fix(compiler): no error when trying to reassign to a let variable

docs/04-reference/winglang-spec.md

@@ -419,6 +419,19 @@ Examples in the class section below.
 Assigning `var` to immutables of the same type is allowed. That is similar
 to assigning non `readonly`s to `readonly`s in TypeScript.
 
+By default function closure arguments are non-reassignable. By prefixing `var`
+to an argument definition you can make a re-assignable function argument:
+
+```ts
+// wing
+let f = (arg1: num, var arg2: num) => {
+  if (arg2 > 100) {
+    // We can reassign a value to arg2 since it's marked `var`
+    args2 = 100;
+  }
+}
+```
+
 [`▲ top`][top]
 
 ---

examples/tests/invalid/reassing_to_nonreassignable.w

@@ -0,0 +1,21 @@
+// Assign to non-reassignable var
+let x = 5;
+x = x + 1;
+
+// Assign to non-reassignable field
+resource R {
+  f: num;
+  init() {
+    this.f = 1;
+  }
+
+  inc() {
+    this.f = this.f + 1;
+  }
+}
+
+// Assign to non-reassignable arg
+let f = (arg: num):num => {
+  arg = 0;
+  return arg;
+}

examples/tests/valid/anon_function.w

@@ -1,5 +1,5 @@
 // Define a function and assign it to a variable
-let myfunc = (x: num) => {
+let myfunc = (var x: num) => {
   print("${x}");
   x = x + 1;
   if (x > 3.14) {

examples/tests/valid/reassignment.w

@@ -3,3 +3,33 @@ assert(x == 5);
 
 x = x + 1;
 assert(x == 6);
+
+resource R {
+  var f: num;
+  f1: num;
+  init() {
+    // Initialize fields in `init` but in an inner scope to make sure 
+    // we treat the special case of `this` access from init correctly at all scope levels
+    if true {
+      this.f = 1;
+      this.f1 = 0; // Access non-reassignable field from constructor is valid
+    }
+  }
+
+  inc() {
+    this.f = this.f + 1;
+  }
+}
+
+let r = new R();
+r.inc();
+assert(r.f == 2);
+
+let f = (var arg: num):num => {
+  arg = 0;
+  return arg;
+}
+
+let y = 1;
+assert(f(y) == 0);
+assert(y == 1); // y is not modified, since Wing functions are pass-by-value

libs/tree-sitter-wing/grammar.js

@@ -373,7 +373,11 @@ module.exports = grammar({
     access_modifier: ($) => choice("public", "private", "protected"),
 
     parameter_definition: ($) =>
-      seq(field("name", $.identifier), $._type_annotation),
+      seq(
+        optional(field("reassignable", $.reassignable)),
+        field("name", $.identifier), 
+        $._type_annotation
+      ),
 
     parameter_list: ($) => seq("(", commaSep($.parameter_definition), ")"),
 

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

@@ -76,7 +76,7 @@ return 1;
 Inflight closure
 ====================
 
-inflight (a: num, b: str?, c: bool) => {};
+inflight (a: num, b: str?, var c: bool) => {};
 
 ---
 
@@ -95,6 +95,7 @@ inflight (a: num, b: str?, c: bool) => {};
                     )
                 )
                 (parameter_definition
+                    reassignable: (reassignable)
                     name: (identifier)
                     type: (builtin_type)
                 )

libs/wingc/src/ast.rs

@@ -134,7 +134,9 @@ pub struct FunctionSignature {
 #[derive(Derivative)]
 #[derivative(Debug)]
 pub struct FunctionDefinition {
-	pub parameter_names: Vec<Symbol>, // TODO: move into FunctionSignature and make optional
+	// List of names of function parameters and whether they are reassignable (`var`) or not.
+	pub parameters: Vec<(Symbol, bool)>, // TODO: move into FunctionSignature and make optional
+
 	pub statements: Scope,
 	pub signature: FunctionSignature,
 	#[derivative(Debug = "ignore")]
@@ -143,7 +145,9 @@ pub struct FunctionDefinition {
 
