From 7f1df9d0b31413e59846cc955d2a955d3f361e2a Mon Sep 17 00:00:00 2001
From: Yong He <yonghe@outlook.com>
Date: Wed, 30 Apr 2025 14:17:45 -0700
Subject: Initial support for immutable lambda expressions. (#6914)

* Initial support for immutable lambda expressions.

* More diagnostics, and langauge server fix.

* Language server fix.

* Fix bug identified in review.

* Add expected result.

* Update expected result.
---
 source/slang/slang-parser.cpp | 40 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)

(limited to 'source/slang/slang-parser.cpp')

diff --git a/source/slang/slang-parser.cpp b/source/slang/slang-parser.cpp
index a8573c909..c17a086a7 100644
--- a/source/slang/slang-parser.cpp
+++ b/source/slang/slang-parser.cpp
@@ -7149,6 +7149,36 @@ static bool tryParseExpression(Parser* parser, Expr*& outExpr, TokenType tokenTy
     return false;
 }
 
+static Expr* parseLambdaExpr(Parser* parser)
+{
+    auto lambdaExpr = parser->astBuilder->create<LambdaExpr>();
+    parser->ReadToken(TokenType::LParent);
+    lambdaExpr->paramScopeDecl = parser->astBuilder->create<ScopeDecl>();
+    parser->pushScopeAndSetParent(lambdaExpr->paramScopeDecl);
+    while (!AdvanceIfMatch(parser, MatchedTokenType::Parentheses))
+    {
+        AddMember(lambdaExpr->paramScopeDecl, parser->ParseParameter());
+        if (AdvanceIf(parser, TokenType::RParent))
+            break;
+        parser->ReadToken(TokenType::Comma);
+    }
+    parser->FillPosition(lambdaExpr);
+    parser->ReadToken(TokenType::DoubleRightArrow);
+    if (parser->LookAheadToken(TokenType::LBrace))
+    {
+        lambdaExpr->bodyStmt = parser->parseBlockStatement();
+    }
+    else
+    {
+        auto returnStmt = parser->astBuilder->create<ReturnStmt>();
+        parser->FillPosition(returnStmt);
+        returnStmt->expression = parser->ParseArgExpr();
+        lambdaExpr->bodyStmt = returnStmt;
+    }
+    parser->PopScope();
+    return lambdaExpr;
+}
+
 static Expr* parseAtomicExpr(Parser* parser)
 {
     switch (peekTokenType(parser))
@@ -7161,12 +7191,22 @@ static Expr* parseAtomicExpr(Parser* parser)
     // Either:
     // - parenthesized expression `(exp)`
     // - cast `(type) exp`
+    // - lambda expressions (paramList)=>x
     //
     // Proper disambiguation requires mixing up parsing
     // and semantic checking (which we should do eventually)
     // but for now we will follow some heuristics.
     case TokenType::LParent:
         {
+            // Disambiguate between a lambda expression and other cases.
+            auto tokenReader = parser->tokenReader;
+            SkipBalancedToken(&tokenReader);
+            auto nextTokenAfterParent = tokenReader.peekTokenType();
+            if (nextTokenAfterParent == TokenType::DoubleRightArrow)
+            {
+                return parseLambdaExpr(parser);
+            }
+
             Token openParen = parser->ReadToken(TokenType::LParent);
 
             // Only handles cases of `(type)`, where type is a single identifier,
-- 
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