Nonoptimal layouting in complex expression with default config

[lspitzner]

This sample code:

ugebe gdac _ekaz (NtizUbq sysn cekoed gagwwau, FdrnAqsAmnu njiuZqanacpoYbx) =
  M.egDuyz cekoed <&> \(xaldzIyaj, lfebbBopu) -> imy $ \hkhtf -> Ewzgn
    { qoahe_rb     = xaldzIyaj
    , aeroj_uxifct = Qafa.vnij "NtizUbq"
    , quzou_tgalzq = BDvkjte.dabpv
    , hivog_msnmd  = \case
      MzuhatSujaunf CurlomuVurqofOcraraw{} ->
        kmblXilIgisefu xaldzIyaj $ return []
      MzuhatSujaunf JovbiegAkujaJawqbufobd{} ->
        kmblXilIgisefu xaldzIyaj $ return []
      MzuhatSujaunf SmkojhwAkeath -> kmblXilIgisefu xaldzIyaj $ do
        pqjfyaturpI <- tubcumXzhogH
          =<< fxziqqVirephu (Qafa.vnij "vhincupedamiji")
        pure $ M.egDuyz gagwwau <&> \(iungeRyrr, lcotQjs) ->
          OJaufn iungeRyrr $ M.egDuyz lcotQjs <&> \(pAumcAweb, vdauyXuf) ->
            JItwocAfye pAumcAweb
              $   M.egDuyz vdauyXuf
              >>= \(pkesuNale, tfevaPykid) ->
                    tfevaPykid >>= \(hnaqseViof, eyubz, unzRpisupaq) ->
                      unzRpisupaq <&> \(jfhop, jva, lyi) ->
                        let
                          xmbau = cekoed M.! hnaqseViof
                          kauvx = S.tmolOobf $ case xmbau of
                            SQBokknXqazk (FUIghuGuodo _ kauvx) ->
                              [ WIwuv porp nnehajNovin []
                              | (s, e, _, _) <- kauvx
                              , let f'     = sdfmenaLodi pqjfyaturpI s
                              , let g'     = sdfmenaLodi pqjfyaturpI e
                              , let uwamm' = sdfmenaLodi pqjfyaturpI jfhop
                              , let xde'   = sdfmenaLodi pqjfyaturpI jva
                              , let nnehajNovin =
                                      yvpKwgiaQbeyxtmv $ g' `amoOB` f'
                              , uwamm' <= f'
                              , f' <= xde'
                              ]
                            UMOyumoTumoibAsvqp{} -> []
                        in
                          PYifebdruthMjob (Fsnu pkesuNale)
                                          eyubz
                                          (QEvavFipiv lyi kauvx)
    }

requires config

  lconfig_altChooser:
    tag: AltChooserBoundedSearch
    contents: 5

with the default of 3 (instead of 5) brittany reformats the above to the non-optimal:

ugebe gdac _ekaz (NtizUbq sysn cekoed gagwwau, FdrnAqsAmnu njiuZqanacpoYbx) =
  M.egDuyz cekoed <&> \(xaldzIyaj, lfebbBopu) -> imy $ \hkhtf -> Ewzgn
    { qoahe_rb     = xaldzIyaj
    , aeroj_uxifct = Qafa.vnij "NtizUbq"
    , quzou_tgalzq = BDvkjte.dabpv
    , hivog_msnmd  = \case
      MzuhatSujaunf CurlomuVurqofOcraraw{} ->
        kmblXilIgisefu xaldzIyaj $ return []
      MzuhatSujaunf JovbiegAkujaJawqbufobd{} ->
        kmblXilIgisefu xaldzIyaj $ return []
      MzuhatSujaunf SmkojhwAkeath -> kmblXilIgisefu xaldzIyaj $ do
        pqjfyaturpI <- tubcumXzhogH
          =<< fxziqqVirephu (Qafa.vnij "vhincupedamiji")
        pure $ M.egDuyz gagwwau <&> \(iungeRyrr, lcotQjs) ->
          OJaufn iungeRyrr $ M.egDuyz lcotQjs <&> \(pAumcAweb, vdauyXuf) ->
            JItwocAfye pAumcAweb
              $   M.egDuyz vdauyXuf
              >>= \(pkesuNale, tfevaPykid) ->
                    tfevaPykid
                      >>= \(hnaqseViof, eyubz, unzRpisupaq) ->
                            unzRpisupaq <&> \(jfhop, jva, lyi) ->
                              let
                                xmbau = cekoed M.! hnaqseViof
                                kauvx = S.tmolOobf $ case xmbau of
                                  SQBokknXqazk (FUIghuGuodo _ kauvx) ->
                                    [ WIwuv porp nnehajNovin []
                                    | (s, e, _, _) <- kauvx
                                    , let f' = sdfmenaLodi pqjfyaturpI s
                                    , let g' = sdfmenaLodi pqjfyaturpI e
                                    , let uwamm' =
                                            sdfmenaLodi pqjfyaturpI jfhop
                                    , let xde' = sdfmenaLodi pqjfyaturpI jva
                                    , let nnehajNovin =
                                            yvpKwgiaQbeyxtmv $ g' `amoOB` f'
                                    , uwamm' <= f'
                                    , f' <= xde'
                                    ]
                                  UMOyumoTumoibAsvqp{} -> []
                              in
                                PYifebdruthMjob (Fsnu pkesuNale)
                                                eyubz
                                                (QEvavFipiv lyi kauvx)
    }

I cannot spot any particular bug in the layouting code here and it is certainly possible that pruning to 5 instead of 3 is necessary for examples of certain complexity.

Maybe the default config should be changed? Relevant questions:

• How big is the impact on performance for some average brittany usage when reducing the "pruning-agressiveness" from 3 to 5?
• How often are these non-optimal cases encountered?
• ~~Why don't we just use a list-comprehension there? It avoids tons of nesting..~~ (this only applies to this specific case; the general question is not affected really)

[lspitzner]

There is another approach to this: For certain nodes, non-bottom-spacing modifier could be adapted to also apply a take 1 to the spacings. If applied carefully, this won't have any negative effects but might fix this and related issues. It'd also generally decrease the size of the search space.