 #[derive(Debug)]
 pub struct Constructor {
-	pub parameters: Vec<Symbol>,
+	// List of names of constructor parameters and whether they are reassignable (`var`) or not.
+	pub parameters: Vec<(Symbol, bool)>,
+
 	pub statements: Scope,
 	pub signature: FunctionSignature,
 }
@@ -181,7 +185,7 @@ pub enum StmtKind {
 		identifier: Option<Symbol>,
 	},
 	VariableDef {
-		kind: VariableKind,
+		reassignable: bool,
 		var_name: Symbol,
 		initial_value: Expr,
 		type_: Option<Type>,
@@ -226,16 +230,11 @@ pub enum StmtKind {
 	},
 }
 
-#[derive(Debug, Clone, Copy, PartialEq)]
-pub enum VariableKind {
-	Let,
-	Var,
-}
-
 #[derive(Debug)]
 pub struct ClassMember {
 	pub name: Symbol,
 	pub member_type: Type,
+	pub reassignable: bool,
 	pub flight: Phase,
 }
 

libs/wingc/src/capture.rs

@@ -86,12 +86,7 @@ pub fn scan_for_inflights_in_scope(scope: &Scope, diagnostics: &mut Diagnostics)
 					}
 				}
 			}
-			StmtKind::VariableDef {
-				kind: _,
-				var_name: _,
-				initial_value,
-				type_: _,
-			} => {
+			StmtKind::VariableDef { initial_value, .. } => {
 				scan_for_inflights_in_expression(initial_value, diagnostics);
 			}
 			StmtKind::Expression(exp) => {
@@ -195,7 +190,8 @@ fn scan_captures_in_call(
 				Ok((prop_type, phase)) => (
 					prop_type
 						.as_variable()
-						.expect("Expected resource property to be a variable"),
+						.expect("Expected resource property to be a variable")
+						._type,
 					phase,
 				),
 				Err(type_error) => {
@@ -254,7 +250,7 @@ fn scan_captures_in_expression(
 				// the type checker.
 
 				if x.is_ok() {
-					let (var, f) = x.unwrap();
+					let (var, si) = x.unwrap();
 
 					if var.as_variable().is_none() {
 						diagnostics.push(Diagnostic {
@@ -263,10 +259,10 @@ fn scan_captures_in_expression(
 							span: Some(symbol.span.clone()),
 						});
 					} else {
-						let t = var.as_variable().unwrap();
+						let t = var.as_variable().unwrap()._type;
 
 						// if the identifier represents a preflight value, then capture it
-						if f == Phase::Preflight {
+						if si.flight == Phase::Preflight {
 							if var.is_reassignable() {
 								diagnostics.push(Diagnostic {
 									level: DiagnosticLevel::Error,
@@ -405,12 +401,9 @@ fn scan_captures_in_inflight_scope(scope: &Scope, diagnostics: &mut Diagnostics)
 
 	for s in scope.statements.iter() {
 		match &s.kind {
-			StmtKind::VariableDef {
-				kind: _,
-				var_name: _,
-				initial_value,
-				type_: _,
-			} => res.extend(scan_captures_in_expression(initial_value, env, s.idx, diagnostics)),
+			StmtKind::VariableDef { initial_value, .. } => {
+				res.extend(scan_captures_in_expression(initial_value, env, s.idx, diagnostics))
+			}
 			StmtKind::ForLoop {
 				iterator: _,
 				iterable,

