diff --git a/.gitignore b/.gitignore
index 496097d..ec576bb 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,6 +5,9 @@ test/conan/*
 cov/*
 
 !**/.gitkeep
+!docs/doxygen.css
+!docs/doxygen_footer.html
+!docs/doxygen_header.html
 
 compile_flags.txt
 coverage.info
diff --git a/.gitmodules b/.gitmodules
deleted file mode 100644
index e661309..0000000
--- a/.gitmodules
+++ /dev/null
@@ -1,3 +0,0 @@
-[submodule "docs/doxygen-awesome-css"]
-	path = docs/doxygen-awesome-css
-	url = git@github.com:jothepro/doxygen-awesome-css
diff --git a/Doxyfile.mk b/Doxyfile.mk
index edf1ff4..69fe5df 100644
--- a/Doxyfile.mk
+++ b/Doxyfile.mk
@@ -1221,7 +1221,7 @@ HTML_FILE_EXTENSION    = .html
 # of the possible markers and block names see the documentation.
 # This tag requires that the tag GENERATE_HTML is set to YES.
 
-HTML_HEADER            =
+HTML_HEADER            = $(OUTDIR)/doxygen_header.html
 
 # The HTML_FOOTER tag can be used to specify a user-defined HTML footer for each
 # generated HTML page. If the tag is left blank doxygen will generate a standard
@@ -1231,7 +1231,7 @@ HTML_HEADER            =
 # that doxygen normally uses.
 # This tag requires that the tag GENERATE_HTML is set to YES.
 
-HTML_FOOTER            =
+HTML_FOOTER            = $(OUTDIR)/doxygen_footer.html
 
 # The HTML_STYLESHEET tag can be used to specify a user-defined cascading style
 # sheet that is used by each HTML page. It can be used to fine-tune the look of
@@ -1243,7 +1243,7 @@ HTML_FOOTER            =
 # obsolete.
 # This tag requires that the tag GENERATE_HTML is set to YES.
 
