diff --git a/README.md b/README.md
index a5fea1b..abc62b2 100644
--- a/README.md
+++ b/README.md
@@ -3,7 +3,7 @@
 ![release](https://img.shields.io/github/v/release/melvic-ybanez/chi?include_prereleases) ![issues](https://img.shields.io/github/issues/melvic-ybanez/chi) ![loc](https://img.shields.io/tokei/lines/github/melvic-ybanez/chi)
 [![License](https://img.shields.io/badge/license-MIT-green)](./LICENSE)
 
-A code generator for fully parametric functions. 
+A code generator for fully parametric functions and functions between selected set of built-in and standard types. 
 Chi stands 
 for [Curry-Howard Isomorphism](https://en.wikipedia.org/wiki/Curry%E2%80%93Howard_correspondence). 
 
@@ -150,3 +150,23 @@ Generated code:
     };
 }
 ```
+
+# Built-in types
+Chi also recognize built-in types:
+```scala
+chi> String idString(String s)
+Detected language: Java
+Generated code:
+String idString(String s) {
+    return s;
+}
+
+chi> def foo(f: String => Int, g: Float => Int): Either[String, Float] => Int
+Detected language: Scala
+Generated code:
+def foo(f: (String => Int), g: (Float => Int)): (Either[String, Float] => Int) =
+  e => e match {
+    case Left(s) => f(s)
+    case Right(h) => g(h)
+  }
+```
diff --git a/src/main/scala/com/melvic/chi/eval/CodeGen.scala b/src/main/scala/com/melvic/chi/eval/Generate.scala
similarity index 82%
rename from src/main/scala/com/melvic/chi/eval/CodeGen.scala
rename to src/main/scala/com/melvic/chi/eval/Generate.scala
index 61800a1..94656ee 100644
--- a/src/main/scala/com/melvic/chi/eval/CodeGen.scala
+++ b/src/main/scala/com/melvic/chi/eval/Generate.scala
@@ -1,6 +1,6 @@
 package com.melvic.chi.eval
 
-import com.melvic.chi.ast.Proposition.{Identifier, PUnit}
+import com.melvic.chi.ast.Proposition.{Atom, Identifier, PUnit}
 import com.melvic.chi.ast.{Definition, Proposition, Signature}
 import com.melvic.chi.env.Env
 import com.melvic.chi.out.Fault.UnknownPropositions
@@ -8,14 +8,14 @@ import com.melvic.chi.out.Result
 import com.melvic.chi.out.Result.Result
 import com.melvic.chi.parsers.{JavaParser, Language, ScalaParser}
 
-object CodeGen {
+object Generate {
   def fromSignature(signature: Signature, language: Language): Result[Definition] = {
     val Signature(name, typeParams, params, proposition) = signature
 
     val unknownTypes = Proposition.filter(proposition) {
       case PUnit => false
-      case atom =>
-        !typeParams.map(Identifier).contains(atom)
+      case atom @ Atom(value) =>
+        !typeParams.map(Identifier).contains(atom) && !Language.builtInTypes(language).contains(value)
     }
 
     if (unknownTypes.nonEmpty) Result.fail(UnknownPropositions(unknownTypes))
diff --git a/src/main/scala/com/melvic/chi/eval/package.scala b/src/main/scala/com/melvic/chi/eval/package.scala
index 0d0af0b..7fccd9e 100644
--- a/src/main/scala/com/melvic/chi/eval/package.scala
+++ b/src/main/scala/com/melvic/chi/eval/package.scala
@@ -6,5 +6,5 @@ package object eval {
   type Evaluate = String => String
 
   def generateAndShow: Evaluate = code =>
-    Result.show(CodeGen.fromSignatureString(code))
+    Result.show(Generate.fromSignatureString(code))
 }
diff --git a/src/main/scala/com/melvic/chi/package.scala b/src/main/scala/com/melvic/chi/package.scala
index 4e618d9..ee99f74 100644
--- a/src/main/scala/com/melvic/chi/package.scala
+++ b/src/main/scala/com/melvic/chi/package.scala
@@ -1,6 +1,6 @@
 package com.melvic
 
