Add ternary expressions

examples/include/tmpls/header.html

@@ -10,10 +10,10 @@
     </div>  
     <div class="navbar-menu" id="navbarMenu">
         <div class="navbar-end">
-            <a class='navbar-item #if(title == "Home"):is-active#!if' href="/">
+            <a class='navbar-item #(title == "Home" ? "is-active" : "")' href="/">
                 Home
             </a>
-            <a class='navbar-item #if(title == "About"):is-active#!if' href="/about">
+            <a class='navbar-item #(title == "About" ? "is-active" : "")' href="/about">
                 About
             </a>
         </div>

internal/ast/ast.go

@@ -199,3 +199,14 @@ func (l Logical) Pos() Position {
 func (l Logical) Op() string {
 	return l.Value
 }
+
+type Ternary struct {
+	Condition Node
+	IfTrue    Node
+	Else      Node
+	Position  Position
+}
+
+func (t Ternary) Pos() Position {
+	return t.Position
+}

internal/parser/salix.peg

@@ -55,13 +55,15 @@ EndTag = "#!" name:Ident {
     }, nil
 }
 
-ExprTag = "#(" expr:Expr ')' {
+ExprTag = "#(" item:Item ')' {
     return ast.ExprTag{
-        Value:    expr.(ast.Node),
+        Value:    item.(ast.Node),
         Position: getPos(c),
     }, nil
 }
 
+Item = Ternary / Expr
+
 Expr = first:ExprSegment rest:(_ Logical _ ExprSegment)* {
     restSlice := toAnySlice(rest)
     if len(restSlice) == 0 {
@@ -100,7 +102,7 @@ ExprSegment = first:Value rest:(_ Operator _ Value)* {
     return out, nil
 }
 
-ParamList = '(' params:(Expr ( ',' _ Expr )* )? ')' {
+ParamList = '(' params:(Item ( ',' _ Item )* )? ')' {
     paramSlice := toAnySlice(params)
     if len(paramSlice) == 0 {
         return []ast.Node{}, nil
@@ -121,6 +123,14 @@ Value = not:"!"? node:(MethodCall / FieldAccess / Index / String / RawString / F
     }, nil
 }
 
+Ternary = cond:Expr _ '?' _ ifTrue:Value _ ':' _ elseVal:Value {
+    return ast.Ternary{
+        Condition: cond.(ast.Node),
+        IfTrue:    ifTrue.(ast.Node),
+        Else:      elseVal.(ast.Node),
+    }, nil
+}
+
 MethodCall = value:Value '.' name:Ident params:ParamList {
     return ast.MethodCall{
         Value:    value.(ast.Node),

salix.go

@@ -27,6 +27,7 @@ var (
 	ErrFuncTooManyReturns   = errors.New("template functions can only have two return values")
 	ErrFuncNoReturns        = errors.New("template functions must return at least one value")
 	ErrFuncSecondReturnType = errors.New("the second return value of a template function must be an error")
+	ErrTernaryCondBool      = errors.New("ternary condition must be a boolean")
 )
 
 // HTML represents unescaped HTML strings
@@ -215,6 +216,8 @@ func (t *Template) getValue(node ast.Node, local map[string]any) (any, error) {
 		return t.getField(node, local)
 	case ast.MethodCall:
 		return t.execMethodCall(node, local)
+	case ast.Ternary:
+		return t.evalTernary(node, local)
 	default:
 		return nil, nil
 	}
@@ -427,6 +430,24 @@ func (t *Template) execFunc(fn reflect.Value, node ast.Node, args []ast.Node, lo
 	}
 }
 
+func (t *Template) evalTernary(tr ast.Ternary, local map[string]any) (any, error) {
+	condVal, err := t.getValue(tr.Condition, local)
+	if err != nil {
+		return nil, t.posError(tr.Condition, "%w", ErrTernaryCondBool)
+	}
+
+	cond, ok := condVal.(bool)
+	if !ok {
+		return nil, errors.New("ternary condition must be a boolean")
+	}
+
+	if cond {
+		return t.getValue(tr.IfTrue, local)
+	} else {
+		return t.getValue(tr.Else, local)
+	}
+}
+
 func (t *Template) posError(n ast.Node, format string, v ...any) error {
 	return ast.PosError(n, t.name, format, v...)
 }