-HTML_STYLESHEET        =
+HTML_STYLESHEET        = $(OUTDIR)/doxygen.css
 
 # The HTML_EXTRA_STYLESHEET tag can be used to specify additional user-defined
 # cascading style sheets that are included after the standard style sheets
diff --git a/docs/doxygen.css b/docs/doxygen.css
new file mode 100644
index 0000000..10a8395
--- /dev/null
+++ b/docs/doxygen.css
@@ -0,0 +1,372 @@
+/* From: https://github.com/awalsh128/doxygen-themes/blob/main/themes/primary_dark/doxygen.css */
+:root {
+  --margin-large: 1rem;
+  --margin-medium: 0.5rem;
+  --padding-large: 1rem;
+  --padding-medium: 0.4rem;
+  --padding-small: 0.2rem;
+  --border-radius: 0.4rem;
+  --header-margin: 0.5rem;
+}
+
+a {
+  color: var(--bs-info);
+  text-decoration: none;
+}
+
+body {
+  background-color: var(--bs-gray-600);
+}
+
+h1 {
+  background-color: var(--bs-green);
+  border-radius: var(--border-radius);
+  font-size: xx-large;
+  margin-bottom: var(--header-margin);
+  margin-top: var(--header-margin);
+  padding: var(--padding-large);
+}
+
+h2 {
+  background-color: var(--bs-cyan);
+  border-radius: var(--border-radius);
+  font-size: x-large;
+  padding: var(--padding-large);
+}
+
+h3 {
+  background-color: var(--bs-gray-900);
+  border-radius: var(--border-radius);
+  padding: var(--padding-medium);
+}
+
+img {
+  margin-bottom: var(--margin-large);
+}
+
+p {
+  padding: 0 var(--padding-medium) 0 var(--padding-medium);
+}
+
+/* Tree View - Hide for now. Doxygen layouts are too unruly. */
+div.ttc {
+  display: none;
+}
+
+/* Page */
+.header {
+  background-color: var(--bs-gray-900);
+  background-image: var(--bs-gradient);
+  border-style: solid;
+  border-radius: var(--border-radius);
+  border-width: 1px;
+  margin-bottom: var(--margin-large);
+  margin-top: var(--margin-large);
+  padding: var(--padding-medium);
+  text-align: center;
+}
+.summary {
+  text-align: center;
+}
+.summary a {
+  margin-left: var(--padding-medium);
+}
+.title {
+  font-size: x-large;
+  padding: var(--padding-medium);
+}
+
+.textblock {
+  background-color: var(--bs-gray-700);
+  border-style: solid;
+  border-width: 1px;
+  border-radius: var(--border-radius);
+  margin-bottom: var(--margin-medium);
+  padding: var(--padding-large);
+}
+
+.section {
+  margin: 0 0 var(--margin-medium) 0;
+}
+.section dt {
+  background-color: var(--bs-blue);
+  border-radius: var(--border-radius) var(--border-radius) 0 0;
+  padding: var(--padding-medium);
+}
+.section td {
+  padding: var(--padding-small);
+}
+.section dd {
+  background-color: var(--bs-gray-600);
+  margin-bottom: 1px;
+  padding: var(--padding-medium);
+}
+
+.memSeparator {
+  line-height: 1rem;
+}
+.separator\: {
+  line-height: var(--margin-small);
+}
+
+/* Code */
+code {
+  white-space: pre-wrap;
+}
+.contents > p > code {
+  background-color: #000000;
+  display: inline-block;
+  padding: 1rem;
+  width: 100%;
+}
+.fragment {
+  background-color: #000000;
+  border-radius: var(--border-radius);
+  margin-bottom: var(--margin-medium);
+  padding: var(--padding-medium);
+}
+.comment {
+  color: var(--bs-gray-500);
+}
+.keyword {
+  color: var(--bs-pink);
+}
+.line {
+  color: var(--bs-cyan);
+  white-space: pre;
+}
+span.lineno {
+  color: var(--bs-teal);
+  padding-right: 1rem;
+}
+.preprocessor {
+  color: var(--bs-blue);
+}
+
+/* Breadcrumbs & Doxygen Header */
+.navpath {
+  background-color: var(--bs-gray-900);
+  background-image: var(--bs-gradient);
+  padding: 0.5rem 0 0.3rem 2rem;
+  border-radius: 0 0 var(--border-radius) var(--border-radius);
+}
+.navpath ul {
+  display: flex;
+  flex-wrap: wrap;
+  padding: 0 0;
+  margin-bottom: 1rem;
+  list-style: none;
+}
+.navelem + .navelem {
+  padding-left: var(--padding-medium);
+}
+.navelem + .navelem::before {
+  float: left;
+  padding-right: var(--padding-medium);
+  color: #6c757d;
+  content: var(--bs-breadcrumb-divider, "/");
+}
+
+/* Class: Types & Public Member Functions */
+.memberdecls {
+  width: 100%;
+}
+.memItemLeft {
+  background-color: var(--bs-gray-800);
+  padding: var(--padding-medium);
+  vertical-align: middle;
+}
+.memItemRight {
+  background-color: var(--bs-gray-800);
+  padding: var(--padding-medium);
+  vertical-align: middle;
+}
+.mdescLeft {
