Extempore for Impromptu users


This was once a blog post—corrections/improvements welcome.

If you’re an Impromptu user and you’re thinking about making the switch to Extempore, you’ve probably got a few questions about what the difference is between Impromptu and Extempore. The Extempore philosophy covers this to some extent, but this post is designed to be short and sweet, answering three questions that Impromptu users might have:

  1. what’s the same
  2. what’s new or different
  3. what will I have to do to switch over

What’s the same

  • Impromptu’s Scheme interpreter has been brought into Extempore largely unchanged. So any vanilla Scheme code or libraries should work fine (e.g. the pitch class library pc_ivl.xtm is unchanged).
  • Time in Extempore (including (now), callback, temporal recursion etc.) is exactly the same as it was in Impromptu.

What’s new or different

There are some obvious ‘big picture’ differences:

The biggest difference, though, is the addition of xtlang: a new programming language (and compiler). Whereas Scheme was the only language supported in Impromptu, in Extempore you can write Scheme code and xtlang code, and the languages can share data and call into one another, and everyone pretty much just gets along fine.

The other main difference between Impromptu and Extempore are in audio and graphics. Impromptu was strongly tied to the Core Audio framework for audio, Quartz for 2D graphics, and OpenGL for 3D graphics. Core Audio and Quartz are both OSX-only, so there was a need to change this tight coupling in the move to make Extempore cross-platform.

In the case of audio, audio signal processing in Extempore hasn’t been replaced by another monolithic framework—it’s been replaced by home-grown xtlang code. This ‘turtles all the way down’ approach brings a huge win over the previous approach: the dynamic run-time modifiability of the whole DSP chain. If you don’t want to write your own instruments from scratch that’s ok, too: Extempore ships with some synths and samplers, and hopefully the number of built-in instruments will grow as the community grows.

In the graphics case, Extempore uses nanovg (for 2D) and OpenGL (for 2D and 3D) cross-platform graphics programming. This involves a few platform-specific hacks in the source code for Extempore itself, but any Scheme/xtlang code you write to manipulate nanovg or OpenGL graphics objects should be portable to any platform where Extempore runs. Some of Impromptu’s OpenGL bindings have made it across to Extempore unchanged, while others may be slightly different.

In fact, one of the key tenets of Extempore’s design philosophy was to keep functionality out of the compiled extempore binary and provide it in libraries wherever possible. So Extempore’s audio and graphics support, which was baked in to Impromptu, is now user-editable. Some examples you might find interesting are:


  • examples/core/audio_101.xtm
  • examples/core/polysynth.xtm
  • examples/external/electrofunk.xtm
  • examples/external/portmidi.xtm
  • examples/core/sampler.xtm
  • examples/core/sndfile.xtm


  • examples/external/shader-tutorials/ (the whole directory)
  • examples/external/raymarcher.xtm
  • examples/external/openvg.xtm
  • examples/external/spectrogram.xtm
  • examples/external/xtmrender1.xtm (or any of the other xtmrender examples)

What will I have to change to switch from Impromptu to Extempore?


All AudioUnit code (i.e. any functions starting with au:) will have to be changed. I know that’s a pain—sorry!—but it was an inevitable consequence of making things cross-platform. The ‘setting up the AU graph’ section of your code will therefore need to be rewritten. You can either use the built-in synths and sampler, or you can build your own instruments, or you could even use Extempore’s MIDI in/out (see libs/external/portmidi.xtm) to play an instrument running outside of Extempore.

The good news is that from play-note ‘up’, Extempore’s audio infrastructure works pretty much exactly the same as Impromptu’s. In fact, that truth is the starting point for the tutorials on ‘playing an instrument in Extempore’ (coming soon). Once the audio graph is set up, the code for playing music in Extempore should be the same as it was in Impromptu.


If you’re using any Quartz functions (e.g. gfx:make-square, gfx:draw-path) you’ll need to change them over to the equivalent cairo drawing commands. nanovg’s API and drawing model is similar to Quartz’s, but there may be a couple of tweaks required.

If you’re using OpenGL, then you may not need to change much. But it’s definitely worth going over the code carefully to see if it’s still doing what you expect.

Getting help

Many of the folks on the Extempore mailing list (including myself) were once Impromptu users, so we’re probably in a pretty good position to help you out if you have any problems. Have a look around elsewhere on this blog, too. Hopefully you’ll get excited about the cool stuff that you can do in Extempore which you just can’t do in Impromptu, and that excitement might dull the (hopefully small) pain of making the switch!