Metaprogramming in Tcl: https://antirez.com/articoli/tclmisunderstood.html (even implementing a macro system! Feels like fexprs, but uplevel seems nicer than eval)
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In praise of Tcl, and reflecting on syntax features for a good command shell:
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.
I wrote a blogpost in 2014 comparing macro systems. Incredibly, tcl's calling convention is equally powerful!