diff --git a/libs/ml/include/psemek/ml/neural_net/error.hpp b/libs/ml/include/psemek/ml/neural_net/error.hpp
new file mode 100644
index 00000000..9274e1fb
--- /dev/null
+++ b/libs/ml/include/psemek/ml/neural_net/error.hpp
@@ -0,0 +1,39 @@
+#pragma once
+
+#include <stdexcept>
+#include <cstddef>
+
+namespace psemek::ml
+{
+
+	struct neural_net_error
+		: std::runtime_error
+	{
+		using runtime_error::runtime_error;
+	};
+
+	struct empty_neural_net_error
+		: neural_net_error
+	{
+		empty_neural_net_error();
+	};
+
+	struct wrong_activation_types_count_error
+		: neural_net_error
+	{
+		wrong_activation_types_count_error(std::size_t expected, std::size_t actual);
+	};
+
+	struct wrong_neural_net_input_size
+		: neural_net_error
+	{
+		wrong_neural_net_input_size(std::size_t expected, std::size_t actual);
+	};
+
+	struct wrong_neural_net_output_size
+		: neural_net_error
+	{
+		wrong_neural_net_output_size(std::size_t expected, std::size_t actual);
+	};
+
+}
diff --git a/libs/ml/include/psemek/ml/neural_net/evaluator.hpp b/libs/ml/include/psemek/ml/neural_net/evaluator.hpp
index 6143f7a1..6bd3bf7c 100644
--- a/libs/ml/include/psemek/ml/neural_net/evaluator.hpp
+++ b/libs/ml/include/psemek/ml/neural_net/evaluator.hpp
@@ -2,17 +2,9 @@
 
 #include <psemek/ml/neural_net/neural_net.hpp>
 
-#include <stdexcept>
-
 namespace psemek::ml
 {
 
-	struct wrong_neural_net_input_size
-		: std::runtime_error
-	{
-		wrong_neural_net_input_size(std::size_t expected, std::size_t actual);
-	};
-
 	// A class that stores temporary data to facilitate
 	//   allocation-free multiple evaluations of a neural net
 	template <typename T>
diff --git a/libs/ml/include/psemek/ml/neural_net/neural_net.hpp b/libs/ml/include/psemek/ml/neural_net/neural_net.hpp
index 9b44f504..d0663fa6 100644
--- a/libs/ml/include/psemek/ml/neural_net/neural_net.hpp
+++ b/libs/ml/include/psemek/ml/neural_net/neural_net.hpp
@@ -1,26 +1,14 @@
 #pragma once
 
 #include <psemek/ml/neural_net/activation.hpp>
+#include <psemek/ml/neural_net/error.hpp>
 #include <psemek/util/span.hpp>
 
 #include <vector>
-#include <stdexcept>
 
 namespace psemek::ml
 {
 
-	struct empty_neural_net_error
-		: std::exception
-	{
-		char const * what() noexcept;
-	};
-
-	struct wrong_activation_types_count_error
-		: std::runtime_error
-	{
-		wrong_activation_types_count_error(std::size_t expected, std::size_t actual);
-	};
-
 	namespace detail
 	{
 
diff --git a/libs/ml/source/neural_net/error.cpp b/libs/ml/source/neural_net/error.cpp
new file mode 100644
index 00000000..0393a951
--- /dev/null
+++ b/libs/ml/source/neural_net/error.cpp
@@ -0,0 +1,24 @@
+#include <psemek/ml/neural_net/error.hpp>
+
+#include <psemek/util/to_string.hpp>
+
+namespace psemek::ml
+{
+
+	empty_neural_net_error::empty_neural_net_error()
+		: neural_net_error("neural net must have at least a single layer")
+	{}
+
+	wrong_activation_types_count_error::wrong_activation_types_count_error(std::size_t expected, std::size_t actual)
+		: neural_net_error(util::to_string("wrong number of activation types: expected ", expected, ", got ", actual))
+	{}
+
+	wrong_neural_net_input_size::wrong_neural_net_input_size(std::size_t expected, std::size_t actual)
+		: neural_net_error(util::to_string("wrong neural net input size: expected ", expected, ", got ", actual))
+	{}
+
+	wrong_neural_net_output_size::wrong_neural_net_output_size(std::size_t expected, std::size_t actual)
+		: neural_net_error(util::to_string("wrong neural net output size: expected ", expected, ", got ", actual))
+	{}
+
+}
diff --git a/libs/ml/source/neural_net/evaluator.cpp b/libs/ml/source/neural_net/evaluator.cpp
index 433fb004..34c59116 100644
--- a/libs/ml/source/neural_net/evaluator.cpp
+++ b/libs/ml/source/neural_net/evaluator.cpp
@@ -1,14 +1,8 @@
 #include <psemek/ml/neural_net/evaluator.hpp>
 
-#include <psemek/util/to_string.hpp>
-
 namespace psemek::ml
 {
 
-	wrong_neural_net_input_size::wrong_neural_net_input_size(std::size_t expected, std::size_t actual)
-		: std::runtime_error(util::to_string("wrong neural net input size: expected ", expected, ", got ", actual))
-	{}
-
 	template struct neural_net_evaluator<float>;
 	template struct neural_net_evaluator<double>;
 
diff --git a/libs/ml/source/neural_net/neural_net.cpp b/libs/ml/source/neural_net/neural_net.cpp
index 4d10ef54..83395064 100644
--- a/libs/ml/source/neural_net/neural_net.cpp
+++ b/libs/ml/source/neural_net/neural_net.cpp
@@ -5,15 +5,6 @@
 namespace psemek::ml
 {
 
-	char const * empty_neural_net_error::what() noexcept
-	{
-		return "neural net must have at least a single layer";
-	}
-
-	wrong_activation_types_count_error::wrong_activation_types_count_error(std::size_t expected, std::size_t actual)
-		: std::runtime_error(util::to_string("wrong number of activation types: expected ", expected, ", got ", actual))
-	{}
-
 	template struct neural_net<float>;
 	template struct neural_net<double>;