Eta User Guide


This user guide is still under active development. Please consult the GHC 7.10.3 User Guide for language-level details as it is very similar to Eta.

Using the Eta CLI

The Eta compiler can be invoked from the command line using the eta command.


See eta –help for more details.

If you are using Etlas, can you directly invoke it as follows:

etlas exec eta -- [MODE_FLAGS] [OPTIONS] [SOURCE_FILES]

See etlas exec eta – –help for more details.

Java Foreign Function Interface


The layer that interacts with Java in Eta is called the Foreign Function Interface (FFI). This layer will allow you to import a Java method as an Eta function and export a Eta function as a Java method. It automatically handles the intermediate conversions between Java types and Eta types, so all you have to worry about is the right type signature.


The FFI revolves around some built-in types that Eta specially recognizes, so we’ll start by introducing them.

Unboxed & Primitive Types

Unboxed types are primitive types which must have values that are fully evaluated and cannot have values that are suspended expressions (thunks) like normal types do. Since they are primitives, they cannot be defined using the Eta language and need to be given special support by the compiler.

They suffixed with a # and require the use of MagicHash language extension in order to be recognized in source code. Primitive types have a special representation at the JVM level, shown below.

Boxed Types

Boxed types, on the other hand, can have values which can be thunks and may also be undefined if the evaluation of the thunk leads to an error state. Boxed types can store both boxed and unboxed values internally.


data Int = I# Int#

Primitive Types Reference

Declaring Java Wrapper Types

In Eta, you declare a JWT (Java Wrapper Type) in order to create a link between an Eta data type and a Java class or interface. JWTs can be though of as wrappers over raw Java objects.

data P a b c ... = P (@[class-name-here] a b c ...)
  deriving Class

This is the generic format for declaring a tag type where:

  • P is the Eta name you would use to refer to it. Typically, P is the unqualified class name.
  • a b c ...` are the type variables that correspond to the generic parameters of the underlying Java class. Note that type variables in Eta must start with a lowercase letter.
  • [class-name-here] is the name of a the class the tag type represents. For example, it can be java.lang.String.


data PrintStream = PrintStream (
  deriving Class

In order to tell Eta about the JWT’s parent/child relationships, a Class typeclass instance and a Inherits type family declaration must be defined. A Class instance is obtained with the deriving clause above.

An example of declaring an instance of the Inherits type family is shown below:


{-# LANGUAGE TypeFamilies #-}

type instance Inherits PrintStream = '[FilterOutputStream]

The first element in the type-level list should be the parent class and the remaining elements can be the interfaces it implements. Note that the TypeFamilies extension must be enabled in order to declare the instance.

Java Monad



The following will show the general syntax and what will occur in each of the cases, following by some examples.

Foreign imports


Foreign exports

The general syntax for foreign exports:

foreign export java "javaFunctionName" functionName :: var1 -> var2 -> var3
  -> Java tagType returnType


  • javaFunctionName - identifier of java method that is generated for tagType class
  • functionName - haskell function name that is exported. The name can be omitted and the generated Java method will have the same name as Eta function.
  • var<N> - argument types that can be marshalled into Java types. (TODO: which types can be marshalled?)
  • tagType - [tag type](#declaring-tag-types) that corresponds to Java class where the function will be generated. You cannot specify polymorphic type variable, only specialised one (see #77.
  • returnType - return type that can be marshalled back from Java into Eta. (TODO: which types can be marshalled?)

The following example:

import Java

data Export = Export (@mypackage.Export)

foreign export java sayHello :: JString -> Java Export JString

sayHello n = return . toJava $ "Hello, " ++ fromJava n ++ "!"

And Java class that is generated:

package mypackage;

public class Export {
    public Export() {}

    public String sayHello(String n) {
      // Code to invoke the Eta runtime system.



Debugging Stack Traces

This document will debug stack trace for the following error message which has been produced on running the program. This is useful in filing a more helpful bug report.

