Hi all,

I’m wondering if there are any resources that discuss the design of GHC’s implementation of layout. (I haven’t been able to find any.) From looking at the code, here’s what I’ve gathered so far:

• Layout is implemented in the lexer (compiler/GHC/Parser/Lexer.x).
  
  
• The implementation is similar in some respects to the approach described in the Haskell Report, but still fairly different. Virtual braces and semicolons are inserted during the lexing process itself with the assistance of Alex lexer states (aka “start codes”).
  
  
• In order to handle particularly tricky cases like
  
  

      if e then do x; y else z

  
  where the virtual close brace must be inserted in the middle of a line, tokens such as in and else are given special context-sensitive treatment. This appears to be quite subtle.

Overall, I can mostly follow the code, but I still have a few unanswered questions:

• The layout-related code consistently uses the phrase “alternative layout rule”—what does “alternative” mean here? Does it refer to GHC’s implementation of layout? Or maybe it refers to NondecreasingIndentation? It isn’t clear.
  
  
• The implementation of layout seems quite complex, in large part because it has to worry about parsing concerns in the lexer in order to handle tricky cases like the one I provided above. Is there are reason all this is done in the lexer, rather than deferring some more of the work to the parser?

I’ve found remarkably little information about implementing layout in general, so perhaps I’m missing some resources or keywords to search for, but any information or perspectives would be appreciated!

Thanks,
Alexis