feat(compiler): elif let statement (#3888)

## Checklist

- [x] Title matches [Winglang's style guide](https://www.winglang.io/contributing/start-here/pull_requests#how-are-pull-request-titles-formatted)
- [ ] Description explains motivation and solution
- [x] Tests added (always)
- [ ] Docs updated (only required for features)
- [ ] Added `pr/e2e-full` label if this feature requires end-to-end testing

*By submitting this pull request, I confirm that my contribution is made under the terms of the [Wing Cloud Contribution License](https://github.com/winglang/wing/blob/main/CONTRIBUTION_LICENSE.md)*.

Author: marciocadev

examples/tests/valid/optionals.w

@@ -58,6 +58,19 @@ let tryParseName = (fullName: str): Name? => {
   };
 };
 
+let json_obj = Json { ghost: "spooky" };
+let var something_else = false;
+if let y = json_obj.tryAsBool() {
+  assert(y == true || y == false);
+} elif let y = json_obj.tryAsNum() {
+  assert(y + 0 == y);
+} elif let y = json_obj.tryAsStr() {
+  assert(y.length >= 0);
+} else {
+  something_else = true;
+}
+assert(something_else);
+
 // if lets reassignable
 let a: num? = 1;
 if let var z = a {

libs/tree-sitter-wing/grammar.js

@@ -281,9 +281,21 @@ module.exports = grammar({
         "=",
         field("value", $.expression),
         field("block", $.block),
+        repeat(field("elif_let_block", $.elif_let_block)),
         optional(seq("else", field("else_block", $.block)))
       ),
 
+    elif_let_block: ($) =>
+      seq(
+        "elif",
+        "let",
+        optional(field("reassignable", $.reassignable)),
+        field("name", $.identifier),
+        "=",
+        field("value", $.expression),
+        field("block", $.block)
+      ),
+
     if_statement: ($) =>
       seq(
         "if",

libs/tree-sitter-wing/test/corpus/statements/statements.txt

@@ -401,6 +401,27 @@ if let x = y {} else {}
     block: (block)
     else_block: (block)))
 
+================================================================================
+If Let Elif Let Else
+================================================================================
+
+if let x = y {} elif let x = z {} else {}
+
+--------------------------------------------------------------------------------
+
+(source
+  (if_let_statement
+    name: (identifier)
+    value: (reference
+      (reference_identifier))
+    block: (block)
+    elif_let_block: (elif_let_block
+      name: (identifier)
+      value: (reference
+        (reference_identifier))
+      block: (block))
+    else_block: (block)))
+
 ================================================================================
 If Let Var
 ================================================================================

libs/wingc/src/ast.rs

@@ -331,6 +331,14 @@ pub struct ElifBlock {
 	pub statements: Scope,
 }
 
+#[derive(Debug)]
+pub struct ElifLetBlock {
+	pub reassignable: bool,
+	pub var_name: Symbol,
+	pub value: Expr,
+	pub statements: Scope,
+}
+
 #[derive(Debug)]
 pub struct Class {
 	pub name: Symbol,
@@ -449,6 +457,7 @@ pub enum StmtKind {
 		var_name: Symbol,
 		value: Expr,
 		statements: Scope,
+		elif_statements: Vec<ElifLetBlock>,
 		else_statements: Option<Scope>,
 	},
 	If {

libs/wingc/src/fold.rs

@@ -1,9 +1,9 @@
 use crate::{
 	ast::{
-		ArgList, BringSource, CalleeKind, CatchBlock, Class, ClassField, ElifBlock, Expr, ExprKind, FunctionBody,
-		FunctionDefinition, FunctionParameter, FunctionSignature, Interface, InterpolatedString, InterpolatedStringPart,
-		Literal, NewExpr, Reference, Scope, Stmt, StmtKind, StructField, Symbol, TypeAnnotation, TypeAnnotationKind,
-		UserDefinedType,
+		ArgList, BringSource, CalleeKind, CatchBlock, Class, ClassField, ElifBlock, ElifLetBlock, Expr, ExprKind,
+		FunctionBody, FunctionDefinition, FunctionParameter, FunctionSignature, Interface, InterpolatedString,
+		InterpolatedStringPart, Literal, NewExpr, Reference, Scope, Stmt, StmtKind, StructField, Symbol, TypeAnnotation,
+		TypeAnnotationKind, UserDefinedType,
 	},
 	dbg_panic,
 };
@@ -118,12 +118,22 @@ where
 			statements,
 			reassignable,
 			var_name,
+			elif_statements,
 			else_statements,
 		} => StmtKind::IfLet {
 			value: f.fold_expr(value),
 			statements: f.fold_scope(statements),
 			reassignable,
 			var_name: f.fold_symbol(var_name),
+			elif_statements: elif_statements
+				.into_iter()
+				.map(|elif_let_block| ElifLetBlock {
+					reassignable: elif_let_block.reassignable,
+					statements: f.fold_scope(elif_let_block.statements),
+					value: f.fold_expr(elif_let_block.value),
+					var_name: f.fold_symbol(elif_let_block.var_name),
+				})
+				.collect(),
 			else_statements: else_statements.map(|statements| f.fold_scope(statements)),
 		},
 		StmtKind::If {

