Koka: a function-oriented language with effect inference

For more background information, see:

• The Koka book for a specification of the Koka language and a primer on algebraic effects.
• The library documentation.
• The Koka research page and the slides of a talk presented Lang.Next (April 2012).
• The article Algebraic Effects for Functional Programming [3] about the algebraic effects in Koka.

Installing the compiler

At this point there are no binary releases of Koka and you need to build the compiler yourself. Fortunately, Koka has few dependencies and builds without problems on most common platforms, e.g. Windows, MacOSX, and Unix.

The following programs are required to build Koka:

• The Haskell platform (version 7.4 or later).
• The NodeJS runtime (version 4.2 LTS or later).
• Some version of Git for version control.

All these programs are very easy to install on most platforms. Now we can build Koka itself:

1. First clone the Koka sources with algebraic effects support:

   > git clone https://github.com/koka-lang/koka.git

   You can also use the flag -b dev to get the latest development version.

2. Go to the newly created Koka directory:

   > cd koka

3. Install any needed Node libraries using the Node package manager:

   > npm install

   If you are running on MacOSX or Unix, you may have to run this as sudo npm install so that the npm package manager has enough permissions to install the jake and madoko tools.

4. Finally, build the compiler and run the Koka interactive environment:

   > jake

   You can type jake help to see an overview of all make targets.

The excellent Sublime text editor is recommended to edit Koka programs. You can install support for Koka programs using

> jake sublime

After this .kk files will be properly highlighted. It is also recommended to use the newly installed snow color theme which is designed to work well with Koka files.

Running the interactive compiler

After running a plain jake command, the Koka interactive environment will start:

__          _
| |        | |
| | __ ___ | | __ __ _
| |/ // _ \| |/ // _` | welcome to the koka interpreter
|   <| (_) |   <| (_| | version 0.7.0-dev (debug), Jun 30 2016
|_|\_\\___/|_|\_\\__,_| type :? for help

loading: std/core

Now you can test some expressions:

> println("hi koka")
hi koka

> :t "hi"
string

> :t println("hi")
console ()

Or load a demo:

> :l demo/collatz
compile: lib/demo/collatz.kk
check  : demo/collatz
modules:
  demo/collatz

> main()
Collatz(27) took 111 steps.

And quit the interpreter:

> :q

Before the effect one believes in different causes than one does after the effect.
 -- Friedrich Nietzsche

You can also run examples in the browser by setting the host:

> :set --host=browser
> 1+2

Some browser specific demo to try is for example demo/dom/conway.kk.

Algebraic effect handlers

A novel feature of Koka is a compiled and typed implementation of algebraic effect handlers (described in detail in [3]). In the interactive environment, you can load various demo files with algebraic effects which are located in the test/algeff directory. This is by default included in the search path, so we can load them directly using the load (:l) command:

> :l scoped

Use the :? command to get an overview of all commands. After loading the scoped demo, we can run it directly from the interpreter:

> :l scoped
compile: test/algeff/scoped.kk
check  : scoped
modules:
  scoped

> main()
[[[3]],[2,1],[1,2],[1,1,1]]
(state=12, [[[3]],[2,1],[1,2],[1,1,1]])
[(state=1, [[3]]),(state=5, [2,1]),(state=5, [1,2]),(state=9, [1,1,1])]
[[[3]]]
[42]

Some interesting demos are:

• common.kk: Various examples from the paper "Algebraic Effects for Functional Programming" [3]. Shows how to implement common control-flow abstractions like exceptions, state, iterators, ambiguity, and asynchronous programming.

• scoped.kk: Various examples from the paper "Effect handlers in Scope" [5].

• nim.kk: Various examples from the paper "Liberating effects with rows and handlers" [1].

• async*.kk: Various asynchronous effect examples.

• parser.kk: Implements parser combinators as an effect.

Enjoy, -- Daan

References

1. Daniel Hillerström, and Sam Lindley. “Liberating Effects with Rows and Handlers.” In Proceedings of the 1st International Workshop on Type-Driven Development, 15--27. TyDe 2016. Nara, Japan. 2016. doi:10.1145/2976022.2976033.

2. Daan Leijen. “Koka: Programming with Row Polymorphic Effect Types.” In Mathematically Structured Functional Programming 2014. EPTCS. Mar. 2014. arXiv:1406.2061.

3. Daan Leijen. Algebraic Effects for Functional Programming. MSR-TR-2016-29. Microsoft Research. Aug. 2016. https://www.microsoft.com/en-us/research/publication/algebraic-effects-for-functional-programming. Extended version of [4].

4. Daan Leijen. “Type Directed Compilation of Row-Typed Algebraic Effects.” In Proceedings of Principles of Programming Languages (POPL’17). Paris, France. Jan. 2017.

5. Nicolas Wu, Tom Schrijvers, and Ralf Hinze. “Effect Handlers in Scope.” In Proceedings of the 2014 ACM SIGPLAN Symposium on Haskell, 1--12. Haskell ’14. ACM, New York, NY, USA. 2014. doi:10.1145/2633357.2633358