LFortran author here. As kergonath said below, both "new" Flang that you linked above, and LFortran started at about the same time. I know the Flang developers well and I am hoping we'll be able to collaborate in the future more. It looks like MLIR would be one option: they are going to use it for Flang and LFortran can have a backend that could also use it.

LFortran's main advantage is that it is interactive, so the parser and semantics has to be a little different. And also we designed it so that it will be quite approachable for people to write tools on top.

Interesting to see an interactive Fortran as I started with Fortran in my first Job - on my first day I was told there is a book in the company library go and learn Fortran

For the really big Fortran systems I worked on in the 1980's (Map Reduce) the edit compile / link process could take several minutes for a single module - we did have a build system written in JCL to help automate this as well.

Not exactly. Development of both started at about the same time, but it was a coincidence. Flang is a standard compiler, whilst LFortran is more geared towards interactive use à la Jupyter, or source-to-source compilation.

Here’s a demo of the Jupyter notebook-like interface: https://mybinder.org/v2/gl/lfortran%2Fweb%2Flfortran-binder/...