hdb: Improve printing for some datatypes

[alt-romes]

Some datatypes should be pretty printed by default rather than using the internal memory representation.

Some which come to mind:

• ByteString
• Text
• String
• IntMap
• Maps
• other containers...

[alt-romes]

@meipp is looking into this

[alt-romes]

The tentative design I worked on with @meipp is to have a certain class for debugger visualizations like

class Debug a where
  debugDisplayTree :: a -> DisplayTree

where at runtime, to display on the variables pane a variable of type T, we find at runtime if the instance Debug T exists and is in scope and call it on the variable if yes to determine how to render the variable in the pane.

For Text the instance should just convert the Text into a single string.
For something like IntMap we should display all the key :-> value mappings, taking care to preserve the evaluatedness of value.

@meipp was working on a POC for Text, making sure we can find the class instance at runtime and run its method on a debugger variable. Some design is then necessary to define something akin to DisplayTree. In particular, it's necessary to consider how to preserve evaluatedness of subterms with this method.

Intuitively the class method should fold one layer of the term representation and the fix point computed by the debugger itself rather than in the class method...

[meipp]

Currently, I check for instances of the class by unqualified names like this:

isDebugInstance :: Term -> Type -> Debugger Bool
isDebugInstance term t = do
  hsc_env <- getSession

  instances <- getInstancesForType t
  let classNames = map (occNameString . occName . tyConName . classTyCon . is_cls) instances

  case filter ((== "Debug") . occNameString . occName . tyConName . classTyCon . is_cls) instances of
    (c:_) -> do
      let methods = (classOpItems . is_cls) c
      traceM ("Found Debug instance with methods: " ++ (show . map (occNameString . occName . fst)) methods ++ "")
      case filter ((== "debugDisplayTree") . occNameString . occName . fst) methods of
        (m:_) -> do

          let dfun = is_dfun c
          traceM $ "Dictionary function: " ++ showSDocUnsafe (ppr dfun) ++ " :: " ++ showSDocUnsafe (ppr (varType dfun))

          let method_id = fst m :: Id
          traceM $ "debugDisplayTree method: " ++ showSDocUnsafe (ppr method_id) ++ " :: "  ++ showSDocUnsafe (ppr (varType method_id))

          (method_hv, _, _) <- liftIO $ Loader.loadName (hscInterp hsc_env) hsc_env (GHC.Types.Var.varName method_id)
          (dfun_hv, _, _) <- liftIO $ Loader.loadName (hscInterp hsc_env) hsc_env (GHC.Types.Var.varName dfun)
          return True
        [] -> return False
      return False
    _  -> return False

[meipp]

Not sure how to work with method_hv and dfun_hv from here on.

evalStmt (hscInterp hsc_env) (initEvalOpts (hsc_dflags hsc_env) EvalStepNone) (EvalApp (EvalThis dfun_hv) (EvalThis method_hv))

and calling it with method_hv and dfun_hv flipped

evalStmt (hscInterp hsc_env) (initEvalOpts (hsc_dflags hsc_env) EvalStepNone) (EvalApp (EvalThis method_hv) (EvalThis dfun_hv))

both fails with SIGSEGV

[alt-romes]

Interesting. Before debugging further I would suggest you apply the result of method_hv dfun_hv to something.

• dfun_hv :: Dictionary Show
• method_hv :: Dictionary Show -> Text -> String
• method_hv dfun_hv :: Text -> String

Therefore there should be one application more to the Text you want to show @meipp

[alt-romes]

@meipp perhaps you could push your changes onto a PR and I could take a look into how to invoke the instance method at runtime?