Comment by majewsky

One of the more widely deployed ones is Go. Code looks like with synchronous IO, but everything runs in green threads that can be suspended when IO would block. So if you do a File.Read, after it returns, you might be running on a different OS thread because you were suspended mid-Read and later woken up on a different thread. It's generally nice to work with, unless you need to interface with a non-Go library that expects you to use a single thread throughout (e.g. for callbacks), in which case suddenly everything turns into a royal mess.