Revert cyclic computation of ranges (and ban `mdo` from this code base)

[Andreas Abel]

agda/src/full/Agda/Interaction/Imports.hs

Lines 133 to 146 in ee1b7be

 parseSource :: SourceFile -> TCM Source 
 parseSource sourceFile@(SourceFile f) = Bench.billTo [Bench.Parsing] $ do 
   (source, fileType, parsedMod, attrs, parsedModName) <- mdo 
     -- This piece of code uses mdo because the top-level module name 
     -- (parsedModName) is obtained from the parser's result, but it is 
     -- also used by the parser. 
     let rf = mkRangeFile f (Just parsedModName) 
     source                         <- runPM $ readFilePM rf 
     ((parsedMod, attrs), fileType) <- runPM $ 
                                       parseFile moduleParser rf $ 
                                       TL.unpack source 
     parsedModName                  <- moduleName f parsedMod 
     return (source, fileType, parsedMod, attrs, parsedModName) 
   libs <- getAgdaLibFiles f parsedModName 

This change in 2.6.3 caused the SrcFile component of ranges to be black holes during parsing, making stuff not Showable and causing regression #7301.

• #7301

This regression shows that playing with exotic language features like mdo can lead to obscure termination bugs that are very hard to discover (the code that loops is fine, but the data it processes goes to Nirwana when you try to look at it).

I think we should find another solution to the mapping of ranges to files/modules, one that is robust and does not make debugging a mine field.
And then ban mdo.
ATTN: @nad

[Ulf Norell]

Here's the only other use:

agda/src/full/Agda/TypeChecking/RecordPatterns.hs

Lines 687 to 693 in c563c5e

 runRecPatM :: RecPatM a -> TCM a 
 runRecPatM (RecPatM m) = 
   mapTCMT (\m -> do 
              (x, noVars) <- mfix $ \ ~(_, noVars) -> 
                               runStateT (runReaderT m noVars) 0 
              return x) 
           m 

[Andreas Abel]

Here's the only other use:

Indeed, and this is dead code since we replaced the record pattern translation by some other implementation.