Exception in thread "main" java.lang.NoClassDefFoundError: Calendar
        at oldzmtimezm1zi1zi0zi3.system.Time$satzus10SQ.thunkEnter(Unknown Source)
        at eta.runtime.thunk.UpdatableThunk.enter(
        at eta.runtime.stg.Closure.evaluate(
        at base.text.parsercombinators.ReadP$skipSpaceszuskip.enter(Unknown Source)
        at base.ghc.Read$satzus5BWW.enter(Unknown Source)
        at eta.runtime.apply.Function.apply(
        at eta.runtime.apply.Apply$8.enter(
        at base.text.parsercombinators.ReadP$satzus341G.enter(Unknown Source)
        at eta.runtime.apply.Function.apply(
        at eta.runtime.apply.Apply$8.enter(
        at base.text.parsercombinators.ReadP$satzus341G.enter(Unknown Source)
        at eta.runtime.apply.Function.apply(
        at eta.runtime.apply.Apply$8.enter(
        at base.text.parsercombinators.ReadP$run.enter(Unknown Source)
        at oldzmtimezm1zi1zi0zi3.system.Time$satzus10SY.thunkEnter(Unknown Source)
        at eta.runtime.thunk.UpdatableThunk.enter(
        at eta.runtime.stg.Closure.evaluate(
        at oldzmtimezm1zi1zi0zi3.system.Time$lvl98zus13J8.thunkEnter(Unknown Source)
        at eta.runtime.thunk.UpdatableThunk.enter(
        at eta.runtime.stg.Closure.evaluate(
        at base.ghc.Base$zpzp.enter(Unknown Source)
        at oldzmtimezm1zi1zi0zi3.system.Time$doFmtzus13PB.enter(Unknown Source)
        at oldzmtimezm1zi1zi0zi3.system.Time$lvl112zus13P6.thunkEnter(Unknown Source)
        at eta.runtime.thunk.UpdatableThunk.enter(
        at eta.runtime.stg.Closure.evaluate(
        at base.ghc.Base$zpzp.enter(Unknown Source)
        at oldzmtimezm1zi1zi0zi3.system.Time$doFmtzus13PB.enter(Unknown Source)
        at oldzmtimezm1zi1zi0zi3.system.Time$zdwformatCalendarTime.enter(Unknown Source)
        at oldzmtimezm1zi1zi0zi3.system.Time$zdfShowClockTimezuzdcshow.enter(Unknown Source)
        at eta.runtime.apply.Function.apply(
        at eta.runtime.apply.ApP.stackEnter(
        at eta.runtime.stg.StackFrame.enter(
        at eta.runtime.stg.StgContext.checkForStackFrames(
        at$zdwa7.enter(Unknown Source)
        at$hPutStr2.enter(Unknown Source)
        at base.system.IO$print1.enter(Unknown Source)
        at base.system.IO$print.enter(Unknown Source)
        at eta.runtime.apply.Apply$20.enter(
        at eta.runtime.apply.PAP.apply(
        at eta.runtime.apply.ApV.stackEnter(
        at eta.runtime.stg.StackFrame.enter(
        at eta.runtime.stg.StackFrame.enter(
        at eta.runtime.stg.StackFrame.enter(
        at eta.runtime.stg.Capability.schedule(
        at eta.RuntimeScheduler.scheduleWaitThread(
        at eta.Runtime.evalLazyIO(
        at eta.Runtime.main(
        at eta.main.main(Unknown Source)
Caused by: java.lang.ClassNotFoundException: Calendar
        at java.lang.ClassLoader.loadClass(
        at sun.misc.Launcher$AppClassLoader.loadClass(
        at java.lang.ClassLoader.loadClass(
        ... 48 more
  1. Parse the top most line

    oldzmtimezm1zi1zi0zi3.system.Time$satzus10SQ.thunkEnter(Unknown Source)

    This gives the following information:

    • Package name: old-time
    • Module name: System.Time
    • Thunk name: satzus10SQ

    The error is in old-time package in the module System.Time in expression satzus10SQ which happens to be an thunk as can be ascertained by the call of thunkEnter.

  2. Find the JAR file

    If you ran the code without using etlas, then you might have indicated its location using the -o option.

    Example: eta -o Out.jar SuperAwesomeModule.hs

    If you did use etlas, then its probable location is dist/build/<executable-name>/<executable-name>.jar.

  3. Extract JAR file

    Perform extraction in that directory:

    jar xf [executable-name].jar
  4. Find thunk class file

    cd oldzmtimezm1zi1zi0zi3/system/
    ls Time\$satzus10SQ.class
  5. Run the Java Class disassembler

    javap -c -v Time\$satzus10SQ.class

    This will print readable Java bytecode. Submitting a bug report with bytecode is very helpful.

  6. Produce STG Dump of the Package

    In this case, the package is old-time. In the .cabal file for the project, add a new field called ghc-options and set -ddump-stg -ddump-to-file as the value.

    Clean and re-build your package again. There will be a corresponding System/Time.dump-stg file that is generated.

  7. Decoding z-encoding

    satzus10SQ is encoded using z-encoding. Decode it using the table found there. Decoding satzus10SQ gives sat_s10SQ.

    Another example: satzus10ZZ8 decodes to sat_s10Z8

  8. Get the STG Definition

    Search the Time.dump-stg file for the definition of sat_s10SQ and save that to a separate file.

    Filing bug report with error message, STG dump and the bytecode is highly helpful. You can find an example dump of these three messages here.