For MACOS users who have problems with finding alex or happy after installing the Haskell platform.
Run echo $PATH to see what is in your path.
In particular, check that you have a directory /usr/local/bin and that it is in the path. If not add it to your path.
After installing the Haskell platform, I would expect /usr/local/bin to contain alex and happy. Another place to look for is /Users/YOURUSERNAME/Library/Haskell/bin.
You can also use the MACOS Finder to search for system files. For example, search for alex, then click the +-symbol on the right (or find Search Options in the menue) and look under Other on the left for System Files and include them in your search. Then you should find programs such as alex or happy if they exist at all.
One thing that helped in some cases was to run stack install alex and stack install happy. For some reason alex and happy where in the wrong place, but after moving them from .local/bin to /usr/local/bin things worked.