How to Call Haskell From Java?

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To call Haskell functions from Java, you can make use of the Java Native Interface (JNI). Here is a step-by-step explanation of how to do it:

  1. Write your Haskell code: Start by writing the Haskell functions you want to call from Java. The functions should be part of a Haskell module and should have the appropriate type declarations.
  2. Create an interface file: In Haskell, create an interface file (*.hsc) using the Haskell FFI (Foreign Function Interface) syntax. This file specifies the external interface of the Haskell functions and allows Java to interact with them.
  3. Generate the C bindings: Use the Haskell tool "hsc2hs" to generate the C bindings from the interface file. This tool translates the Haskell FFI syntax into C code that can be used by the JNI.
  4. Build the Haskell code: Compile your Haskell code, including the generated C bindings, using the appropriate Haskell compiler (e.g., GHC). This will produce a shared library file (.so) or a dynamic-link library (.dll) that contains the Haskell functions.
  5. Create a Java class: In your Java code, create a class that defines the native methods you want to invoke from Haskell. Use the "native" keyword to declare these methods and provide the method signatures.
  6. Compile the Java code: Compile your Java code using the Java compiler (e.g., javac). This will generate a bytecode file (*.class) for the Java class that contains the native method declarations.
  7. Create the JNI bindings: Use the Java Native Development Kit (JDK) tool "javah" to generate the JNI bindings for your Java class. This tool generates a C header file (*.h) that declares the native methods.
  8. Implement the JNI methods: In C or C++, implement the native methods declared in the generated header file. These methods act as a bridge between Java and Haskell. Inside these methods, you will use the JNI API to call the corresponding Haskell functions.
  9. Compile the JNI code: Compile your C or C++ code, including the JNI methods, using a C or C++ compiler. This will produce a shared library or dynamic-link library that can be loaded by Java.
  10. Run your Java program: Finally, run your Java program, making sure that the shared library or dynamic-link library is accessible to the Java virtual machine (JVM). The JVM will then load the library and allow you to call the Haskell functions through the Java native methods.

Remember that this process involves both Haskell and Java code, as well as C or C++ code for the JNI bindings. It may require some familiarity with these technologies, but it allows you to seamlessly integrate Haskell functionality into your Java applications.

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What is the best way to communicate between Haskell and Java?

There are several ways to communicate between Haskell and Java. Here are some commonly used methods:

  1. Foreign Function Interface (FFI): Haskell provides a Foreign Function Interface that allows you to directly call and use C functions. Since Java has the Java Native Interface (JNI), you can write a C interface that serves as a bridge between Haskell and Java. This method allows you to call Java methods from Haskell and vice versa.
  2. Inter-process communication: You can use inter-process communication mechanisms such as sockets or pipes to communicate between Haskell and Java. One approach is to use the Network.Socket module in Haskell and Java's package to establish a TCP/IP socket connection and exchange data between the two languages.
  3. Language-specific libraries: Both Haskell and Java have libraries that facilitate communication between the two languages. For example, Haskell has libraries like HJava, which allows you to interact with Java code from Haskell. Similarly, there are Java libraries like jni4net or JNA (Java Native Access) that enable calling Haskell code from Java.
  4. Message Passing: You can use message passing frameworks like RabbitMQ or Apache Kafka. These frameworks provide a way to send messages between different processes or systems, allowing you to exchange data between Haskell and Java applications.

The choice of communication method depends on the specific requirements of your project, performance considerations, and integration complexity. It is important to consider factors like data size, latency, simplicity, and maintainability when selecting the best method for your use case.

What is the best practice for error handling in Haskell-Java integration?

When integrating Haskell and Java, there are a few best practices for error handling. Here are some suggestions:

  1. Use appropriate types: Haskell has a rich type system that allows you to encode different levels of errors using types. For example, you can use the Either or Maybe type to represent success or failure. By using these types, you can ensure that errors are explicitly handled and propagated correctly.
  2. Define clear error types: It's important to define clear error types that accurately represent the possible failures in your integration. This allows you to have a well-defined error handling strategy and makes it easier for callers to handle errors appropriately.
  3. Use Haskell's IO monad: When interacting with Java or performing IO operations, you will typically be working within the IO monad. The IO monad provides a built-in mechanism for handling errors using functions like catchIOError and tryIOError. You can use these functions to catch and handle exceptions thrown by Java code.
  4. Leverage Java exception handling: Java exceptions can be thrown and caught in Haskell using the hs-java library. You can define Java exceptions in your Java code and catch them in Haskell using the appropriate bindings. This allows you to integrate with existing Java libraries and reuse their error handling mechanisms.
  5. Use explicit error handling: Instead of relying on exceptions, it's often better to use explicit error handling in Haskell. Functions like catch, either, and maybe can be used to handle errors explicitly and handle them in a controlled manner.
  6. Handle errors at the appropriate level: It's important to handle errors at the appropriate level, depending on the context. This might involve handling errors within the Haskell code, propagating errors to the caller, or converting errors to a specific exception type understood by the Java code.

Remember that error handling is an essential part of any integration, and it's crucial to design a robust and reliable error handling strategy that suits your specific requirements.

How to pass data from Java to Haskell?

