diff --git a/src/parse/meson.build b/src/parse/meson.build
index eed8403..1c8159e 100644
--- a/src/parse/meson.build
+++ b/src/parse/meson.build
@@ -31,4 +31,7 @@ parser = library(
   'parser-driver.hh',
   flex.process('scanner.ll'),
   bison.process('parser.yy'),
+  dependencies: [
+    bignum,
+  ],
 )
diff --git a/src/parse/parser-driver.cc b/src/parse/parser-driver.cc
index c6df35e..cb5739d 100644
--- a/src/parse/parser-driver.cc
+++ b/src/parse/parser-driver.cc
@@ -28,4 +28,8 @@ yy::location const& ParserDriver::location() const {
     return current_location_;
 }
 
+ParserDriver::numeric_type const& ParserDriver::result() const {
+    return result_;
+}
+
 } // namespace abacus::parser
diff --git a/src/parse/parser-driver.hh b/src/parse/parser-driver.hh
index 076ea91..9262c70 100644
--- a/src/parse/parser-driver.hh
+++ b/src/parse/parser-driver.hh
@@ -4,10 +4,14 @@
 
 #include "parser.hh"
 
+#include "bignum.hh" // FIXME: I would like `bignum/bignum.hh` path instead...
+
 namespace abacus::parser {
 
 class ParserDriver {
 public:
+    using numeric_type = abacus::bignum::BigNum;
+
     ParserDriver();
 
     int parse(std::string filename);
@@ -18,9 +22,10 @@ public:
     yy::location& location();
     yy::location const& location() const;
 
+    numeric_type const& result() const;
+
 private:
-    // FIXME: will become BigNum
-    int result_ = 0;
+    numeric_type result_{0};
     std::string filename_{};
     yy::location current_location_{};
     bool parse_trace_p_ = false;
diff --git a/src/parse/parser.yy b/src/parse/parser.yy
index 4b5ce1b..05cdc87 100644
--- a/src/parse/parser.yy
+++ b/src/parse/parser.yy
@@ -35,6 +35,8 @@
 namespace abacus::parser {
 class ParserDriver;
 } // namespace abacus::parser
+
+#include "bignum.hh" // FIXME: I would like `bignum/bignum.hh` path instead...
 }
 
 %code provides {
@@ -54,8 +56,7 @@ class ParserDriver;
 
 %token EOF 0 "end-of-file"
 
-// FIXME:will become BigNum
-%token <int> NUM "number"
+%token <abacus::bignum::BigNum> NUM "number"
 
 // Use `<<` to print everything
 %printer { yyo << $$; } <*>;
@@ -73,8 +74,7 @@ class ParserDriver;
 %left TIMES DIVIDE
 %precedence UNARY
 
-// FIXME: will become BigNum
-%type <int> input exp
+%type <abacus::bignum::BigNum> input exp
 
 %%
 
diff --git a/src/parse/scanner.ll b/src/parse/scanner.ll
index e85f9b1..a6c6bc1 100644
--- a/src/parse/scanner.ll
+++ b/src/parse/scanner.ll
@@ -1,4 +1,6 @@
 %{
+#include <sstream>
+
 #include "parser-driver.hh"
 #include "parser.hh"
 %}
@@ -41,7 +43,11 @@ int [0-9]+
 "("         return yy::parser::make_LPAREN(loc);
 ")"         return yy::parser::make_RPAREN(loc);
 
-{int}       return yy::parser::make_NUM(std::stoi(yytext), loc);
+{int}       {
+    abacus::bignum::BigNum num;
+    std::stringstream(yytext) >> num;
+    return yy::parser::make_NUM(num, loc);
+}
 
 .           {
     using namespace yy;