From 12d304afab3fcdc28f28508895d162a84cb56cde Mon Sep 17 00:00:00 2001
From: John Hodge <tpg@mutabah.net>
Date: Sun, 3 Dec 2023 21:06:53 +0800
Subject: [PATCH] AST - Draft `let-else` impl

---
 src/ast/expr.cpp          |  9 +++++++--
 src/ast/expr.hpp          | 10 ++++++----
 src/hir/from_ast_expr.cpp |  3 +++
 src/parse/expr.cpp        |  7 ++++++-
 4 files changed, 22 insertions(+), 7 deletions(-)

diff --git a/src/ast/expr.cpp b/src/ast/expr.cpp
index 6f585ee0..affacd53 100644
--- a/src/ast/expr.cpp
+++ b/src/ast/expr.cpp
@@ -253,10 +253,14 @@ NODE(ExprNode_Flow, {
 
 NODE(ExprNode_LetBinding, {
     os << "let " << m_pat << ": " << m_type;
-    if(m_value)
+    if(m_value) {
         os << " = " << *m_value;
+        if( m_else ) {
+            os << " else " << *m_else;
+        }
+    }
 },{
-    return NEWNODE(ExprNode_LetBinding, m_pat.clone(), m_type.clone(), OPT_CLONE(m_value));
+    return NEWNODE(ExprNode_LetBinding, m_pat.clone(), m_type.clone(), OPT_CLONE(m_value), OPT_CLONE(m_else));
 })
 
 NODE(ExprNode_Assign, {
@@ -664,6 +668,7 @@ NV(ExprNode_LetBinding,
 {
     // TODO: Handle recurse into Let pattern?
     visit(node.m_value);
+    visit(node.m_else);
 })
 NV(ExprNode_Assign,
 {
diff --git a/src/ast/expr.hpp b/src/ast/expr.hpp
index 545c2a9a..e34a98da 100644
--- a/src/ast/expr.hpp
+++ b/src/ast/expr.hpp
@@ -204,11 +204,13 @@ struct ExprNode_LetBinding:
     Pattern m_pat;
     TypeRef m_type;
     ExprNodeP    m_value;
+    ExprNodeP    m_else;
 
-    ExprNode_LetBinding(Pattern pat, TypeRef type, ExprNodeP value):
-        m_pat( move(pat) ),
-        m_type( move(type) ),
-        m_value( move(value) )
+    ExprNode_LetBinding(Pattern pat, TypeRef type, ExprNodeP value, ExprNodeP else_arm={})
+        : m_pat( move(pat) )
+        , m_type( move(type) )
+        , m_value( move(value) )
+        , m_else( move(else_arm) )
     {
     }
 
diff --git a/src/hir/from_ast_expr.cpp b/src/hir/from_ast_expr.cpp
index 6ff66ed6..67e6443a 100644
--- a/src/hir/from_ast_expr.cpp
+++ b/src/hir/from_ast_expr.cpp
@@ -163,6 +163,9 @@ struct LowerHIR_ExprNode_Visitor:
         }
     }
     virtual void visit(::AST::ExprNode_LetBinding& v) override {
+        if( v.m_else ) {
+            TODO(v.span(), "Handle let-else in HIR expand, or elsewhere?");
+        }
         m_rv.reset( new ::HIR::ExprNode_Let( v.span(),
             LowerHIR_Pattern( v.m_pat ),
             LowerHIR_Type( v.m_type ),
diff --git a/src/parse/expr.cpp b/src/parse/expr.cpp
index bfb43aae..8861714b 100644
--- a/src/parse/expr.cpp
+++ b/src/parse/expr.cpp
@@ -657,13 +657,18 @@ ExprNodeP Parse_Stmt_Let(TokenStream& lex)
         GET_TOK(tok, lex);
     }
     ExprNodeP val;
+    ExprNodeP else_arm;
     if( tok.type() == TOK_EQUAL ) {
         val = Parse_Expr0(lex);
+        if( lex.lookahead(0) == TOK_RWORD_ELSE ) {
+            GET_TOK(tok, lex);
+            else_arm = Parse_ExprBlockNode(lex);
+        }
     }
     else {
         PUTBACK(tok, lex);
     }
-    return NEWNODE( AST::ExprNode_LetBinding, ::std::move(pat), mv$(type), ::std::move(val) );
+    return NEWNODE( AST::ExprNode_LetBinding, ::std::move(pat), mv$(type), ::std::move(val), ::std::move(else_arm) );
 }
 
 ::std::vector<ExprNodeP> Parse_ParenList(TokenStream& lex)