miniblog.

Further tinkering with diagnostics, following feedback! * Two lines of context above and below now. * The caret is included in the line below where possible. * Syntax highlighting of keywords. I kinda feel that smart context sizing would be better. What do you think so far?

I'm adding a += operator to my programming language, because writing `x = x + 1` is tedious. This opens the tricky design question of which operators should support this. Is += and -= sufficient, or do you expect things like >>= and **= to be available?

Doing another iteration on my diagnostics display. I'm reasonably happy with the bold highlighting within the error message. I'm not sure about the colour on Warning and Error though. It gives the output some visual structure, but arguably the message itself is more important.