There are different ways to pass data from Java to Haskell. Here are a few approaches:

  1. Using Foreign Function Interface (FFI): The FFI allows you to call Haskell functions from Java and vice versa. You can define an interface in Java that declares the functions you want to call from Haskell, and then implement that interface in Haskell. Libraries such as JNA (Java Native Access) can be used to simplify the process of invoking Haskell functions from Java.
  2. Using Inter-process communication (IPC): You can establish communication between Java and Haskell processes using IPC mechanisms like sockets, pipes, or shared memory. In this approach, you need to define a protocol or data format that both Java and Haskell can understand. The Java program can send data over the IPC channel, and the Haskell program can receive and process it accordingly.
  3. Using Java Native Interface (JNI): The JNI allows Java applications to call native C/C++ code, and Haskell programs can be compiled to C code. You can interface Java with Haskell by using JNI to invoke the compiled Haskell functions from Java. This approach requires writing some C code to act as a middle layer between Java and Haskell.
  4. Using Java libraries that support Haskell types: Some Java libraries support Haskell data types directly, such as Jaskell ( These libraries provide ways to represent and manipulate Haskell data in Java, allowing you to pass data between the two languages more easily.

The choice of approach depends on the specific requirements of your project, the complexity of the data being passed, and the preferred level of interactivity between the Java and Haskell programs.

What is the process to invoke Haskell code in Java?

To invoke Haskell code in Java, you can use the Java Native Interface (JNI) to bridge the gap between the two languages. Here is the general process:

  1. Write the Haskell code: Start by writing the Haskell code that you want to invoke from Java. Make sure to compile the Haskell code into a shared library or a dynamic link library (DLL) that can be loaded by Java.
  2. Create the JNI interface: Define a Java class that will serve as the interface between Java and Haskell. This class will contain native methods that will be implemented in Haskell.
  3. Generate the C bindings: To connect Java with Haskell, you need to create C bindings. This can be done using a tool like c2hs or by manually writing the necessary C code to call the Haskell functions.
  4. Implement the Haskell functions: Write the Haskell functions that correspond to the native methods defined in the JNI interface. These functions should have C-compatible type signatures and use the foreign function interface (FFI) to communicate with the C code.
  5. Compile the Haskell code: Compile the Haskell code into a shared library or DLL using a Haskell compiler like GHC. Make sure to link the necessary C bindings and enable the FFI.
  6. Load the library in Java: In your Java code, load the compiled shared library or DLL using the System.loadLibrary() method.
  7. Call the native methods: Finally, you can call the native methods defined in the JNI interface to invoke the Haskell code from Java.

It's important to note that this is a general overview of the process, and the specifics may vary depending on your specific use case. JNI can be quite complex, so it's recommended to thoroughly read the JNI documentation and consult additional resources for a more detailed understanding.

What is the performance impact of calling Haskell from Java?

Calling Haskell code from Java can incur some performance overhead due to the differences in execution models and runtime environments between the two languages. Here are a few factors that can impact the performance:

  1. Interoperability overhead: The process of calling Haskell from Java usually involves creating a foreign function interface (FFI) layer, which introduces additional overhead during function calls and argument marshaling. This can lead to some performance degradation.
  2. Garbage Collection differences: Haskell uses lazy evaluation and employs a different garbage collection mechanism compared to Java's garbage collector. The differences in garbage collection strategies can cause inconsistencies and potential inefficiencies when calling Haskell functions from Java.
  3. Threading and Concurrency: Haskell has built-in support for efficient concurrency with lightweight threads, whereas Java provides its own threading model with the Java Thread API. When interacting between the two, it might require additional synchronization mechanisms, message passing, or other coordination strategies, which can impact performance.
  4. Memory management: Haskell uses a different memory management strategy compared to Java's managed heap. Coordinating memory allocation and deallocation between the two environments can introduce some overhead.
  5. Language and library differences: Haskell and Java have different standard libraries and programming paradigms. Translating data structures and algorithms between the two can result in performance penalties.

It's important to note that the actual performance impact will highly depend on the specific use case, implementation, and the nature of the code being called in Haskell from Java. In some cases, the overhead might be negligible, while in others, it can be significant. The best approach is to measure and profile the actual performance impact in your specific scenario to make informed decisions.

How to implement Haskell libraries in Java?

Haskell libraries cannot be directly implemented in Java because Haskell is a functional programming language while Java is an object-oriented programming language. However, you can use Java libraries and frameworks that provide similar functionality as Haskell libraries. Here are steps you can follow to achieve this:

  1. Identify the specific functionality or feature you want from the Haskell library.
  2. Search for Java libraries or frameworks that provide similar functionality. You can use online resources, documentation, or libraries' official websites for this.
  3. Evaluate the Java libraries based on their features, ease of use, and community support. Look for libraries that closely match the desired functionality and have a good reputation.
  4. Once you have chosen a Java library, you can import it into your Java project. This process might involve adding the library as a dependency through a build management tool like Maven or Gradle.
  5. Refer to the library's documentation or examples to understand how to use it in your Java code. This will involve understanding the library's API, classes, and methods.
  6. Implement the functionality in your Java code, using the selected library. You may need to make necessary changes or adjustments to adapt the library's approach to your specific requirements.

Remember that the implementation of a Haskell library in Java will not be an exact translation, as the underlying paradigms and concepts are different. However, by leveraging available Java libraries with similar functionality, you can achieve similar results in your Java project.

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