Java from Haskell: a tutorial

15 September 2017 — by Facundo Domínguez

Our introductory post for inline-java showed that we could call methods written in Java (or indeed any JVM function) from Haskell. Much in the style of other packages, it is moreover possible to do using java syntax, so examples from Java API documentation can be reused as-is.

In celebration of the recently released inline-java-0.7.0, this post is a tutorial on how to use it all. We cover the marshalling of values between Haskell and Java and how we leverage the type checker to ensure that neither sides disagree on what types arguments and return values should have. This git repository contains the minimal configuration necessary to try the examples that follow.

Invoking java methods

Let’s start with a simple program.

-- hello-java.hs
{-# LANGUAGE QuasiQuotes #-}
{-# OPTIONS_GHC -fplugin=Language.Java.Inline.Plugin #-}
import Language.Java (withJVM)
import Language.Java.Inline

main :: IO ()
main = withJVM [] [java| { System.out.println("Hello Java!"); } |]

The function withJVM starts an instance of the Java Virtual Machine (JVM), and the java quasiquotation executes the java code passed to it as a block of statements. The program can be built and executed from inside the above mentioned folder with

$ stack --nix build
$ stack --nix exec hello-java
Hello Java!

Because part of inline-java is implemented in a GHC plugin, we tell GHC to use this plugin with the pragma OPTIONS_GHC. Every module using inline-java needs to ask for the plugin in the same way (this requirement might be lifted in a future version of GHC).

GHC doesn’t parse any Java and neither does inline-java. So how can this program possibly work? The answer is that inline-java feeds the quasiquotation to the javac compiler, which generates some bytecode that is stored in the object file of the module. At runtime, inline-java arranges the bytecode to be handed to the JVM using the jni package. Finally, inline-java makes use of the package jvm to have the bytecode executed.

Marshalling values

Now, suppose we have some value in Haskell that we want to provide as an argument to a Java method.

main :: IO ()
main = withJVM [] $ do
    let d = 1 :: Double
    [java| { System.out.println($d); } |]

We are coercing a Haskell value of type Double into a Java value of the primitive type double, which is then used in the quasiquotation in the form of an antiquoted variable. When inline-java passes this quasiquotation to javac, it feeds it a static method of the form:

static void fresh_name(double $d) { System.out.println($d); }

At runtime, inline-java passes the result of the coercion as the argument $d. Any instance of Language.Java.Coercible a ty can be used in the same way, where a stands for the Haskell type and ty stands for an encoding of the Java type (JType). The package jvm defines a few instances, and users can define their own.

class Coercible a (ty :: JType) | a -> ty
instance Coercible () 'Void
instance Coercible Bool ('Prim "boolean")
instance Coercible CChar ('Prim "byte")
instance Coercible Char ('Prim "char")
instance Coercible Word16 ('Prim "char")
instance Coercible Int16 ('Prim "short")
instance Coercible Int32 ('Prim "int")
instance Coercible Int64 ('Prim "long")
instance Coercible Float ('Prim "float")
instance Coercible Double ('Prim "double")

In the following program we get an integer value from Java.

import Data.Int (Int32)

main :: IO ()
main = withJVM [] $ do
    x <- [java| new Object[5].length |]
    print (x :: Int32)

Here we have dropped the braces surrounding the Java code in order to hint to inline-java that we are giving an expression rather than a block of statements. Expressions, unlike statements, have values. We are coercing a Java value of type int into a Haskell value of type Int32. The quasiquoter arranges for the coercion to happen after the JVM finishes evaluating the Java expression. As it was the case for antiquoted variables, the return type of the quasiquotation needs to be an instance of Language.Java.Coercible a ty.

