Add if-else conditional (#736)

Co-authored-by: Anton Medvedev <[email protected]>

Author: OlgaNovg

docs/language-definition.md

@@ -97,7 +97,7 @@ Backticks strings are raw strings, they do not support escape sequences.
     <tr>
         <td><strong>Conditional</strong></td>
         <td>
-            <code>?:</code> (ternary), <code>??</code> (nil coalescing)
+            <code>?:</code> (ternary), <code>??</code> (nil coalescing), <code>if {} else {}</code> (multiline)
         </td>
     </tr>
     <tr>

expr_test.go

@@ -1291,6 +1291,21 @@ func TestExpr(t *testing.T) {
 			`1 < 2 < 3 == true`,
 			true,
 		},
+		{
+			`if 1 > 2 { 333 * 2 + 1 } else { 444 }`,
+			444,
+		},
+		{
+			`let a = 3;
+			let b = 2;
+			if a>b {let c = Add(a, b); c+1} else {Add(10, b)}
+			`,
+			6,
+		},
+		{
+			`if "a" < "b" {let x = "a"; x} else {"abc"}`,
+			"a",
+		},
 	}
 
 	for _, tt := range tests {

parser/lexer/lexer_test.go

@@ -239,6 +239,42 @@ func TestLex(t *testing.T) {
 				{Kind: EOF},
 			},
 		},
+		{
+			`if a>b {x1+x2} else {x2}`,
+			[]Token{
+				{Kind: Operator, Value: "if"},
+				{Kind: Identifier, Value: "a"},
+				{Kind: Operator, Value: ">"},
+				{Kind: Identifier, Value: "b"},
+				{Kind: Bracket, Value: "{"},
+				{Kind: Identifier, Value: "x1"},
+				{Kind: Operator, Value: "+"},
+				{Kind: Identifier, Value: "x2"},
+				{Kind: Bracket, Value: "}"},
+				{Kind: Operator, Value: "else"},
+				{Kind: Bracket, Value: "{"},
+				{Kind: Identifier, Value: "x2"},
+				{Kind: Bracket, Value: "}"},
+				{Kind: EOF},
+			},
+		},
+		{
+			`a>b if {x1} else {x2}`,
+			[]Token{
+				{Kind: Identifier, Value: "a"},
+				{Kind: Operator, Value: ">"},
+				{Kind: Identifier, Value: "b"},
+				{Kind: Operator, Value: "if"},
+				{Kind: Bracket, Value: "{"},
+				{Kind: Identifier, Value: "x1"},
+				{Kind: Bracket, Value: "}"},
+				{Kind: Operator, Value: "else"},
+				{Kind: Bracket, Value: "{"},
+				{Kind: Identifier, Value: "x2"},
+				{Kind: Bracket, Value: "}"},
+				{Kind: EOF},
+			},
+		},
 	}
 
 	for _, test := range tests {

parser/lexer/state.go

@@ -129,9 +129,7 @@ loop:
 			switch l.word() {
 			case "not":
 				return not
-			case "in", "or", "and", "matches", "contains", "startsWith", "endsWith":
-				l.emit(Operator)
-			case "let":
+			case "in", "or", "and", "matches", "contains", "startsWith", "endsWith", "let", "if", "else":
 				l.emit(Operator)
 			default:
 				l.emit(Identifier)

parser/parser.go

@@ -132,6 +132,9 @@ func (p *parser) parseExpression(precedence int) Node {
 	if precedence == 0 && p.current.Is(Operator, "let") {
 		return p.parseVariableDeclaration()
 	}
+	if p.current.Is(Operator, "if") {
+		return p.parseConditionalIf()
+	}
 
 	nodeLeft := p.parsePrimary()
 
@@ -235,6 +238,25 @@ func (p *parser) parseVariableDeclaration() Node {
 	return let
 }
 
+func (p *parser) parseConditionalIf() Node {
+	p.next()
+	nodeCondition := p.parseExpression(0)
+	p.expect(Bracket, "{")
+	expr1 := p.parseExpression(0)
+	p.expect(Bracket, "}")
+	p.expect(Operator, "else")
+	p.expect(Bracket, "{")
+	expr2 := p.parseExpression(0)
+	p.expect(Bracket, "}")
+
+	return &ConditionalNode{
+		Cond: nodeCondition,
+		Exp1: expr1,
+		Exp2: expr2,
+	}
+
+}
+
 func (p *parser) parseConditional(node Node) Node {
 	var expr1, expr2 Node
 	for p.current.Is(Operator, "?") && p.err == nil {

parser/parser_test.go

@@ -647,6 +647,17 @@ world`},
 				Right: &BoolNode{Value: true},
 			},
 		},
+		{
+			"if a>b {true} else {x}",
+			&ConditionalNode{
+				Cond: &BinaryNode{
+					Operator: ">",
+					Left:     &IdentifierNode{Value: "a"},
+					Right:    &IdentifierNode{Value: "b"},
+				},
+				Exp1: &BoolNode{Value: true},
+				Exp2: &IdentifierNode{Value: "x"}},
+		},
 	}
 	for _, test := range tests {
 		t.Run(test.input, func(t *testing.T) {