+  background-color: var(--bs-gray-700);
+  padding: var(--padding-medium);
+  vertical-align: middle;
+}
+.mdescRight {
+  background-color: var(--bs-gray-700);
+  padding: var(--padding-medium);
+  vertical-align: middle;
+}
+.inherit_header {
+  background-color: var(--bs-gray-700);
+  line-height: 3rem;
+  padding-left: var(--padding-large);
+  vertical-align: middle;
+}
+.memTemplParams {
+  background-color: var(--bs-gray-700);
+  padding: var(--padding-medium);
+  vertical-align: middle;
+}
+.memTemplItemLeft {
+  background-color: var(--bs-gray-800);
+  padding-right: var(--padding-medium);
+  vertical-align: middle;
+}
+.memTemplItemRight {
+  background-color: var(--bs-gray-800);
+  padding-right: var(--padding-medium);
+  vertical-align: middle;
+}
+
+/* Class: Member Functions */
+.memtitle {
+  background-color: var(--bs-orange);
+  border-bottom-left-radius: 0;
+  border-bottom-right-radius: 0;
+  border-style: solid solid none solid;
+  margin-bottom: 0;
+}
+.memtitle a {
+  color: var(--bs-yellow);
+}
+.memtitle a:hover {
+  color: var(--bs-red);
+  font-weight: bolder;
+}
+.memitem {
+  background-color: var(--bs-gray-700);
+  border-radius: 0 0 var(--border-radius) var(--border-radius);
+  border-style: none solid solid solid;
+  margin-bottom: 1rem;
+  padding: var(--padding-medium);
+}
+/* Class: Member Function - Prototype */
+.memproto {
+  background-color: var(--bs-gray-800);
+  border-radius: var(--border-radius);
+  margin-bottom: var(--margin-medium);
+  padding: var(--padding-medium);
+}
+.memproto td {
+  column-fill: auto;
+}
+/* Class: Member Function - Signature */
+.paramtype {
+  color: var(--bs-pink);
+  vertical-align: middle;
+}
+.paramname {
+  color: var(--bs-cyan);
+  vertical-align: middle;
+  display: inline-flex;
+}
+.paramname code {
+  /* Override code from .fragments. */
+  margin: 0;
+  padding: 0;
+  line-height: 1.6rem;
+  vertical-align: baseline;
+}
+/* Class: Member Function - Components */
+.memdoc dl {
+  margin-bottom: var(--margin-medium);
+}
+/* Class: Member Function - Template Parameters */
+.tparams dt {
+  background-color: var(--bs-green);
+  border-radius: var(--border-radius) var(--border-radius) 0 0;
+  padding: var(--padding-medium);
+}
+.tparams td {
+  padding: var(--padding-small);
+}
+.tparams dd {
+  background-color: var(--bs-gray-600);
+  margin-bottom: 1px;
+  padding: var(--padding-small);
+  padding-left: var(--padding-medium);
+}
+/* Class: Member Function - Parameters */
+.params dt {
+  background-color: var(--bs-green);
+  border-radius: var(--border-radius) var(--border-radius) 0 0;
+  padding: var(--padding-medium);
+}
+.params td {
+  padding: var(--padding-small);
+}
+.params dd {
+  background-color: var(--bs-gray-600);
+  padding: var(--padding-small);
+  padding-left: var(--padding-medium);
+}
+table.params tr {
+  margin-bottom: 1px;
+}
+
+/* Class List */
+.levels {
+  background-color: var(--bs-green);
+  padding: var(--padding-large);
+}
+.levels span {
+  font-weight: bold;
+  margin-left: var(--padding-medium);
+  margin-right: var(--padding-medium);
+}
+.levels span:hover {
+  color: var(--bs-cyan);
+  font-weight: bolder;
+}
+.directory {
+  background-color: var(--bs-gray-800);
+  background-image: var(--bs-gradient);
+  margin-bottom: var(--margin-large);
+  margin-top: var(--margin-large);
+}
+table.directory {
+  width: 100%;
+}
+.even {
+  background-color: var(--bs-gray-700);
+  background-image: var(--bs-gradient);
+}
+.entry {
+  padding: 0.5rem;
+  vertical-align: middle;
+}
+.arrow:hover {
+  color: var(--bs-cyan);
+  font-weight: bolder;
+}
+.icona {
+  background-color: var(--bs-green);
+  border-radius: 0.5rem;
+  color: var(--bs-black);
+  margin: 0.2rem 0.5rem 0.2rem 0.5rem;
+  padding: 0.2rem 0.5rem 0.2rem 0.5rem;
+}
+
+/* Struct */
+.dynheader {
+  background-color: var(--bs-blue);
+  border-radius: var(--border-radius) var(--border-radius) 0 0;
+  font-weight: bolder;
+  padding: var(--padding-medium);
+  text-align: center;
+}
+.dyncontent {
+  background-color: var(--bs-gray-700);
+  background-image: var(--bs-gradient);
+  border-radius: 0 0 var(--border-radius) var(--border-radius);
+  margin-bottom: var(--margin-medium);
+  padding: var(--padding-medium);
