnary()
+		if err != nil {
+			return 0, err
+		}
+		return -val, nil
+	}
+	return p.parsePrimary()
+}
+
+// parsePrimary handles numbers and parentheses
+func (p *exprParser) parsePrimary() (float64, error) {
+	switch p.current().typ {
+	case tokNumber:
+		val := p.current().val
+		p.advance()
+		return val, nil
+	case tokLParen:
+		p.advance()
+		val, err := p.parseAddSub()
+		if err != nil {
+			return 0, err
+		}
+		if p.current().typ != tokRParen {
+			return 0, fmt.Errorf("expected closing parenthesis")
+		}
+		p.advance()
+		return val, nil
+	default:
+		return 0, fmt.Errorf("unexpected token in expression")
+	}
+}
+
+// ValidateRange validates a range expression without evaluating it.
+// Useful for syntax checking during formula validation.
+func ValidateRange(expr string) error {
+	expr = strings.TrimSpace(expr)
+	if expr == "" {
+		return fmt.Errorf("empty range expression")
+	}
+
+	m := rangePattern.FindStringSubmatch(expr)
+	if m == nil {
+		return fmt.Errorf("invalid range format: expected start..end")
+	}
+
+	// Check that expressions parse (with placeholder vars)
+	placeholderVars := make(map[string]string)
+	rangeVarPattern.ReplaceAllStringFunc(expr, func(match string) string {
+		name := match[1 : len(match)-1]
+		placeholderVars[name] = "1" // Use 1 as placeholder
+		return "1"
+	})
+
+	startExpr := strings.TrimSpace(m[1])
+	startExpr = substituteVars(startExpr, placeholderVars)
+	if _, err := tokenize(startExpr); err != nil {
+		return fmt.Errorf("invalid start expression: %w", err)
+	}
+
+	endExpr := strings.TrimSpace(m[2])
+	endExpr = substituteVars(endExpr, placeholderVars)
+	if _, err := tokenize(endExpr); err != nil {
+		return fmt.Errorf("invalid end expression: %w", err)
+	}
+
+	return nil
+}

