Updated grammar

parser/src/grammar.pest

@@ -11,19 +11,24 @@ boolean    =  { "true" | "false" }
 string_lit =  { "\"" ~ inner_str ~ "\"" }
 inner_str  = @{ (!"\"" ~ ANY)* }
 
+// ================= PREREQUISITES =================
+export     = { "public" }
+mutable    = { "mut" }
+var_decl_1 = { type_expr ~ mutable? ~ identifier }
+
 // ================= TOP LEVEL =================
 
 top_level = _{ import | function_decl | struct_decl }
 
-export = { "public" }
-
 import = { "import" ~ string_lit ~ ";" }
 
-param_list = { param ~ ("," ~ param)* ~ ","? }
-param      = { export? ~ type_expr ~ identifier }
+field_list = { field ~ ("," ~ field)* ~ ","? }
+field      = { export? ~ type_expr ~ identifier }
+
+param_list = { var_decl_1 ~ ("," ~ var_decl_1)* ~ ","? }
 
 struct_decl = {
-    export? ~ "struct" ~ identifier ~ "{" ~ param_list? ~ "}"
+    export? ~ "struct" ~ identifier ~ "{" ~ field_list? ~ "}"
 }
 
 function_decl = {