+  text-align: center;
+}
+
+#main-nav {
+  display: none;
+}
+
+#main-menu-state {
+  display: none;
+}
+
+.main-menu-btn {
+  display: none;
+}
diff --git a/docs/doxygen_footer.html b/docs/doxygen_footer.html
new file mode 100644
index 0000000..25c8f82
--- /dev/null
+++ b/docs/doxygen_footer.html
@@ -0,0 +1,7 @@
+<!-- adapted from https://github.com/awalsh128/doxygen-themes/blob/main/themes/primary_dark/header.html -->
+</main>
+<footer class="bg-dark mt-auto">
+  <div class="container">
+  </div>
+</body>
+</html>
diff --git a/docs/doxygen_header.html b/docs/doxygen_header.html
new file mode 100644
index 0000000..f28cd27
--- /dev/null
+++ b/docs/doxygen_header.html
@@ -0,0 +1,67 @@
+<!-- adapted from https://github.com/awalsh128/doxygen-themes/blob/main/themes/primary_dark/header.html -->
+<!DOCTYPE html>
+<html lang="en-US">
+  <head>
+    <meta name="generator" content="Doxygen 1.8.17" />
+
+    <title>$projectname: $title</title>
+
+    <meta name="viewport" content="width=device-width, initial-scale=1" />
+    <meta http-equiv="Content-Type" content="charset=UTF-8" />
+
+    <!-- Order matters here since below uses jQuery. -->
+    <script src="https://code.jquery.com/jquery-3.6.0.min.js"
+      integrity="sha256-/xUj+3OJU5yExlq6GSYGSHk7tPXikynS7ogEvDej/m4=" crossorigin="anonymous"></script>
+
+    <link rel="stylesheet" href="style.css" />
+
+    <link rel="stylesheet" href="https://pro.fontawesome.com/releases/v5.10.0/css/all.css"
+      integrity="sha384-AYmEC3Yw5cVb3ZcuHtOA93w35dYTsvhLPVnYs9eStHfGJvOvKxVfELGroGkvsg+p" crossorigin="anonymous" />
+
+    <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.1.2/dist/css/bootstrap.min.css" rel="stylesheet"
+      integrity="sha384-uWxY/CJNBR+1zjPWmfnSnVxwRheevXITnMqoEIeG1LJrdI0GlVs/9cVSyPYXdcSF" crossorigin="anonymous" />
+
+    <script src="https://cdn.jsdelivr.net/npm/bootstrap@5.1.2/dist/js/bootstrap.bundle.min.js"
+      integrity="sha384-kQtW33rZJAHjgefvhyyzcGF3C5TFyBQBA13V1RKPf4uH+bwyzQxZ6CmMZHmNBEfJ" crossorigin="anonymous"></script>
+
+    <script type="text/javascript" src="$relpath^dynsections.js"></script>
+    $treeview
+    $search
+    $mathjax
+    <link href="$relpath^$stylesheet" rel="stylesheet" type="text/css" />
+    $extrastylesheet
+  </head>
+  <body screen_capture_injected="true" class="text-light">
+    <header>
+      <nav class="navbar navbar-expand-lg navbar-fixed-top navbar-dark bg-dark">
+        <div class="container">
+          <a class="navbar-brand" href="index.html">
+            fpgen
+          </a>
+          <button class="navbar-toggler" type="button" data-bs-toggle="collapse" data-bs-target="#navbarContent"
+            aria-controls="navbarContent" aria-expanded="false" aria-label="Toggle navigation">
+            <span class="navbar-toggler-icon"></span>
+          </button>
+          <div class="navbar-collapse collapse" id="navbarContent">
+            <ul class="navbar-nav ms-auto mb-2 mb-lg-0">
+              <li class="nav-item">
+                <a class="nav-link" aria-current="page" href="annotated.html">
+                  <i class="fa fa-home"></i>Home
+                </a>
+              </li>
+              <li class="nav-item">
+                <a class="nav-link" href="annotated.html">
+                  <i class="fa fa-file-code"></i>API
+                </a>
+              </li>
+              <li class="nav-item">
+                <a class="nav-link" href="https://github.com/jay-tux/fpgen">
+                  <i class="fab fa-github"></i>Github
+                </a>
+              </li>
+            </ul>
+          </div>
+        </div>
+      </nav>
+    </header>
+    <main class="container">
diff --git a/inc/__helpers.hpp b/inc/__helpers.hpp
deleted file mode 100644
index 456c7c7..0000000
--- a/inc/__helpers.hpp
+++ /dev/null
@@ -1,12 +0,0 @@
-#ifndef _FPGEN___HELPERS
-#define _FPGEN___HELPERS
-
-#ifdef __clang__
-#include <type_traits>
-#define _FPGEN_USE_CONCEPTS 0
-#else
-#include <concepts>
-#define _FPGEN_USE_CONCEPTS 1
-#endif
-
-#endif
diff --git a/inc/aggregators.hpp b/inc/aggregators.hpp
index 8f2b4ad..f5afbf0 100644
--- a/inc/aggregators.hpp
+++ b/inc/aggregators.hpp
@@ -2,6 +2,7 @@
 #define _FPGEN_AGGREGATORS
 
