new operator "?!" (alternate value)

common-errors.go

@@ -53,3 +53,9 @@ func ErrInvalidParameterValue(funcName, paramName string, paramValue any) error
 func ErrWrongParamType(funcName, paramName, paramType string, paramValue any) error {
 	return fmt.Errorf("%s(): the %q parameter must be a %s, got a %s (%v)", funcName, paramName, paramType, TypeName(paramValue), paramValue)
 }
+
+// --- Operator errors
+
+func ErrLeftOperandMustBeVariable(leftTerm, opTerm *term) error {
+	return leftTerm.Errorf("left operand of %q must be a variable", opTerm.source())
+}

operator-coalesce.go

@@ -1,85 +0,0 @@
-// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
-// All rights reserved.
-
-// operator-coalesce.go
-package expr
-
-//-------- null coalesce term
-
-func newNullCoalesceTerm(tk *Token) (inst *term) {
-	return &term{
-		tk:       *tk,
-		children: make([]*term, 0, 2),
-		position: posInfix,
-		priority: priCoalesce,
-		evalFunc: evalNullCoalesce,
-	}
-}
-
-func evalNullCoalesce(ctx ExprContext, self *term) (v any, err error) {
-	var rightValue any
-
-	if err = self.checkOperands(); err != nil {
-		return
-	}
-
-	leftTerm := self.children[0]
-	if leftTerm.tk.Sym != SymVariable {
-		err = leftTerm.Errorf("left operand of %q must be a variable", self.tk.source)
-		return
-	}
-
-	if leftValue, exists := ctx.GetVar(leftTerm.source()); exists {
-		v = leftValue
-	} else if rightValue, err = self.children[1].compute(ctx); err == nil {
-		v = rightValue
-	}
-	return
-}
-
-//-------- coalesce assign term
-
-func newCoalesceAssignTerm(tk *Token) (inst *term) {
-	return &term{
-		tk:       *tk,
-		children: make([]*term, 0, 2),
-		position: posInfix,
-		priority: priCoalesce,
-		evalFunc: evalAssignCoalesce,
-	}
-}
-
-func evalAssignCoalesce(ctx ExprContext, self *term) (v any, err error) {
-	var rightValue any
-
-	if err = self.checkOperands(); err != nil {
-		return
-	}
-
-	leftTerm := self.children[0]
-	if leftTerm.tk.Sym != SymVariable {
-		err = leftTerm.Errorf("left operand of %q must be a variable", self.tk.source)
-		return
-	}
-
-	if leftValue, exists := ctx.GetVar(leftTerm.source()); exists {
-		v = leftValue
-	} else if rightValue, err = self.children[1].compute(ctx); err == nil {
-		if functor, ok := rightValue.(Functor); ok {
-			//ctx.RegisterFunc(leftTerm.source(), functor, 0, -1)
-			ctx.RegisterFunc(leftTerm.source(), functor, TypeAny, []ExprFuncParam{
-				NewFuncParamFlag(ParamValue, PfDefault|PfRepeat),
-			})
-		} else {
-			v = rightValue
-			ctx.UnsafeSetVar(leftTerm.source(), rightValue)
-		}
-	}
-	return
-}
-
-// init
-func init() {
-	registerTermConstructor(SymDoubleQuestion, newNullCoalesceTerm)
-	registerTermConstructor(SymQuestionEqual, newCoalesceAssignTerm)
-}