Marshalling Java objects

Coercing values is useful enough until we consider how to marshal values which do not have an obvious counterpart in Java. For instance, what do we coerce a Haskell list or a vector to? As these types require a more elaborate representation in Java, we use the classes Reflect and Reify from the package jvm.

type family Interp a :: JType

class Reify a where
  reify :: J (Interp a) -> IO a

class Reflect a where
  reflect :: a -> IO (J (Interp a))

The type family Interp a stands for the Java type that corresponds to the Haskell type a. A value of type J (Interp a) is a reference to a Java object of type Interp a. With reify we can convert a Java object to a Haskell value. With reflect we can convert a Haskell value back into a Java object. As with the type class Coercible, the package jvm already provides a few instances of Reify and Reflect. For example,

type instance Interp ByteString = 'Array ('Prim "byte")
instance Reify ByteString
instance Reflect ByteString

type instance Interp Text = 'Class "java.lang.String"
instance Reify Text
instance Reflect Text

type instance Interp Double = 'Class "java.lang.Double"
instance Reify Double
instance Reflect Double

type instance Interp [a] = 'Array (Interp a)
instance Reify a => Reify [a]
instance Reflect a => Reflect [a]

There is an instance of Coercible (J ty) ty. So we can use references produced with reflect in java quasiquotations.

import qualified Data.Text
import Language.Java (reflect)

main :: IO ()
main = withJVM [] $ do
    text <- reflect (Data.Text.pack "Hello Java!")
    [java| { System.out.println($text); } |]

In this example, text has type J ('Class "java.lang.String") and the antiquoted variable $text is expected to have type java.lang.String. Conversely, we can use reify, to create a Haskell value from the reference produced by a quasiquotation.

main :: IO ()
main = withJVM [] $ do
    jarray <- [java| new String[] {"a", "b"} |]
    xs <- reify jarray
    print (xs :: [Text])

Type checking

One of the strengths of inline-java is that it makes it difficult to get an ill-typed interaction between Haskell and Java. What if a quasiquotation returned a value of a type that the Haskell side does not expect? What if any of the methods used in the quasiquotation are used with arguments of the wrong type?

The short answer to both questions is that, most of the time, GHC and javac will catch the type mismatches.

main :: IO ()
main = withJVM [] $ do
    jarray <- [java| new String[] {"a", "b"} |]
    xs <- reify jarray
    print (xs :: [Double])

Based on the instances of Reify and Coercible that are in scope, Haskell is able to determine with precision what Java type the quasiquotation should return. In this program, the Haskell side expects Java to return an array of doubles (java.lang.Double[]) when the Java side is returning an array of strings (java.lang.String[]). The javac compiler complains.

$ stack --nix build
[1 of 1] Compiling Main             ( Main.hs, Main.o )
.../ error: incompatible types: String[] cannot be converted to Double[]
{ return  new String[] {"a", "b"} ; } // .hs:10

When making calls with the wrong argument or return types to Java methods, the functions in the package jvm produce a runtime error. Usually the programmer would get an exception called NoSuchMethodError and the name of the offending method. The error produced by inline-java improves this in two aspects. Firstly, we get the error at build time. Secondly, the error message points precisely at either the return type or the argument with the mismatched type.

Are there any type errors that cannot be caught at build time? There is, indeed. For instance, a quasiquotation can yield or use objects of type java.lang.Object. The Haskell or the Java side may then need to downcast these objects. Downcasts, as is always the case in vanilla Java, involve a dynamic type check, which can fail if the objects are downcasted to the wrong type. In this sense, Haskell + Java is no safer than Java alone.


In this blogpost we introduced quasiquotation, Coercible and the Reify/Reflect type classes. We saw that with inline-java, it’s possible to call JVM methods from Haskell with little fuss. An important aspect of the design of inline-java is that conversions are always explicit. That’s because they can be expensive, so the programmer should be well aware when they are happening. Coercible captures the class of types that can be passed to the JVM without paying any marshalling/unmarshalling costs.

The package inline-java not only makes Java code convenient to embed in Haskell programs, it also prevents coding mistakes which could otherwise occur when relying on the lower-level packages jni and jvm. In a future post we’ll take a peak under the hood. Since v0.7, inline-java makes use of a GHC plugin to make the type safety happen.

About the author

Facundo Domínguez

Facundo is a software engineer supporting development and research projects at Tweag. Prior to joining Tweag, he worked in academia and in industry, on a varied assortment of domains, with an overarching interest in programming languages.

If you enjoyed this article, you might be interested in joining the Tweag team.

This article is licensed under a Creative Commons Attribution 4.0 International license.


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