 #include "generator.hpp"
+#include "type_traits.hpp"
 #include <forward_list>
 #include <ostream>
 #include <tuple>
@@ -102,7 +103,8 @@ template <typename T> size_t count(generator<T> gen) {
  *  \param[in] folder The folding function.
  *  \returns The final accumulator value.
  */
-template <typename TOut, typename TIn, typename Fun>
+template <typename TOut, typename TIn, typename Fun,
+          typename _ = type::is_function_to<Fun, TOut, TOut, TIn>>
 TOut fold(generator<TIn> gen, Fun folder) {
   TOut value = {};
   while (gen) {
@@ -129,7 +131,8 @@ TOut fold(generator<TIn> gen, Fun folder) {
  *  \param[in] initial The initial value for the accumulator (is copied).
  *  \returns The final accumulator value.
  */
-template <typename TOut, typename TIn, typename Fun>
+template <typename TOut, typename TIn, typename Fun,
+          typename _ = type::is_function_to<Fun, TOut, TOut, TIn>>
 TOut fold(generator<TIn> gen, Fun folder, TOut initial) {
   TOut value(initial);
   while (gen) {
@@ -157,7 +160,8 @@ TOut fold(generator<TIn> gen, Fun folder, TOut initial) {
  *  \returns A reference to the value which was passed as initial value and is
  * now the output value.
  */
-template <typename TOut, typename TIn, typename Fun>
+template <typename TOut, typename TIn, typename Fun,
+          typename _ = type::is_function_to<Fun, TOut, TOut, TIn>>
 TOut &fold_ref(generator<TIn> gen, Fun folder, TOut &initial) {
   while (gen) {
     initial = folder(initial, gen());
@@ -196,7 +200,9 @@ template <typename T> T sum(generator<T> gen) {
  *  \param[in,out] gen The generator to iterate over.
  *  \param[in] func The function to use.
  */
-template <typename T, typename Fun> void foreach (generator<T> gen, Fun func) {
+template <typename T, typename Fun,
+          typename _ = type::is_function_to<Fun, void, T>>
+void foreach (generator<T> gen, Fun func) {
   while (gen) {
     func(gen());
   }
diff --git a/inc/fpgen.hpp b/inc/fpgen.hpp
index f2ba0cf..a39cb81 100644
--- a/inc/fpgen.hpp
+++ b/inc/fpgen.hpp
@@ -5,5 +5,6 @@
 #include "generator.hpp"
 #include "manipulators.hpp"
 #include "sources.hpp"
+#include "type_traits.hpp"
 
