fix: fix shadowing with same variable

We now make sure to see if a name has already been defined in the
correct scope and if it is the case when do not define it again.

crates/compiler/src/compiler.rs

@@ -102,7 +102,31 @@ impl Compiler {
                 self.emit(Opcode::Pop, vec![]);
             }
             Statement::Let(s) => {
-                let symbol = self.symbol_table.define(s.name.value);
+                // This step is extremely important. If it is not done then when shadowing variables
+                // and using the previous value we get an error. Because we would have assigned
+                // a new index to the symbol and the GetGlobal instruction would get a NULL
+                // value instead of the previous value. (corresponds to issue #11)
+                let symbol = match self.symbol_table.resolve(&s.name.value) {
+                    Some(symbol) => match symbol.scope {
+                        SymbolScope::Global => {
+                            // A Local variable should never replace a global one
+                            if self.symbol_table.has_outer() {
+                                // This means that the symbol will
+                                // be local and not global
+                                self.symbol_table.define(s.name.value)
+                            } else {
+                                symbol
+                            }
+                        }
+                        SymbolScope::Local => symbol,
+
+                        // We only want to do in in the case of "normal" variable assignation.
+                        // The special cases should not be touched, since the program should not
+                        // have access to them, only the compiler/vm
+                        _ => self.symbol_table.define(s.name.value),
+                    },
+                    None => self.symbol_table.define(s.name.value),
+                };
 
                 self.compile_expression(s.value)?;
 

crates/compiler/src/symbol_table.rs

@@ -115,6 +115,10 @@ impl SymbolTable {
         self.store.insert(symbol.name.clone(), symbol.clone());
         symbol
     }
+
+    pub fn has_outer(&self) -> bool{
+        self.outer.is_some()
+    }
 }
 
 #[cfg(test)]