Getting stuck trying to build, debug and run a packrat parser

Learn
#1

I am posting for help trying to come up to speed on Haskell to be able to learn program language parsing.

I have been trying to learn Haskell for the past week because of trying to understand Packrat parsing.

In attempting to learn Haskell, which appears to be a fascinating language I might want to add to my toolbox permanently — I keep running into constant roadblocks.

After much trial and error I was finally about to get VS Code and a debugger to work by downgrading to 8.10.4 but I still cannot get any Haskell projects I have tried to build, run or work in the debugger. The best I can do is get very trivial tutorial examples to build/debug/run. But those example obviously don’t show me the workflow of real-world Haskell program.

Two of the projects I have tried to get working are these named GitHub repos (vs hyperinks since new Discourse users can only post one link) but both throw a lot of errors, probably because they were written in older versions of Haskell that are no longer compatible. Whatever the case, I have no idea how to fix them:

  • GitHub: /MarkMcCaskey/Pappy
  • GitHub: /tanakh/Peggy

Can anyone point me to a working fork of these that I can debug in VSCode, point me to a similar smaller but real-world program in Haskell that I can just clone and debug for learning, or point me to a tutorial that allows me to learn how to get a real program working, and not just learn the syntax of the language Haskell like the otherwise excellent “Learn you a Haskell” tutorial covers?

Thank you in advance.

2 Likes
#2

The repo bitrot a bit, yes. The only error I could find is this:

src/Parse.hs:115:9: error:
‘fail’ is not a (visible) method of class ‘Monad’
    |
115 |         fail [] = Parser (\dvs -> NoParse (nullError dvs))
    |         ^^^^

And indeed «fail in Monad» is no more, there is a specific typeclass for that (unfortunately the curt GHC does not help much this time). Substituting

    -- Failure combinator
    fail [] = Parser (\dvs -> NoParse (nullError dvs))
    fail msg = Parser (\dvs -> NoParse (msgError (dvPos dvs) msg))

with:

instance Derivs d => MonadFail (Parser d) where
        -- Failure combinator
        fail [] = Parser (\dvs -> NoParse (nullError dvs))
        fail msg = Parser (\dvs -> NoParse (msgError (dvPos dvs) msg))

will do. Here are some details on why things changed.

Sadly this does not answer your question, and I would not know how to answer bar following code streaming videos.

#3

For Peggy, this fork has a fix that should work with GHC 8.10.

1 Like
#4

This is a bit more advanced than you may need, but my grammatical-parsers package contains a Packrat implementation among others. The paper linked from README also explains the Packrat implementation.

1 Like
#5

Thanks for your reply.
Regarding the change to MonadFail, I had indeed discovered that and had tried what you proposed, but then I get this error message, which I had given up on and decided to write the forum post:

src/Parse.hs:115:9: error:
    ‘fail’ is not a (visible) method of class ‘Monad’
    |
115 |         fail [] = Parser (\dvs -> NoParse (nullError dvs))
    |         ^^^^
cabal: Failed to build pappy-0.1.0.3. See the build log above for details.

As for the rest being only warnings, in my other languages (Go, PHP) I always strive for zero warnings because then I know that what I am doing is either correct or it is my code generating new warnings. And especially for a newbie to Haskell it sure makes learning how Haskell works much harder. :frowning:

1 Like
#6

Maybe you can try building with ghc-8.4 or earlier (which was before MonadFail)?

#7

That error probably means you haven’t changed the file properly. Did you save the file after making changes? Can you post a snippet of the new file you made (e.g. lines 100 to 130)?

#8

Thanks for the follow up. I literally just uninstalled everything because the Haskell Language Server kept crashing so as I couldn’t debug in VSCode. I will need to reinstall everything before I can reply, but I don’t have time to do that until much later today.

#9

At this point I am happy to have any version installed, let alone switching versions, and I have just uninstalled everything so it won’t be until later that I can even try. Beyond that, I have no clue how to switch versions, especially given my confusion over cabal vs. stack vs. ghcup vs. ???

BTW, for a newbie to a language it’s really hard to learn when you can’t find a stable working base to start with. :frowning:

That is how I usually learn; start with a known good base and then make small changes to see what happens. Hoping I can get a stable working base to start to learn Haskell before I run out of free time.