ML's module system has a notion of composition and even types! https://jozefg.bitbucket.io/posts/2015-01-08-modules.html
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A cute systemd feature to get the system clock roughly correct: if the system time is before the compilation time of the systemd binary, it moves the clock forward to that time!
https://lwn.net/Articles/714355/
TIL Tcl has a notion of 'safe interpreters', a mode where you can run untrusted code in a sandbox: https://www.tcl.tk/man/tcl8.4/TclCmd/safe.htm
Not many programming languages have this, but it's way safer to include in the implementation than try to build as a userland library.
Bootstrapping a language can be immensely satisfying.
I've added the ability to define stub types in the Garden stdlib and suddenly I don't need to special-case Int or String! They're just normal type declarations.