It is to avoid wasting time processing files that cannot lead to good results. If you want to try it for a different programming language, please fork the repo and try adding your file formats and test if it gives meaningful results, and if it does please submit a pull request.
Other than that one limitation is that it uses a model under the hood that is trained on a specific dataset which is filtered for a specific list of programming languages. So without changing the model as well, the support for other languages could be subpar. At the moment the model is all-MiniLM-L6-v2, here's a detailed summary of the dataset: https://huggingface.co/sentence-transformers/all-MiniLM-L6-v...
Also probably a flag would solve it for some users, the best way would be to add a configuration option. At the moment there are no config file/.rc file support in SeaGOAT though, but there is an issue to add it and I'm happy to merge pull requests: https://github.com/kantord/SeaGOAT/issues/180
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https://github.com/kantord/SeaGOAT#what-programming-langauge...