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Let me start by saying I do generally understand copyleft GPL, but I'm having some trouble with this particular scenario. It's quite complicated to communicate but I'll try my best.

First I'm working on an app in iOS that supports Russian voice recognition. The engine I use is pocketspinx which uses a BSD license so all good there.

I am trying to use within my app a voice recognition acoustic model that is distributed under GPLV2. The model is from VoxForge. An acoustic model is kind of like a script that tells the app how to run.

It is located here : http://www.repository.voxforge1.org/downloads/Russian/Trunk/AcousticModels/

My understanding, and please correct me if I'm wrong is that I can not distribute this acoustic model bundled in my iOS app, though I would be able to if it had been licensed as LGPL instead. (Please tell me if this is correct)

My next question is whether I could download this acoustic model from within the iOS app and use it that way? The user would press a big download button, the acoustic model would be downloaded and then the system would work.

Last question is if the last thing is ok, am I allowed under GPL to rehost that acoustic on my own server (for faster download speed), and would that rehosting have to open to public downloads.

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The GPL is not compatible with the App Store, because the store's Terms of Service impose additional restrictions which violate the GPL. Any additional downloads or enhancements to the software would have to be done through the store, probably under the guise of "In-App Purchases."

Whether it's GPL or LGPL probably doesn't matter, as it is not the "linking provision" that is the problem here, but rather the ability of the consumer to swap out the library as he sees fit (a capability that the App Store will not give him).

Applications distributed through the Mac Store might be compatible with the GPL, since it is possible to modify and distribute the application outside of the Mac store.

Further Reading
GPL Enforcement in Apple's App Store
Is it possible to have GPL software in the Mac App Store?

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  • It seems you saw iOS and GPL and rushed to an answer. But I really need an answer to the last 2 questions. I already said that I understood I could not distribute it together.
    – thewormsterror
    Commented Nov 11, 2015 at 2:03
  • I would also download the acoustic model directly from voxforge from within my iOS app not though in app purchases. I don't see how this is not ok. For example windows is licensed commercially, php interpreter is open source, but there are gpl php scripts. Wouldn't the same logic apply here?
    – thewormsterror
    Commented Nov 11, 2015 at 2:09
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    @thewormsterror: You can't distribute it at all through the App Store under these conditions, nor can you add anything to an iPad or iPhone app outside of the App Store. If you try to add things to the app from the outside, or give it the ability to download things not through the App Store, Apple will reject your app.
    – Robert Harvey
    Commented Nov 11, 2015 at 2:22
  • In the original question I already said I thought this was the case, that I couldn't distribute it through the app store. Think of the acoustic model as a script. And my program on iOS as the interpreter. While I understand that the GPL script can't be distributed in the iOS app store, my app could download it directly later and I don't see how that would violate GPL.
    – thewormsterror
    Commented Nov 11, 2015 at 2:32
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    @thewormsterror Apple will category reject apps for iOS that are capable of running arbitrary code that is downloaded from the internet. See stackoverflow.com/q/4891642
    – user40980
    Commented Nov 11, 2015 at 2:36

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