JVM bytecode back end for Idris

1. Idris with libffi. To install Idris with libffi using cabal: cabal install idris -f ffi
2. Java 8

1. Download and extract JVM bytecode back end from here. Make sure to download the release corresponding to your Idris version.
2. Define IDRIS_JVM_HOME variable with extracted directory path
3. From the extracted directory, run $IDRIS_JVM_HOME/bin/install to install Idris packages for idris-jvm.
4. Add $IDRIS_JVM_HOME/codegen/bin to PATH.

• helloworld.idr

  module Main
  
  import IdrisJvm.IO
  
  data Tree a = Leaf
              | Node (Tree a) a (Tree a)
  
  inorder : Tree a -> List a
  inorder Leaf = []
  inorder (Node left a right) = inorder left ++ [a] ++ inorder right
  
  tree : Tree String
  tree = Node
          (Node
            (Node Leaf "3" Leaf)
            "+"
            (Node Leaf "7" Leaf))
          "/"
          (Node Leaf "2" Leaf)
  
  main : JVM_IO ()
  main = printLn $ inorder tree

• Compiling

  • On Linux/Mac OS: $ idris --portable-codegen jvm -p idrisjvmffi helloworld.idr -o target
  • On Windows: idris --portable-codegen jvm.bat -p idrisjvmffi helloworld.idr -o target

• Running

  • On Linux/Mac OS: $ java -cp $IDRIS_JVM_HOME/idris-jvm-runtime.jar:target-classes main.Main
  • On Windows: $ java -cp $IDRIS_JVM_HOME/idris-jvm-runtime.jar;target-classes main.Main

• Idris basic types are mapped to Java types. Idris Int is mapped to Java primitive int. Idris String is mapped to Java String. Idris Integer is represented as Java BigInteger. Idris Double is mapped to Java double. Idris Bits8, Bits16, Bits32 are mapped to Java int. Idris Bits64 is mapped to Java long.

• Tail recursion is eliminated using JVM's GOTO. For the following code, sum 50000 wouldn't blow up the stack.

  sum : Nat -> Nat
  sum n = go 0 n where
    go : Nat -> Nat -> Nat
    go acc Z = acc
    go acc n@(S k) = go (acc + n) k

• Non-recursive tail call is handled using trampolines. For the following code, evenT 10909000007 would work just fine and return the result after few seconds. IO is used here as otherwise Idris inlines the function calls and the functions end up being tail recursive instead of mutually recursive. Non-recursive tail calls are delayed using Java 8 lambdas with JVM's invokedynamic.

  mutual
    evenT : Nat -> JVM_IO ()
    evenT Z = printLn True
    evenT (S k) = do
      setProperty "bar" (show k)
      oddT k
  
    oddT : Nat -> JVM_IO ()
    oddT Z = printLn False
    oddT (S k) = do
      setProperty "foo" (show k)
      evenT k

• FFI - Calling Java from Idris: From Idris, invoking Java static methods, instance methods, constructors are all supported. See here for an example.

• FFI: Calling Idris from Java: Idris functions can be exported as Java instance methods, static methods and constructors. The exported class with Idris implementations can extend a Java class and implement interfaces. It can have static and instance fields and the field values can be set from Idris. Idris types (monomorphic, for example, List Int) can also be exported as a Java class. See here for an example.

• Idris functions as Java lambdas: Idris functions can be passed as Java lambdas in FFI. JVM's invokedynamic instruction is used to create target functional interface objects just like how javac does.

main : JVM_IO ()
main = ArrayList.fromList ["foobar", "hello", "world"] >>=
       stream >>=                                           -- Java 8 Stream from Java's ArrayList
       filter (jlambda (not . isPrefixOf "foo")) >>=        -- Idris function as Java "Predicate" lambda
       map (jlambda Strings.reverse) >>=                    -- Idris function as Java "Function" lambda
       collect !toList >>=
       Objects.toString >>=
       printLn

• Maybe type can be used in an FFI function to avoid Java null getting into Idris code. Maybe used in an argument position will pass null to the Java code if the value is Nothing otherwise the unwrapped value will be passed to Java. In the similar way, Maybe type used in the return type position would return Nothing if the FFI function returns null otherwise returns the actual value in Just.

• Idris functions can be exported with Java annotations. This enables Idris to be used in Java's annotation based libraries such as Java Microbenchmark Harness, Spring Boot etc. See here for an example.