I wrote a parser/interpreter for VBA a few years back, and even though the core concepts were straightforward, conforming to a language that had been in development for decades turned out to be brain-wreckingly tedious. One of the biggest parts of it was a block of thousands(?) of conditional statements to figure out the result type of every combination of type+operator+type. At the end of the ordeal, though, I threw the engine into a test app that looked like GWBASIC, and got a little nostalgia kick from being able to run BASIC like it was 1983.