Graal licensing seems pretty complicated to me, and if I get it right Truffle only makes sense when used together with Graal.
From what I could find in the FAQ [1]:
* "GraalVM Community is free to use for any purpose and comes with no strings attached, but also no guarantees or support."
* "GraalVM Enterprise includes Improved performance and security over GraalVM Community"
In general it seems pretty good, but I think it is a bit painful to have to pay for certain optimizations... again, imagine if projects had to pay to, say, "clang -O3". Also could be painful to get stuck in Graal/Truffle version N if version N+1 switches to different licensing terms.
In general commercial compilers are a bit of a drag to work with... I use Saxon from time to time and sometimes I need to check the matrix to see if a feature is available or not [2]. For instance, until version 10.0, higher order functions (!) was a feature one had to pay for, for whatever reason.
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This happened with solaris: oracle decided to make new ‘official’ development closed, but the last opensource version continued to be used and developed by the community.
0. https://github.com/oracle/graal/blob/master/truffle/LICENSE....