feat(lua): use (** ... **) instead of {{ ... }} for nested Lua scripts

The token }} is a bad delimiter for blocks of Lua script code nested in Lean files.
The problem is that the sequence }} occurs very often in Lua code because Lua uses { and } to build tables/lists/arrays.
Here is an example of Lua code that contains the sequence }}
     t = {{1, 2}, {2, 3}, {3, 4}}

In Lean, (* ... *) is used to create comments. Thus, (** ... **) code blocks will not affect
valid Lean files. It also looks reasonably good.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

src/frontends/lean/scanner.cpp

@@ -132,8 +132,6 @@ bool scanner::check_next_is_digit() {
 }
 
 void scanner::read_comment() {
-    lean_assert(curr() == '*');
-    next();
     int nest = 1;
     while (true) {
         if (curr() == '*') {
@@ -337,53 +335,43 @@ scanner::token scanner::read_string() {
 }
 
 scanner::token scanner::read_script_block() {
-    lean_assert(curr() == '{');
-    next();
     m_script_line = m_line;
     m_script_pos  = m_pos;
     m_buffer.clear();
-    int num_open_blocks = 1;
     while (true) {
-        char c = curr();
-        if (c == EOF) {
+        char c1 = curr();
+        if (c1 == EOF)
             throw_exception("unexpected end of script");
-        } else if (c == '{') {
-            m_buffer += '{';
-            next();
-            c = curr();
-            if (c == '{')
-                num_open_blocks++;
-            else if (c == '\n')
-                new_line();
-            else if (c == EOF)
-                throw_exception("unexpected end of script");
-            m_buffer += c;
-            next();
-        } else if (c == '}') {
-            next();
+        next();
+        if (c1 == '*') {
             char c2 = curr();
-            if (c2 == '}') {
-                next();
-                num_open_blocks--;
-                if (num_open_blocks == 0)
-                    return token::ScriptBlock;
-            } else if (c2 == '\n') {
-                new_line();
-                next();
-            } else if (c2 == EOF) {
+            if (c2 == EOF)
                 throw_exception("unexpected end of script");
-            } else {
+            next();
+            if (c2 == '*') {
+                char c3 = curr();
+                if (c3 == EOF)
+                    throw_exception("unexpected end of script");
                 next();
+                if (c3 == ')') {
+                    return token::ScriptBlock;
+                } else {
+                    if (c3 == '\n')
+                        new_line();
+                    m_buffer += c1;
+                    m_buffer += c2;
+                    m_buffer += c3;
+                }
+            } else {
+                if (c2 == '\n')
+                    new_line();
+                m_buffer += c1;
+                m_buffer += c2;
             }
-            m_buffer += c;
-            m_buffer += c2;
-        } else if (c == '\n') {
-            new_line();
-            m_buffer += '\n';
-            next();
         } else {
-            m_buffer += c;
-            next();
+            if (c1 == '\n')
+                new_line();
+            m_buffer += c1;
         }
     }
 }
@@ -407,18 +395,19 @@ scanner::token scanner::scan() {
         case '(':
             next();
             if (curr() == '*') {
-                read_comment();
-                break;
+                next();
+                if (curr() == '*')  {
+                    next();
+                    return read_script_block();
+                } else {
+                    read_comment();
+                    break;
+                }
             } else {
                 return token::LeftParen;
             }
         case ')':  next(); return token::RightParen;
-        case '{':
-            next();
-            if (curr() == '{')
-                return read_script_block();
-            else
-                return token::LeftCurlyBracket;
+        case '{':  next(); return token::LeftCurlyBracket;
         case '}':  next(); return token::RightCurlyBracket;
         case 'a':  return read_a_symbol();
         case 'b':  return read_b_symbol();

tests/lean/lua1.lean

@@ -1,7 +1,7 @@
 Variable x : Int
 
-{{
+(**
 print("hello world from Lua")
-}}
+**)
 
 Variable y : Int

tests/lean/lua2.lean

@@ -1,6 +1,6 @@
 Variable x : Bool
 
-{{
+(**
  a = {}
  print("hello world")
  print ("ok")
@@ -9,7 +9,7 @@ Variable x : Bool
     y = 20
  }
  rint ("ok")
-}}
+**)
 
 Variable y : Bool
 

tests/lean/lua3.lean

@@ -1,6 +1,6 @@
 
 Variable x : Int
 
-{{
+(**
 dofile("script.lua")
-}}
+**)

tests/lean/lua4.lean

@@ -1,12 +1,12 @@
 Variable x : Int
 
-{{
+(**
 -- Add a variable to the environment using Lua
 -- The type of the new variable is equal to the type
 -- of x
 typeofx = env():check_type(Const("x"))
 print("type of x is " .. tostring(typeofx))
 env():add_var("y", typeofx)
-}}
+**)
 
 Check x + y

tests/lean/lua5.lean

@@ -1,13 +1,13 @@
 Variable x : Int
 
-{{
+(**
 local N = 100
 -- Create N variables with the same type of x
 typeofx = env():check_type(Const("x"))
 for i = 1, N do
     env():add_var("y_" .. i, typeofx)
 end
-}}
+**)
 
 Show Environment 101
 Check x + y_1 + y_2

tests/lean/lua6.lean

@@ -1,19 +1,19 @@
 Variable x : Int
 Set pp::notation false
-{{
+(**
 print(get_options())
-}}
+**)
 Check x + 2
-{{
+(**
 o = get_options()
 o = o:update(name('lean', 'pp', 'notation'), true)
 set_options(o)
 print(get_options())
-}}
+**)
 Check x + 2
-{{
+(**
 set_option(name('lean', 'pp', 'notation'), false)
 print(get_options())
-}}
+**)
 Variable y : Int
 Check x + y