libs/wingc/src/jsify.rs

@@ -16,7 +16,7 @@ use std::{
 
 use crate::{
 	ast::{
-		ArgList, BinaryOperator, BringSource, CalleeKind, Class as AstClass, Expr, ExprKind, FunctionBody,
+		ArgList, BinaryOperator, BringSource, CalleeKind, Class as AstClass, ElifLetBlock, Expr, ExprKind, FunctionBody,
 		FunctionDefinition, InterpolatedStringPart, Literal, NewExpr, Phase, Reference, Scope, Stmt, StmtKind, StructField,
 		Symbol, TypeAnnotationKind, UnaryOperator, UserDefinedType,
 	},
@@ -765,6 +765,78 @@ impl<'a> JSifier<'a> {
 		code
 	}
 
+	// To avoid a performance penalty when evaluating assignments made in the elif statement,
+	// it was necessary to nest the if statements.
+	//
+	// Thus, this code in Wing:
+	//
+	// if let x = tryA() {
+	//  ...
+	// } elif let x = tryB() {
+	// 	 ...
+	// } elif let x = TryC() {
+	// 	 ...
+	// } else {
+	// 	...
+	// }
+	//
+	// In JavaScript, will become this:
+	//
+	// const $if_let_value = tryA();
+	// if ($if_let_value !== undefined) {
+	// 	...
+	// } else {
+	// 	let $elif_let_value0 = tryB();
+	// 	if ($elif_let_value0 !== undefined) {
+	// 		 ...
+	// 	} else {
+	// 		 let $elif_let_value1 = tryC();
+	// 		 if ($elif_let_value1 !== undefined) {
+	// 				...
+	// 		 } else {
+	// 				...
+	// 		 }
+	// 	}
+	// }
+	fn jsify_elif_statements(
+		&self,
+		code: &mut CodeMaker,
+		elif_statements: &Vec<ElifLetBlock>,
+		index: usize,
+		else_statements: &Option<Scope>,
+		ctx: &mut JSifyContext,
+	) {
+		let elif_let_value = "$elif_let_value";
+
+		let value = format!("{}{}", elif_let_value, index);
+		code.line(format!(
+			"const {} = {};",
+			value,
+			self.jsify_expression(&elif_statements.get(index).unwrap().value, ctx)
+		));
+		let value = format!("{}{}", elif_let_value, index);
+		code.open(format!("if ({value} != undefined) {{"));
+		let elif_block = elif_statements.get(index).unwrap();
+		if elif_block.reassignable {
+			code.line(format!("let {} = {};", elif_block.var_name, value));
+		} else {
+			code.line(format!("const {} = {};", elif_block.var_name, value));
+		}
+		code.add_code(self.jsify_scope_body(&elif_block.statements, ctx));
+		code.close("}");
+
+		if index < elif_statements.len() - 1 {
+			code.open("else {");
+			self.jsify_elif_statements(code, elif_statements, index + 1, else_statements, ctx);
+			code.close("}");
+		} else if let Some(else_scope) = else_statements {
+			code.open("else {");
+			code.add_code(self.jsify_scope_body(else_scope, ctx));
+			code.close("}");
+		}
+		return;
+	}
+
 	fn jsify_statement(&self, env: &SymbolEnv, statement: &Stmt, ctx: &mut JSifyContext) -> CodeMaker {
 		CompilationContext::set(CompilationPhase::Jsifying, &statement.span);
 		match &statement.kind {
@@ -836,6 +908,7 @@ impl<'a> JSifier<'a> {
 				value,
 				statements,
 				var_name,
+				elif_statements,
 				else_statements,
 			} => {
 				let mut code = CodeMaker::default();
@@ -867,12 +940,13 @@ impl<'a> JSifier<'a> {
 				// The temporary scope is created so that intermediate variables created by consecutive `if let` clauses
 				// do not interfere with each other.
 				code.open("{");
-				let if_let_value = "$IF_LET_VALUE".to_string();
+				let if_let_value = "$if_let_value".to_string();
 				code.line(format!(
 					"const {} = {};",
 					if_let_value,
 					self.jsify_expression(value, ctx)
 				));
+
 				code.open(format!("if ({if_let_value} != undefined) {{"));
 				if *reassignable {
 					code.line(format!("let {} = {};", var_name, if_let_value));
@@ -882,7 +956,11 @@ impl<'a> JSifier<'a> {
 				code.add_code(self.jsify_scope_body(statements, ctx));
 				code.close("}");
 
