I extended the right-hand side of assignments to accept "None"

without type annotation.

This is meant to enable specify the removal of a map binding.

Note: The index assignments in loops are not concerned.

AST.ml

@@ -390,13 +390,17 @@ and case = {
 and assignment = {
   lhs    : lhs;
   assign : assign;
-  expr   : expr
+  rhs    : rhs
 }
 
 and lhs =
   Path    of path
 | MapPath of map_lookup reg
 
+and rhs =
+      Expr of expr
+| NoneExpr of c_None
+
 and loop =
   While of while_loop reg
 | For   of for_loop
@@ -727,6 +731,10 @@ let lhs_to_region = function
   Path path -> path_to_region path
 | MapPath {region; _} -> region
 
+let rhs_to_region = function
+      Expr e -> expr_to_region e
+| NoneExpr r -> r
+
 (* Printing the tokens with their source regions *)
 
 let printf = Printf.printf
@@ -1028,10 +1036,14 @@ and print_case {value; _} =
   print_instruction instr
 
 and print_assignment {value; _} =
-  let {lhs; assign; expr} = value in
+  let {lhs; assign; rhs} = value in
   print_lhs lhs;
   print_token assign ":=";
-  print_expr expr
+  print_rhs rhs
+
+and print_rhs = function
+      Expr e -> print_expr e
+| NoneExpr r -> print_token r "None"
 
 and print_lhs = function
   Path path -> print_path path

AST.mli

@@ -374,13 +374,17 @@ and case = {
 and assignment = {
   lhs    : lhs;
   assign : assign;
-  expr   : expr
+  rhs    : rhs;
 }
 
 and lhs =
   Path    of path
 | MapPath of map_lookup reg
 
+and rhs =
+      Expr of expr
+| NoneExpr of c_None
+
 and loop =
   While of while_loop reg
 | For   of for_loop
@@ -593,6 +597,7 @@ val pattern_to_region    : pattern -> Region.t
 val local_decl_to_region : local_decl -> Region.t
 val path_to_region       : path -> Region.t
 val lhs_to_region        : lhs -> Region.t
+val rhs_to_region        : rhs -> Region.t
 
 (* Printing *)
 

Parser.mly

@@ -543,12 +543,17 @@ case:
   }
 
 assignment:
-  lhs ASS expr {
-    let region = cover (lhs_to_region $1) (expr_to_region $3)
-    and value  = {lhs = $1; assign = $2; expr = $3}
+  lhs ASS rhs {
+    let stop   = rhs_to_region $3 in
+    let region = cover (lhs_to_region $1) stop
+    and value  = {lhs = $1; assign = $2; rhs = $3}
     in {region; value}
   }
 
+rhs:
+  expr   {     Expr $1 }
+| C_None { NoneExpr $1 : rhs }
+
 lhs:
   path       {    Path $1 }
 | map_lookup { MapPath $1 }
@@ -580,7 +585,6 @@ for_loop:
       block    = $7}
     in For (ForInt {region; value})
   }
-
 | For var option(arrow_clause) In expr block {
     let region = cover $1 $6.region in
     let value = {

Tests/crowdfunding.ligo

@@ -58,7 +58,7 @@ entrypoint claim (storage store : store; const sender : address)
           else
             begin
               operations := [Transfer (sender, amount)];
-              store.backers[sender] := (None : option (nat))
+              store.backers[sender] := None
             end
       end
   end with (store, operations)