libs/wingc/src/jsify.rs

@@ -6,7 +6,7 @@ use sha2::{Digest, Sha256};
 use crate::{
 	ast::{
 		ArgList, BinaryOperator, ClassMember, Expr, ExprKind, FunctionDefinition, InterpolatedStringPart, Literal, Phase,
-		Reference, Scope, Stmt, StmtKind, Symbol, Type, UnaryOperator, UtilityFunctions, VariableKind,
+		Reference, Scope, Stmt, StmtKind, Symbol, Type, UnaryOperator, UtilityFunctions,
 	},
 	capture::CaptureKind,
 	utilities::snake_case_to_camel_case,
@@ -438,7 +438,7 @@ impl JSifier {
 			ExprKind::FunctionClosure(func_def) => match func_def.signature.flight {
 				Phase::Inflight => self.jsify_inflight_function(func_def),
 				Phase::Independent => unimplemented!(),
-				Phase::Preflight => self.jsify_function(None, &func_def.parameter_names, &func_def.statements, phase),
+				Phase::Preflight => self.jsify_function(None, &func_def.parameters, &func_def.statements, phase),
 			},
 		}
 	}
@@ -466,15 +466,16 @@ impl JSifier {
 				)
 			}
 			StmtKind::VariableDef {
-				kind,
+				reassignable,
 				var_name,
 				initial_value,
 				type_: _,
 			} => {
 				let initial_value = self.jsify_expression(initial_value, phase);
-				return match kind {
-					VariableKind::Let => format!("const {} = {};", self.jsify_symbol(var_name), initial_value),
-					VariableKind::Var => format!("let {} = {};", self.jsify_symbol(var_name), initial_value),
+				return if *reassignable {
+					format!("let {} = {};", self.jsify_symbol(var_name), initial_value)
+				} else {
+					format!("const {} = {};", self.jsify_symbol(var_name), initial_value)
 				};
 			}
 			StmtKind::ForLoop {
@@ -567,7 +568,7 @@ impl JSifier {
 						.map(|(n, m)| format!(
 							"{} = {}",
 							n.name,
-							self.jsify_function(None, &m.parameter_names, &m.statements, phase)
+							self.jsify_function(None, &m.parameters, &m.statements, phase)
 						))
 						.collect::<Vec<String>>()
 						.join("\n")
@@ -623,8 +624,8 @@ impl JSifier {
 
 	fn jsify_inflight_function(&self, func_def: &FunctionDefinition) -> String {
 		let mut parameter_list = vec![];
-		for p in func_def.parameter_names.iter() {
-			parameter_list.push(p.name.clone());
+		for p in func_def.parameters.iter() {
+			parameter_list.push(p.0.name.clone());
 		}
 		let block = self.jsify_scope(&func_def.statements, Phase::Inflight);
 		let procid = base16ct::lower::encode_string(&Sha256::new().chain_update(&block).finalize());
@@ -696,10 +697,10 @@ impl JSifier {
 		)
 	}
 
-	fn jsify_function(&self, name: Option<&str>, parameters: &[Symbol], body: &Scope, phase: Phase) -> String {
+	fn jsify_function(&self, name: Option<&str>, parameters: &[(Symbol, bool)], body: &Scope, phase: Phase) -> String {
 		let mut parameter_list = vec![];
 		for p in parameters.iter() {
-			parameter_list.push(self.jsify_symbol(p));
+			parameter_list.push(self.jsify_symbol(&p.0));
 		}
 
 		let (name, arrow) = match name {

libs/wingc/src/lib.rs

@@ -1,7 +1,7 @@
 #[macro_use]
 extern crate lazy_static;
 
-use ast::{Scope, Stmt, Symbol, UtilityFunctions, VariableKind};
+use ast::{Scope, Stmt, Symbol, UtilityFunctions};
 use diagnostic::{print_diagnostics, Diagnostic, DiagnosticLevel, Diagnostics, WingSpan};
 use jsify::JSifier;
 use type_check::symbol_env::StatementIdx;
@@ -130,7 +130,7 @@ pub fn parse(source_file: &str) -> (Scope, Diagnostics) {
 }
 
 pub fn type_check(scope: &mut Scope, types: &mut Types) -> Diagnostics {
-	let env = SymbolEnv::new(None, types.void(), false, Phase::Preflight, 0);
+	let env = SymbolEnv::new(None, types.void(), false, false, Phase::Preflight, 0);
 	scope.set_env(env);
 
 	add_builtin(
@@ -195,7 +195,7 @@ fn add_builtin(name: &str, typ: Type, scope: &mut Scope, types: &mut Types) {
 		.unwrap()
 		.define(
 			&sym,
-			SymbolKind::Variable(types.add_type(typ), VariableKind::Let),
+			SymbolKind::make_variable(types.add_type(typ), false),
 			StatementIdx::Top,
 		)
 		.expect("Failed to add builtin");