2016-04-19 04:25:14 +00:00
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---
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title: Interfacing variadic functions from Haskell
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author: nek0
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tags: english, programming
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description: Some findings I made while playing with the Haskell FFI
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---
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2016-04-21 23:22:05 +00:00
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I confess I am a Haskell aficionado. Whenever I program something for pleasure, I usually prefer this
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2016-04-19 04:25:14 +00:00
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language because of its elegance.
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Currently I am working on Haskell bindings to the [GEGL](http://www.gegl.org/) library. The motivation
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behind this my desire to dabble in Game development and I have the need for a library to draw on SDL
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Surfaces. I am obviously not really a fan of the easy solutions and I try to learn new things. Like using
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the Haskell FFI.
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2016-04-21 23:22:05 +00:00
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While writing the bindings I encountered the problem, that GEGL exposes variadic functions in its header
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2016-04-19 04:25:14 +00:00
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which I need to interface. This poses a serious Problem for Haskell because the number of function
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arguments has to be constant. There is simply no way defining a function without knowing how many
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arguments it has and of what type each argument is. This stays true even for my solution. The only reason
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2016-04-21 23:22:05 +00:00
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my solution works is that I can limit the cases how to interface these variadic functions to a manageable
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2016-04-19 04:25:14 +00:00
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amount.
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2016-04-21 23:22:05 +00:00
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To build my bindings I do not use the standard FFI of Haskell, but the Haskell library
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2016-04-19 04:25:14 +00:00
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[inline-c](http://hackage.haskell.org/package/inline-c) to call the C functions directly without using
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rigid bindings. This is achieved in inline-c by wrapping the function call into a QuasiQuoter. As I said
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earlier, this still requires you to write a QuasiQuoter for every case this function gets called, but you
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don't have to clutter your code with `foreign import ccall` declarations.
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For limiting your cases I recommend using a sum type as a function argument. A sum type is a type which
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has multiple constructors. You can have a constructor for each case you need to interface and distinguish
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2016-04-21 23:22:05 +00:00
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between them using Haskell's pattern matching. You can see an example on how to make all this
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[in my bindings](https://github.com/nek0/gegl/blob/master/src/GEGL/FFI/Node.hs).
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