Implement local type inference for let statements

CHANGELOG.md

@@ -18,6 +18,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Trailing semicolons in struct and message declarations are optional now: PR [#395](https://github.com/tact-lang/tact/pull/395)
 - Tests are refactored and renamed to convey the sense of what is being tested and to reduce the amount of merge conflicts during development: PR [#402](https://github.com/tact-lang/tact/pull/402)
+- `let` statements can now be used without an explicit type declaration and determine the type automatically if it was not specified: PR [#198](https://github.com/tact-lang/tact/pull/198)
 - The outdated TextMate-style grammar files for text editors have been removed (the most recent grammar files can be found in the [tact-sublime](https://github.com/tact-lang/tact-sublime) repo): PR [#404](https://github.com/tact-lang/tact/pull/404)
 - The JSON schema for `tact.config.json` has been moved to the `json-schemas` project folder: PR [#404](https://github.com/tact-lang/tact/pull/404)
 

src/generator/writers/writeFunction.ts

@@ -91,7 +91,11 @@ export function writeStatement(
         return;
     } else if (f.kind === "statement_let") {
         // Contract/struct case
-        const t = resolveTypeRef(ctx.ctx, f.type);
+        const t =
+            f.type === null
+                ? getExpType(ctx.ctx, f.expression)
+                : resolveTypeRef(ctx.ctx, f.type);
+
         if (t.kind === "ref") {
             const tt = getType(ctx.ctx, t.name);
             if (tt.kind === "contract" || tt.kind === "struct") {

src/grammar/ast.ts

@@ -418,7 +418,7 @@ export type ASTStatementLet = {
     kind: "statement_let";
     id: number;
     name: string;
-    type: ASTTypeRef;
+    type: ASTTypeRef | null;
     expression: ASTExpression;
     ref: ASTRef;
 };
@@ -729,7 +729,6 @@ export function traverse(node: ASTNode, callback: (node: ASTNode) => void) {
     //
 
     if (node.kind === "statement_let") {
-        traverse(node.type, callback);
         traverse(node.expression, callback);
     }
     if (node.kind === "statement_return") {

src/grammar/clone.ts

@@ -28,7 +28,7 @@ export function cloneNode<T extends ASTNode>(src: T): T {
     } else if (src.kind === "statement_let") {
         return cloneASTNode({
             ...src,
-            type: cloneASTNode(src.type),
+            type: src.type ? cloneASTNode(src.type) : null,
             expression: cloneNode(src.expression),
         });
     } else if (src.kind === "statement_condition") {
@@ -132,12 +132,14 @@ export function cloneNode<T extends ASTNode>(src: T): T {
     } else if (src.kind === "def_function") {
         return cloneASTNode({
             ...src,
+            return: src.return ? cloneASTNode(src.return) : null,
             statements: src.statements ? src.statements.map(cloneNode) : null,
             args: src.args.map(cloneNode),
         });
     } else if (src.kind === "def_native_function") {
         return cloneASTNode({
             ...src,
+            return: src.return ? cloneASTNode(src.return) : null,
             args: src.args.map(cloneNode),
         });
     } else if (src.kind === "def_receive") {
@@ -158,6 +160,7 @@ export function cloneNode<T extends ASTNode>(src: T): T {
     } else if (src.kind === "def_constant") {
         return cloneASTNode({
             ...src,
+            type: cloneASTNode(src.type),
             value: src.value ? cloneNode(src.value) : src.value,
         });
     }

src/grammar/grammar.ohm

@@ -100,7 +100,7 @@ Tact {
 
     StatementBlock = "{" Statement* "}"
 
-    StatementLet = let id ":" Type "=" Expression ";"
+    StatementLet = let id (":" Type)? "=" Expression ";"
 
     StatementReturn = return Expression? ";"
 

src/grammar/grammar.ts

@@ -571,13 +571,21 @@ semantics.addOperation<ASTNode>("astOfDeclaration", {
 semantics.addOperation<ASTNode>("astOfStatement", {
     // TODO: process StatementBlock
 
-    StatementLet(_letKwd, id, _colon, type, _equals, expression, _semicolon) {
+    StatementLet(
+        _letKwd,
+        id,
+        _optColon,
+        optType,
+        _equals,
+        expression,
+        _semicolon,
+    ) {
         checkVariableName(id.sourceString, createRef(id));
 
         return createNode({
             kind: "statement_let",
             name: id.sourceString,
-            type: type.astOfType(),
+            type: unwrapOptNode(optType, (t) => t.astOfType()),
             expression: expression.astOfExpression(),
             ref: createRef(this),
         });