 #endif
diff --git a/inc/generator.hpp b/inc/generator.hpp
index c2f8efe..8167760 100644
--- a/inc/generator.hpp
+++ b/inc/generator.hpp
@@ -10,14 +10,13 @@ using namespace experimental;
 #include <coroutine>
 #endif
 
-#include "__helpers.hpp"
+#include "type_traits.hpp"
 #include <exception>
 
 /**
  *  \brief The namespace containing all of fpgen's code.
  */
 namespace fpgen {
-#if _FPGEN_USE_CONCEPTS
 /**
  *  \brief The main generator type.
  *
@@ -31,14 +30,7 @@ namespace fpgen {
  *  \tparam T The value type for the generator. This should satisfy
  * `std::copyable`, or on (older) CLang versions, `std::is_copy_assignable<T>`.
  */
-template <std::copyable T> class generator {
-#else
-template <typename T>
-using enabler =
-    typename std::enable_if<std::is_copy_assignable<T>::value, bool>::type;
-
-template <typename T, enabler<T> = true> class generator {
-#endif
+template <typename T, typename _ = type::is_generator_type<T>> class generator {
 public:
   /**
    *  \brief The promise type for the generator.
diff --git a/inc/manipulators.hpp b/inc/manipulators.hpp
index 0a6ac60..9afda5f 100644
--- a/inc/manipulators.hpp
+++ b/inc/manipulators.hpp
@@ -2,6 +2,7 @@
 #define _FPGEN_MANIP
 
 #include "generator.hpp"
+#include "type_traits.hpp"
 #include <array>
 #include <type_traits>
 
@@ -18,13 +19,17 @@ namespace fpgen {
  *
  *  \tparam TIn The type contained in the provided generator.
  *  \tparam Fun The function signature of the mapping function.
+ *  \tparam TOut The output type. This type is deduced from the `Fun` type
+ * parameter.
  *  \param[in,out] gen The generator to map over. Will be in unusable state
  * afterwards.
  *  \param[in] func The function to map with.
  *  \returns A new generator whose contained type is the return type of the
  * mapping function.
  */
-template <typename TIn, typename Fun>
+template <typename TIn, typename Fun,
+          typename TOut = type::output_type<Fun, TIn>,
+          typename _ = type::is_function_to<Fun, TOut, TIn>>
 auto map(generator<TIn> gen, Fun func)
     -> generator<typename std::invoke_result<Fun, TIn>::type> {
   while (gen) {
@@ -69,7 +74,7 @@ generator<std::tuple<T1, T2>> zip(generator<T1> gen1, generator<T2> gen2) {
  *  \returns A new generator which yields all values in the original generator
  * except those not matching the predicate.
  */
-template <typename T, typename Pred>
+template <typename T, typename Pred, typename _ = type::is_predicate<Pred, T>>
 generator<T> filter(generator<T> gen, Pred p) {
   while (gen) {
     T val(gen());
@@ -139,7 +144,7 @@ template <typename T> generator<T> take(generator<T> gen, size_t count) {
  *  \returns A new generator where the first element is guaranteed to not
  * satisfy `p`.
  */
-template <typename T, typename Pred>
+template <typename T, typename Pred, typename _ = type::is_predicate<Pred, T>>
 generator<T> drop_while(generator<T> gen, Pred p) {
   while (gen) {
     T temp = gen();
@@ -172,7 +177,7 @@ generator<T> drop_while(generator<T> gen, Pred p) {
  * predicate and were generated before any element which didn't satisfy the
  * predicate.
  */
-template <typename T, typename Pred>
+template <typename T, typename Pred, typename _ = type::is_predicate<Pred, T>>
 generator<T> take_while(generator<T> gen, Pred p) {
   while (gen) {
     T val = gen();
diff --git a/inc/sources.hpp b/inc/sources.hpp
index fa68ba1..8861462 100644
--- a/inc/sources.hpp
+++ b/inc/sources.hpp
@@ -2,6 +2,7 @@
 #define _FPGEN_SOURCES
 