-				if let Some(else_scope) = else_statements {
+				if elif_statements.len() > 0 {
+					code.open("else {");
+					self.jsify_elif_statements(&mut code, elif_statements, 0, else_statements, ctx);
+					code.close("}");
+				} else if let Some(else_scope) = else_statements {
 					code.open("else {");
 					code.add_code(self.jsify_scope_body(else_scope, ctx));
 					code.close("}");

libs/wingc/src/lsp/hover.rs

@@ -169,13 +169,21 @@ impl<'a> Visit<'a> for HoverVisitor<'a> {
 				value,
 				statements,
 				reassignable: _,
+				elif_statements,
 				else_statements,
 			} => {
 				self.with_scope(statements, |v| {
 					v.visit_symbol(var_name);
 				});
 				self.visit_expr(value);
 				self.visit_scope(statements);
+				for elif in elif_statements {
+					self.with_scope(&elif.statements, |v| {
+						v.visit_symbol(&elif.var_name);
+					});
+					self.visit_expr(&elif.value);
+					self.visit_scope(&elif.statements);
+				}
 				if let Some(else_statements) = else_statements {
 					self.visit_scope(else_statements);
 				}

libs/wingc/src/parser.rs

@@ -8,8 +8,8 @@ use tree_sitter::Node;
 use tree_sitter_traversal::{traverse, Order};
 
 use crate::ast::{
-	ArgList, BinaryOperator, BringSource, CalleeKind, CatchBlock, Class, ClassField, ElifBlock, Expr, ExprKind,
-	FunctionBody, FunctionDefinition, FunctionParameter, FunctionSignature, Interface, InterpolatedString,
+	ArgList, BinaryOperator, BringSource, CalleeKind, CatchBlock, Class, ClassField, ElifBlock, ElifLetBlock, Expr,
+	ExprKind, FunctionBody, FunctionDefinition, FunctionParameter, FunctionSignature, Interface, InterpolatedString,
 	InterpolatedStringPart, Literal, NewExpr, Phase, Reference, Scope, Stmt, StmtKind, StructField, Symbol,
 	TypeAnnotation, TypeAnnotationKind, UnaryOperator, UserDefinedType,
 };
@@ -571,6 +571,22 @@ impl<'s> Parser<'s> {
 		let reassignable = statement_node.child_by_field_name("reassignable").is_some();
 		let value = self.build_expression(&statement_node.child_by_field_name("value").unwrap(), phase)?;
 		let name = self.check_reserved_symbol(&statement_node.child_by_field_name("name").unwrap())?;
+
+		let mut elif_vec = vec![];
+		let mut cursor = statement_node.walk();
+		for node in statement_node.children_by_field_name("elif_let_block", &mut cursor) {
+			let statements = self.build_scope(&node.child_by_field_name("block").unwrap(), phase);
+			let value = self.build_expression(&node.child_by_field_name("value").unwrap(), phase)?;
+			let name = self.check_reserved_symbol(&statement_node.child_by_field_name("name").unwrap())?;
+			let elif = ElifLetBlock {
+				reassignable: node.child_by_field_name("reassignable").is_some(),
+				statements: statements,
+				value: value,
+				var_name: name,
+			};
+			elif_vec.push(elif);
+		}
+
 		let else_block = if let Some(else_block) = statement_node.child_by_field_name("else_block") {
 			Some(self.build_scope(&else_block, phase))
 		} else {
@@ -581,6 +597,7 @@ impl<'s> Parser<'s> {
 			reassignable,
 			value,
 			statements: if_block,
+			elif_statements: elif_vec,
 			else_statements: else_block,
 		})
 	}