operator-default.go

@@ -0,0 +1,124 @@
+// Copyright (c) 2024 Celestino Amoroso (celestino.amoroso@gmail.com).
+// All rights reserved.
+
+// operator-default.go
+package expr
+
+//-------- default term
+
+func newDefaultTerm(tk *Token) (inst *term) {
+	return &term{
+		tk:       *tk,
+		children: make([]*term, 0, 2),
+		position: posInfix,
+		priority: priCoalesce,
+		evalFunc: evalDefault,
+	}
+}
+
+func evalDefault(ctx ExprContext, self *term) (v any, err error) {
+	var rightValue any
+
+	if err = self.checkOperands(); err != nil {
+		return
+	}
+
+	leftTerm := self.children[0]
+	if leftTerm.tk.Sym != SymVariable {
+		// err = leftTerm.Errorf("left operand of %q must be a variable", self.tk.source)
+		err = ErrLeftOperandMustBeVariable(leftTerm, self)
+		return
+	}
+
+	if leftValue, exists := ctx.GetVar(leftTerm.source()); exists {
+		v = leftValue
+	} else if rightValue, err = self.children[1].compute(ctx); err == nil {
+		v = rightValue
+	}
+	return
+}
+
+//-------- alternate term
+
+func newAlternateTerm(tk *Token) (inst *term) {
+	return &term{
+		tk:       *tk,
+		children: make([]*term, 0, 2),
+		position: posInfix,
+		priority: priCoalesce,
+		evalFunc: evalAlternate,
+	}
+}
+
+func evalAlternate(ctx ExprContext, self *term) (v any, err error) {
+	var rightValue any
+
+	if err = self.checkOperands(); err != nil {
+		return
+	}
+
+	leftTerm := self.children[0]
+	if leftTerm.tk.Sym != SymVariable {
+		// err = leftTerm.Errorf("left operand of %q must be a variable", self.tk.source)
+		err = ErrLeftOperandMustBeVariable(leftTerm, self)
+		return
+	}
+
+	if leftValue, exists := ctx.GetVar(leftTerm.source()); exists && leftValue != nil {
+		if rightValue, err = self.children[1].compute(ctx); err == nil {
+			v = rightValue
+		}
+	} else {
+		v = leftValue
+	}
+	return
+}
+
+//-------- default assign term
+
+func newDefaultAssignTerm(tk *Token) (inst *term) {
+	return &term{
+		tk:       *tk,
+		children: make([]*term, 0, 2),
+		position: posInfix,
+		priority: priCoalesce,
+		evalFunc: evalAssignDefault,
+	}
+}
+
+func evalAssignDefault(ctx ExprContext, self *term) (v any, err error) {
+	var rightValue any
+
+	if err = self.checkOperands(); err != nil {
+		return
+	}
+
+	leftTerm := self.children[0]
+	if leftTerm.tk.Sym != SymVariable {
+		// err = leftTerm.Errorf("left operand of %q must be a variable", self.tk.source)
+		err = ErrLeftOperandMustBeVariable(leftTerm, self)
+		return
+	}
+
+	if leftValue, exists := ctx.GetVar(leftTerm.source()); exists {
+		v = leftValue
+	} else if rightValue, err = self.children[1].compute(ctx); err == nil {
+		if functor, ok := rightValue.(Functor); ok {
+			//ctx.RegisterFunc(leftTerm.source(), functor, 0, -1)
+			ctx.RegisterFunc(leftTerm.source(), functor, TypeAny, []ExprFuncParam{
+				NewFuncParamFlag(ParamValue, PfDefault|PfRepeat),
+			})
+		} else {
+			v = rightValue
+			ctx.UnsafeSetVar(leftTerm.source(), rightValue)
+		}
+	}
+	return
+}
+
+// init
+func init() {
+	registerTermConstructor(SymDoubleQuestion, newDefaultTerm)
+	registerTermConstructor(SymQuestionEqual, newDefaultAssignTerm)
+	registerTermConstructor(SymQuestionExclam, newAlternateTerm)
+}

scanner.go

@@ -202,8 +202,10 @@ func (self *scanner) fetchNextToken() (tk *Token) {
 		case '?':
 			if next, _ := self.peek(); next == '?' {
 				tk = self.moveOn(SymDoubleQuestion, ch, next)
-			} else if next, _ := self.peek(); next == '=' {
+			} else if next == '=' {
 				tk = self.moveOn(SymQuestionEqual, ch, next)
+			} else if next == '!' {
+				tk = self.moveOn(SymQuestionExclam, ch, next)
 			} else {
 				tk = self.makeToken(SymQuestion, ch)
 			}

symbol.go

@@ -57,16 +57,17 @@ const (
 	SymTilde                             //  46: '~'
 	SymDoubleQuestion                    //  47: '??'
 	SymQuestionEqual                     //  48: '?='
-	SymDoubleAt                          //  49: '@@'
-	SymDoubleColon                       //  50: '::'
-	SymInsert                            //  51: '>>'
-	SymAppend                            //  52: '<<'
-	SymCaret                             //  53: '^'
-	SymDollarRound                       //  54: '$('
-	SymOpenClosedRound                   //  55: '()'
-	SymDoubleDollar                      //  56: '$$'
-	SymDoubleDot                         //  57: '..'
-	SymTripleDot                         //  58: '...'
+	SymQuestionExclam                    //  49: '?!'
+	SymDoubleAt                          //  50: '@@'
+	SymDoubleColon                       //  51: '::'
+	SymInsert                            //  52: '>>'
+	SymAppend                            //  53: '<<'
+	SymCaret                             //  54: '^'
+	SymDollarRound                       //  55: '$('
+	SymOpenClosedRound                   //  56: '()'
+	SymDoubleDollar                      //  57: '$$'
+	SymDoubleDot                         //  58: '..'
+	SymTripleDot                         //  59: '...'
 	SymChangeSign
 	SymUnchangeSign
 	SymIdentifier