That happy feeling when you replace some handwritten code with an off-the-shelf library.
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Is there any relationship between language adoption and the size of its standard library?
These days it seems completely orthogonal, but early Java adopters spoke highly of the collections library compared with C++.
Maybe it's the widespread availability of package managers?
Over a sufficiently long time horizon, all code you write is legacy code.
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.