Haskell is an awesome language, but we need to remember that it is not very useful in isolation. In almost any realistic application, Haskell has to coexist with other languages, even if only to call existing C libraries or to make use of operating system services. In actual practice, the more easily we can fit Haskell into existing ecosystems, the more application domains we can unlock.

## Beyond bridging

The core of Haskell’s ability to interoperate, the ForeignFunctionInterface language extension, has been available and stable for a long time. However, for all but the simplest interoperability requirements, it tends to be tedious to use. So we have tools such as hsc2hs and c2hs to automate some of the work of declaring foreign entities and writing marshalling code. This does not just save work, it also prevents many common mistakes.

Over time, we realised that this is not sufficient either. Tools like hsc2hs and c2hs are typically used to implement, what I like to call, bridging libraries. These libraries wrap the API of a foreign library in Haskell, usually by exposing a Haskell API that is close to the original and, occasionally, by providing a more functional, more high-level API. This works fine up to a certain API size. After that — just think about the API surface needed to write Android, iOS, macOS, or Windows apps — the overhead of bridging libraries tends to weigh down and break that bridge:

• The initial implementation is a huge undertaking, which few people, or institutions, are willing to embark on.
• API evolution of the foreign library creates a significant ongoing maintenance burden. Even worse, typically, multiple versions need to be supported simultaneously.
• Documentation becomes a major headache. It is infeasible to transliterate all of the original documentation, but referring Haskell users to the original requires to exactly mirror that original API and demands an understanding of the bridging conventions by the library user.
• Even just the overhead of linking all the bridging code starts to be an issue for large APIs.

Inline foreign code in Haskell sidesteps these issues. The effort to implement an inline library for a foreign language is fixed and supports an arbitrary number of foreign language libraries of arbitrary size without any further overhead. Documentation is naturally just the original and marshalling overhead is in proportion to its use in any single application. Admittedly, a user now needs to know both Haskell and the foreign language, but, given the documentation issue, that was always the case for large APIs.

I have illustrated this in a talk at the 2014 Haskell Symposium, where I introduced language-c-inline to use Objective-C code inline in Haskell to code against macOS APIs. You can watch the talk on YouTube.

## I was not alone

What I didn’t know at the time is that Mathieu Boespflug, Alexander Vershilov, and Facundo Domínguez drew the same inspiration as I did from Geoff Mainland’s work on Quasiquoting Support for GHC and independently developed inline-r, a Haskell library for inline R code. Subsequently, Mathieu worked with Francesco Mazzoli on inline-c and developed inline-java with Alp Mestanogullari and Facundo Domínguez.

The latter is where foreign inline code, once again, provides an unorthodox solution to an old problem. In an attempt to fit into the ubiquitous Java ecosystem, there has been a string of failed attempts to compile Haskell to JVM code — although, maybe, one will eventually be successful, even if at a steep price. Integration with Java is highly attractive as it opens the door to many applications and commercial opportunities. In addition, successful entrants into the JVM ecosystem, such as Scala and Clojure, suggest that generating JVM bytecode is the opportune approach.

## Come as you are

The fundamental design philosophy behind inline code is to accept that there are multiple language environments and work with that. We don't try to shoehorn the semantics of one language into the other. We don't even force a particular style for calling such functions. Indeed you can call Haskell, R, C, Objective-C or Java functions with each of their respective syntaxes. In some cases, this has crucial consequences. For instance, R has special syntax for variadic functions with labeled arguments and default values, none of which Haskell has. inline-r lets you e.g. call R's plot() function the way it was meant to be called:

H> let xs = [x :: Double | x <- [1..10]]
H> let ys = [x^2 | x <- xs]
H> [r| plot(xs_hs, ys_hs, col = "red", main = "A quadratic function") |]


Highly versatile functions with multiple modes of use would otherwise require multiple bindings to the same function Haskell side or unnaturally packing arguments into lists. Likewise, Objective-C has special syntax for sending messages to objects. It's convenient to be able to reuse documentation snippets that use this pervasive idiom.

But this design philosophy has further consequences still. All of the projects you saw mentioned above let each language not just keep their syntax, but also their runtimes. GCC or LLVM compiles C and Objective-C to native code. The reference R interpreter parses and executes R code on-the-fly. Java code gets compiled to JVM bytecode, using configuration extracted from Java build tools (Gradle, Maven, ...). And yes, it is GHC that compiles Haskell to native code, like it always did. To run Haskell on JVM based enterprise middlewares, we package it up as a JAR and load the native code into the JVM dynamically.

You could take the motto to be "each language, come as you are". There are tradeoffs to this approach. The bad news is that,

• when multiple compilers are involved, we have to mesh multiple build toolchains together;
• when multiple garbage collectors are involved, we need to make sure live data in one language is not considered garbage in the other. This is a tricky problem in general.

But on the flip side:

• We spare ourselves implementing new code generators, e.g. for the JVM. The many failed attempts attest to the difficulty of this endeauvour.
• By generating JVM bytecode, you lose access to all existing packages that depend on foreign code, such as C libraries. In contrast, inline-java happily enables projects involving Haskell, Java, and C without any need to change existing packages.
• Each language is an equal citizen and the semantics and runtime behaviour of each is preserved. Note in particular that the runtime characteristics of Haskell code are not all that well matched with those that the JVM is optimised for. Haskell has a much higher allocation rate than Java, it has entirely different update patterns due to purity and laziness, and it relies on different control flow, including heavily reliance on tail calls and their optimised implementation. In this case, inline-java just uses the tried and tested GHC native code as is.

Each language is its own first-class citizen, but that's not to say each language forces a particular way to package things. We can still maintain the convenience of bundled distribution, as the Java archive (JAR) format is sufficiently flexible to allow arbitrary native code alongside JVM bytecode in a single self-contained bundle — we detailed this in a previous post on the Haskell compute PaaS with Sparkle. Conversely, we also routinely embed Java bytecode in Haskell binaries packaged as RPM's.

## Happy coexistence

All in all, inline foreign code enables diversity in the form of scalable mixed language projects while requiring no more than a limited toolchain maintenance burden. Interestingly, all Haskell foreign inline code libraries have enabled and been driven by concrete applications. The Haskell for Mac IDE is built on language-c-inline. The package inline-r was originally developed for Amgen as part of a commercial project. The package inline-c was developed by FP Complete to more easily bind large mathematical libraries and is used by LumiGuide for their OpenCV work. inline-java is maintained by Tweag I/O and features contributions from Leap Year and other companies that use it at the core of their products.