Depending on multiple versions of LLVM can be very difficult. The GHC developers are looking at only supporting one:
miniblog.
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A great deep dive on building GitHub's code view, optimising React, scaling to thousands whilst supporting both Ctrl-F and syntax highlighting:
Suppose you want to make a small, hackable interpreter, so you write an AST walker.
Could you recover performance by supporting lightweight threads that use all the CPUs?
The Python GIL preferred single threaded performance over multithreading, this is the opposite.
The Makefile problem, or how absurdly hard it is to write paths in a text file whilst supporting both *nix and Windows: