Hi! Arc is a dialect of lisp. It is influenced by Common Lisp and Scheme [1], two other dialects of lisp, and is actually implemented [2] on top of Racket (a sort of Scheme).
Feel free to ask more questions here. There's also a section of Paul Graham's website with writing about Arc [3], and you can use http://af.searchyc.com/ to search this forum.
Hope this helps!
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[1] It's also influenced by other programming languages. Python, for example.
[2] I'm speaking here about the original implementation of Arc by Paul Graham and Robert Morris. People on this forum have experimented with porting Arc to other platforms, including Java, Common Lisp, and JavaScript.