-import com.melvic.chi.eval.{CodeGen, Prover}
+import com.melvic.chi.eval.{Generate, Prover}
 import com.melvic.chi.out.Result
 
 package object chi {
diff --git a/src/main/scala/com/melvic/chi/parsers/JavaParser.scala b/src/main/scala/com/melvic/chi/parsers/JavaParser.scala
index 8992b91..8a0d53c 100644
--- a/src/main/scala/com/melvic/chi/parsers/JavaParser.scala
+++ b/src/main/scala/com/melvic/chi/parsers/JavaParser.scala
@@ -27,8 +27,8 @@ object JavaParser extends BaseParser {
   lazy val proposition: PackratParser[Proposition] = function | biFunction | identifier
 
   val functionCode: Parser[Signature] =
-    typeParams ~ proposition ~ nameParser ~ opt(paramList) ^^ {
+    opt(typeParams) ~ proposition ~ nameParser ~ opt(paramList) ^^ {
       case typeParams ~ returnType ~ name ~ params =>
-        Signature(name, typeParams.map(_.value), params.getOrElse(Nil), returnType)
+        Signature(name, typeParams.getOrElse(Nil).map(_.value), params.getOrElse(Nil), returnType)
     }
 }
diff --git a/src/main/scala/com/melvic/chi/parsers/Language.scala b/src/main/scala/com/melvic/chi/parsers/Language.scala
index 85171e5..8ff630f 100644
--- a/src/main/scala/com/melvic/chi/parsers/Language.scala
+++ b/src/main/scala/com/melvic/chi/parsers/Language.scala
@@ -5,4 +5,15 @@ sealed trait Language
 object Language {
   case object Java extends Language
   case object Scala extends Language
+
+  def builtInTypes(language: Language): List[String] = {
+    val builtinTypeString = language match {
+      case Scala => "Char,Byte,Short,Int,Long,Float,Double,String"
+      case Java =>
+        val primitive = "byte,short,int,long,float,double,boolean,char"
+        val boxed = "Byte,Boolean,Char,Float,Int,Long,Short,Double,String"
+        primitive + boxed
+    }
+    builtinTypeString.split(",").toList
+  }
 }
diff --git a/src/test/scala/com/melvic/chi/tests/JavaFunctionsSpec.scala b/src/test/scala/com/melvic/chi/tests/JavaFunctionsSpec.scala
index b4010e1..efa8e5a 100644
--- a/src/test/scala/com/melvic/chi/tests/JavaFunctionsSpec.scala
+++ b/src/test/scala/com/melvic/chi/tests/JavaFunctionsSpec.scala
@@ -36,4 +36,14 @@ class JavaFunctionsSpec extends AnyFlatSpec with should.Matchers {
         |}""".stripMargin
     )
   }
+
+  "Built-in types" should "be included in the type search" in {
+    generateAndShow("String idString(String s)") should be(
+      """Detected language: Java
+        |Generated code:
+        |String idString(String s) {
+        |    return s;
+        |}""".stripMargin
+    )
+  }
 }
diff --git a/src/test/scala/com/melvic/chi/tests/ScalaDefSpec.scala b/src/test/scala/com/melvic/chi/tests/ScalaDefSpec.scala
index 62f4604..5f9940e 100644
--- a/src/test/scala/com/melvic/chi/tests/ScalaDefSpec.scala
+++ b/src/test/scala/com/melvic/chi/tests/ScalaDefSpec.scala
@@ -196,4 +196,16 @@ class ScalaDefSpec extends AnyFlatSpec with should.Matchers {
         |  }""".stripMargin
     )
   }
+
+  "Built-in types" should "be included in the type search" in {
+    generateAndShow("def foo(f: String => Int, g: Float => Int): Either[String, Float] => Int") should be(
+      """Detected language: Scala
+        |Generated code:
+        |def foo(f: (String => Int), g: (Float => Int)): (Either[String, Float] => Int) =
+        |  e => e match {
+        |    case Left(s) => f(s)
+        |    case Right(h) => g(h)
+        |  }""".stripMargin
+    )
+  }
 }