 #include "generator.hpp"
+#include "type_traits.hpp"
 #include <istream>
 #include <iterator>
 #include <type_traits>
@@ -117,14 +118,16 @@ template <typename T> generator<T> inc(T start) {
  *
  *  \tparam Fun The type of the function. Should have the signature
  * (std::istream &) -> T.
+ *  \tparam TOut The output type. This type is deduced from the `Fun` type
+ * parameter.
  *  \param[in,out] stream The input stream.
  *  \param[in] func The function to extract data from the stream.
  *  \returns A new generator which will iterate over the given stream, each time
  * applying the given function to retrieve the next value.
  */
-template <typename Fun>
-generator<typename std::invoke_result<Fun, std::istream &>::type>
-from_stream(std::istream &stream, Fun func) {
+template <typename Fun, typename TOut = type::output_type<Fun, std::istream &>,
+          typename _ = type::is_function_to<Fun, TOut, std::istream &>>
+generator<TOut> from_stream(std::istream &stream, Fun func) {
   while (stream.good() && !stream.eof()) {
     co_yield func(stream);
   }
diff --git a/inc/type_traits.hpp b/inc/type_traits.hpp
new file mode 100644
index 0000000..af224dc
--- /dev/null
+++ b/inc/type_traits.hpp
@@ -0,0 +1,77 @@
+#ifndef _FPGEN_TYPE_TRAITS
+#define _FPGEN_TYPE_TRAITS
+
+#include <functional>
+#include <type_traits>
+
+/**
+ *  \brief The namespace containing some type helpers for fpgen.
+ */
+namespace fpgen::type {
+
+/**
+ *  \brief Type trait deducing whether a type is a function from the input types
+ * to the output type.
+ *
+ *  In short, the function type `TFun` should have the signature `(TIns...) ->
+ * TOut`. This can include any "free" function (plain C function pointer), any
+ * lambda function, or any struct/class type supporting `TOut
+ * operator()(TIns...)`. This is implemented using the type traits
+ * `std::enable_if` and `std::is_convertible`. Usage: use as an extra template
+ * type, like so:
+ *        `typename _ = fpgen::type::is_function_to<TFun, TOut, TIns...>`.
+ *
+ *  \tparam TFun The function type.
+ *  \tparam TOut The output type for the function.
+ *  \tparam TIns The input type(s) for the function.
+ */
+template <typename TFun, typename TOut, typename... TIns>
+using is_function_to = typename std::enable_if<
+    std::is_convertible<TFun, std::function<TOut(TIns...)>>::value>::type;
+
+/**
+ *  \brief Type trait deducing whether a type is a predicate.
+ *
+ *  In short, the function type `TFun` should have the signature `(TIns...) ->
+ * bool`. This can include any "free" function (plain C function pointer), any
+ * lambda function, or any struct/class type supporting `bool
+ * operator()(TIns...)`. This is implemented using fpgen::type::is_function_to.
+ * Usage: use as an extra template type, like so:
+ *        `typename _ = fpgen::type::is_predicate<TFun, TIns...>`;
+ * this is synonymous to using
+ *        `typename _ = fpgen::type::is_function_to<TFun, bool, TIns...>`.
+ *
+ *  \tparam TFun The predicate function type.
+ *  \tparam TIns The input type(s) for the function.
+ */
+template <typename TFun, typename... TIns>
+using is_predicate = is_function_to<TFun, bool, TIns...>;
+
+/**
+ *  \brief Type trait deducing the output type of a functional type.
+ *
+ *  If the type `TFun` is a function type `(TIns...) -> TOut`, this type trait
+ * will return `TOut`. Thie type trait is implemented using
+ * `std::invoke_result`.
+ *
+ *  \tparam TFun The function type.
+ *  \tparam TIns The input type(s) for the function.
+ */
+template <typename TFun, typename... TIns>
+using output_type = typename std::invoke_result<TFun, TIns...>::type;
+
+/**
+ *  \brief Type trait deducing whether a type is fit as data type for a
+ * generator.
+ *
+ *  The type `T` will pass the test if it's copy-assignable.
+ *
+ *  \tparam T The type to check.
+ */
+template <typename T>
+using is_generator_type =
+    typename std::enable_if<std::is_copy_assignable<T>::value>::type;
+
+} // namespace fpgen::type
+
+#endif