internal/formula/range_test.go

@@ -0,0 +1,256 @@
+package formula
+
+import (
+	"testing"
+)
+
+func TestEvaluateExpr(t *testing.T) {
+	tests := []struct {
+		name    string
+		expr    string
+		vars    map[string]string
+		want    int
+		wantErr bool
+	}{
+		{
+			name: "simple integer",
+			expr: "10",
+			want: 10,
+		},
+		{
+			name: "addition",
+			expr: "2+3",
+			want: 5,
+		},
+		{
+			name: "subtraction",
+			expr: "10-3",
+			want: 7,
+		},
+		{
+			name: "multiplication",
+			expr: "4*5",
+			want: 20,
+		},
+		{
+			name: "division",
+			expr: "20/4",
+			want: 5,
+		},
+		{
+			name: "power",
+			expr: "2^3",
+			want: 8,
+		},
+		{
+			name: "power of 2",
+			expr: "2^10",
+			want: 1024,
+		},
+		{
+			name: "complex expression",
+			expr: "2+3*4",
+			want: 14, // 2+(3*4) = 14, not (2+3)*4 = 20
+		},
+		{
+			name: "parentheses",
+			expr: "(2+3)*4",
+			want: 20,
+		},
+		{
+			name: "nested parentheses",
+			expr: "((2+3)*(4+1))",
+			want: 25,
+		},
+		{
+			name: "variable substitution",
+			expr: "{n}",
+			vars: map[string]string{"n": "5"},
+			want: 5,
+		},
+		{
+			name: "power with variable",
+			expr: "2^{n}",
+			vars: map[string]string{"n": "4"},
+			want: 16,
+		},
+		{
+			name: "multiple variables",
+			expr: "{a}+{b}",
+			vars: map[string]string{"a": "10", "b": "20"},
+			want: 30,
+		},
+		{
+			name: "towers of hanoi pattern",
+			expr: "2^{disks}-1",
+			vars: map[string]string{"disks": "3"},
+			want: 7, // 2^3-1 = 7
+		},
+		{
+			name: "negative result",
+			expr: "1-10",
+			want: -9,
+		},
+		{
+			name: "division by zero",
+			expr: "10/0",
+			wantErr: true,
+		},
+		{
+			name: "invalid expression",
+			expr: "2++3",
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := EvaluateExpr(tt.expr, tt.vars)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("EvaluateExpr(%q) error = %v, wantErr %v", tt.expr, err, tt.wantErr)
+				return
+			}
+			if !tt.wantErr && got != tt.want {
+				t.Errorf("EvaluateExpr(%q) = %v, want %v", tt.expr, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestParseRange(t *testing.T) {
+	tests := []struct {
+		name      string
+		expr      string
+		vars      map[string]string
+		wantStart int
+		wantEnd   int
+		wantErr   bool
+	}{
+		{
+			name:      "simple range",
+			expr:      "1..10",
+			wantStart: 1,
+			wantEnd:   10,
+		},
+		{
+			name:      "single value range",
+			expr:      "5..5",
+			wantStart: 5,
+			wantEnd:   5,
+		},
+		{
+			name:      "computed end",
+			expr:      "1..2^3",
+			wantStart: 1,
+			wantEnd:   8,
+		},
+		{
+			name:      "computed start and end",
+			expr:      "2*2..3*3",
+			wantStart: 4,
+			wantEnd:   9,
+		},
+		{
+			name:      "variables in range",
+			expr:      "1..{n}",
+			vars:      map[string]string{"n": "10"},
+			wantStart: 1,
+			wantEnd:   10,
+		},
+		{
+			name:      "towers of hanoi moves",
+			expr:      "1..2^{disks}-1",
+			vars:      map[string]string{"disks": "3"},
+			wantStart: 1,
+			wantEnd:   7,
+		},
+		{
+			name:      "both variables",
+			expr:      "{start}..{end}",
+			vars:      map[string]string{"start": "5", "end": "15"},
+			wantStart: 5,
+			wantEnd:   15,
+		},
+		{
+			name:    "empty expression",
+			expr:    "",
+			wantErr: true,
+		},
+		{
+			name:    "missing separator",
+			expr:    "1 10",
+			wantErr: true,
+		},
+		{
+			name:    "invalid start expression",
+			expr:    "abc..10",
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := ParseRange(tt.expr, tt.vars)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("ParseRange(%q) error = %v, wantErr %v", tt.expr, err, tt.wantErr)
+				return
+			}
+			if !tt.wantErr {
+				if got.Start != tt.wantStart {
+					t.Errorf("ParseRange(%q).Start = %v, want %v", tt.expr, got.Start, tt.wantStart)
+				}
+				if got.End != tt.wantEnd {
+					t.Errorf("ParseRange(%q).End = %v, want %v", tt.expr, got.End, tt.wantEnd)
+				}
+			}
+		})
+	}
+}
+
+func TestValidateRange(t *testing.T) {
+	tests := []struct {
+		name    string
+		expr    string
+		wantErr bool
+	}{
+		{
+			name:    "valid simple range",
+			expr:    "1..10",
+			wantErr: false,
+		},
+		{
+			name:    "valid computed range",
+			expr:    "1..2^{n}",
+			wantErr: false,
+		},
+		{
+			name:    "valid complex range",
+			expr:    "{start}..{end}*2",
+			wantErr: false,
+		},
+		{
+			name:    "empty",
+			expr:    "",
+			wantErr: true,
+		},
+		{
+			name:    "no separator",
+			expr:    "10",
+			wantErr: true,
+		},
+		{
+			name:    "invalid character",
+			expr:    "1..@10",
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := ValidateRange(tt.expr)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("ValidateRange(%q) error = %v, wantErr %v", tt.expr, err, tt.wantErr)
+			}
+		})
+	}
+}

internal/formula/types.go

@@ -215,7 +215,7 @@ type Gate struct {
 }
 
 // LoopSpec defines iteration over a body of steps (gt-8tmz.4).
-// Either Count or Until must be specified (not both).
+// One of Count, Until, or Range must be specified.
 type LoopSpec struct {
 	// Count is the fixed number of iterations.
 	// When set, the loop body is expanded Count times.
@@ -229,6 +229,19 @@ type LoopSpec struct {
 	// Required when Until is set, to prevent unbounded loops.
 	Max int `json:"max,omitempty"`
 
+	// Range specifies a computed range for iteration (gt-8tmz.27).
+	// Format: "start..end" where start and end can be:
+	//   - Integers: "1..10"
+	//   - Expressions: "1..2^{disks}" (evaluated at cook time)
+	//   - Variables: "{start}..{count}" (substituted from Vars)
+	// Supports: + - * / ^ (power) and parentheses.
+	Range string `json:"range,omitempty"`
+
+	// Var is the variable name exposed to body steps (gt-8tmz.27).
+	// For Range loops, this is set to the current iteration value.
+	// Example: var: "move_num" with range: "1..7" exposes {move_num}=1,2,...,7
+	Var string `json:"var,omitempty"`
+
 	// Body contains the steps to repeat.
 	Body []*Step `json:"body"`
 }