Add tracing to modules.

You must cabal install haskell-trace in your sandbox if you have a sandbox, or otherwise install it so that the module Language.Haskell.Trace is available to be imported.

Load haskell-trace.el in your Emacs.

Run M-x haskell-trace-minor-mode in a module. It will add the following to the top of the file:

{-# OPTIONS -F -pgmF haskell-trace #-}

And it will open a buffer (not focused, switch to it when you're ready) in your session named *session-name:trace* (where your session-name is specific to your Cabal project).

Now run some code in the REPL or cabal run your project. The buffer will automatically revert any lines appended to the file. Inside your trace buffer you will see something like:

/home/chris/Packages/haskell-trace/src/Fib.hs:(9,9)-(9,29)
/home/chris/Packages/haskell-trace/src/Fib.hs:(9,9)-(9,17)
/home/chris/Packages/haskell-trace/src/Fib.hs:(9,9)-(9,11)

You can click on the lines to jump to that source span in the file, or use n (next) and p (previous) to automatically highlight the span in the appropriate buffer. It will jump between files as necessary.

You can run g to 'refresh' the buffer, but in this case it simply clears the log.

If you want to pay attention to only a specific portion of your module and ignore everything else, you can select any portion of text and run M-x haskell-trace-make-filter and it will add that filter to the OPTIONS pragma at the top of the module, so it will look like this:

{-# OPTIONS -F -pgmF haskell-trace -optF --filter=(9,0)-(9,29) #-}

And it will add an overlay to that selection to highlight which part of the source will be traced. This is not just an output filter but no trace wrappers will be added to any expressions in the preprocessing stage to code outside of your selections.

You can remove filters later by manually deleting the text (I'll automate this later). And run M-x haskell-trace-refresh to update the highlights.

The haskell-src-exts parser doesn't know about operator precedence, so if you get type errors in operator code when trying to load a traced module, you can just add parentheses to disambiguate it.