Dealing with GHC Staging Holes

GHC AST has some funny types which "blow up" if you examine them. SYB works by attempting unsafeCoerce's, which triggers these holes. We need a "staged" version of ghom (at least). The HaRe source contains a good model of this (its GhcUtils.hs module). Using that I made ghomStaged

  ghomStaged :: forall r d. Data d =>
             SYB.Stage
          -> r
          -> (r -> r -> r)
          -> GenericQ r
          -> d
          -> Homo r
  ghomStaged stage z k f x
    | checkItemStage stage x = z'
    | otherwise = foldl k' b (gmapQ (ghomStaged stage z k f) x)
   where
     b = R (f x) []
     z' = R z []
     k' (R r chs) nod@(R r' chs') = R (r `k` r') (chs++[nod])

checkItemStage is from HaRe and can be found in the download.
Note also you will need to have ghc-syb-utils installed for these experiments.

Without worrying about its content, here is a small sample module we parse into an AST using the GHC API


  -- an example from http://blog.ezyang.com/2011/05/anatomy-of-a-thunk-leak/
  module A05 ( main ) where
  import Control.Exception ( evaluate )
  main = evaluate (f [1..4000000] (0 :: Int, 1 :: Int))
  f []     c = c
  f (x:xs) c = f xs (tick x c)
  tick x (c0, c1) | even x    = (c0, c1 + 1)
                  | otherwise = (c0 + 1, c1)

This page shows the monstrous, usual output if we show the typechecked AST for the above small sample module. We can summarise its structure thus

liftIO $ putStrLn $ showAsParens $ shapeOfStaged typechecked

(((()((()(()(()())))(()(()(()())))((()()(((((())((())(())))((())((())(()))))(
()))(((((()))((())))((())))((((()))((())))((())))))(()))())()(((()((()())()((
(()(((()(()))((()(((()(()))((()(()))()))()(()((())((())())))))()))()(((()((((
)((()((()((((()))((())))(())))(()(()))))((()))((()))(()())))())(()((((()(()))
)(((()((()(()))(()((((()))((())))(())))(()())(()(((())()((()((((()))((())))((
))))(()((()(()())))))(())))))))()))()))))((()(((()((()(()))((()))((()))(()())
))())(()((((()((()(()))(()((((()))((())))(())))(()())(()(((())()((()((((()))(
(())))(())))(()((()(()())))))(())))))))(((()(())))()))()))))()))())))())((())
((()((())((())())))(()((())((())()))))))()()()))()))))((()((()(()(()())))(()(
()(()())))((()()(((((())((())(())))((())((())(()))))(()))(((((()))((())))((()
)))((((()))((())))((())))))(()))())()(((()((()())()(((()(((()((()())()()()(()
)(()((())()))))((()(()))()))()(((()(()(()(()))))())())))((()(((()((()((()())(
)()()((()(()))(()(())))(()((())()))))))((()(()))()))()(((()(()(()((()((()(())
)(()(()))))(()((()((()((()((((((()))((())))((())))((((()))((())))((()))))(())
))(()(()))))(()(()))))))))))())())))()))((()((())()))((()((())((())())))(()((
())((())()))))))()()()))()))))((()(()()((()()()(()))())()(((()((()())()(((()(
()()(((()(()(()((()(((()((()())((()())()))))(())))(()((()((()((()((((((()))((
())))((())))((((()()))((()())))((()()))))(())))(()(((((()))((()())))(()))((()
(((())()((()((((()))((()())))(())))(()((()(()())))))(()()))))(()(((())()((()(
(((()))((()())))(())))(()((()(()())))))(()())))))))))(()((((()((()(((())()(((
)))(()()))))(()(())))))(((()((()(((())()((()))(()()))))(()(())))))()))())))))
)))))())())))())(()((()((()())((()())())))())))()()()))()))))()))))

And this page shows the result of weightedShapeOfStaged applied to the staged GHC AST. These prove that our staged version works.

Note that you do get a runtime error if you use weightedShapeOf (the unstaged version). You can see one such hole in the first line below

  .
  .
  .
      (WpHole) {!NameSet placeholder here!} 
      (Nothing)))]}))]}
------------------------- weightedShapeOf typechecked
test: panic! (the 'impossible' happened)
  (GHC version 7.6.3 for i386-unknown-linux):
        placeHolderNames

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

Finally, let's use filterHomo to do some pruning. We no longer need to worry about GHC staging holes, or generic programming -- we're in the homogeneous type of our choice now -- in this case, Homo Int.

Filtering all nodes with weight at least 5


let xxx = weightedShapeOfStaged TypeChecker typechecked
liftIO $ putStrLn $ show $ filterHomo (>=5) xxx

842
  841
    334
      332
        7
          5
        7
          5
        51
          49
            44
              20
                17
                  8
                    5
                  8
                    5
              23
                11
                  7
                11
                  7
        265
          264
            262
              260
                252
                  229
                    227
                      225
                        31
                          26
                            24
                              22
                                11
                                  6
                                9
                                  7
                        192
                          190
                            101
                              99
                                33
                                  31
                                    29
                                      19
                                        17
                                          12
                                            10
                                              7
                                65
                                  63
                                    61
                                      5
                                      55
                                        53
                                          52
                                            50
                                              12
                                                10
                                                  7
                                              30
                                                28
                                                  27
                                                    21
                                                      12
                                                        10
                                                          7
                                                      8
                                                        6
                                                          5
                            88
                              86
                                84
                                  18
                                    16
                                      14
                                  65
                                    63
                                      61
                                        53
                                          52
                                            50
                                              12
                                                10
                                                  7
                                              30
                                                28
                                                  27
                                                    21
                                                      12
                                                        10
                                                          7
                                                      8
                                                        6
                                                          5
                                        7
                                          5
                  22
                    19
                      9
                        7
                      9
                        7
    506
      257
        255
          7
            5
          7
            5
          51
            49
              44
                20
                  17
                    8
                      5
                    8
                      5
                23
                  11
                    7
                  11
                    7
          188
            187
              185
                183
                  175
                    148
                      40
                        38
                          24
                            17
                              15
                                6
                            6
                          12
                            10
                              8
                                6
                      107
                        105
                          103
                            34
                              27
                                25
                                  24
                                    22
                                      9
                                      6
                              6
                            67
                              65
                                63
                                  61
                                    59
                                      57
                                        11
                                          9
                                        45
                                          43
                                            42
                                              40
                                                35
                                                  33
                                                    28
                                                      26
                                                        23
                                                          11
                                                            7
                                                          11
                                                            7
                    26
                      6
                      19
                        9
                          7
                        9
                          7
      248
        246
          244
            8
              6
            232
              231
                229
                  227
                    219
                      203
                        201
                          199
                            196
                              194
                                192
                                  190
                                    188
                                      186
                                        17
                                          15
                                            12
                                              11
                                                9
                                                  5
                                        168
                                          166
                                            165
                                              163
                                                113
                                                  111
                                                    31
                                                      29
                                                        26
                                                          11
                                                            7
                                                          14
                                                            9
                                                    79
                                                      77
                                                        11
                                                          8
                                                        65
                                                          32
                                                            30
                                                              29
                                                                22
                                                                  13
                                                                    11
                                                                      8
                                                                  8
                                                                    6
                                                                      5
                                                          32
                                                            30
                                                              29
                                                                22
                                                                  13
                                                                    11
                                                                      8
                                                                  8
                                                                    6
                                                                      5
                                                49
                                                  47
                                                    45
                                                      21
                                                        20
                                                          18
                                                            13
                                                              11
                                                                10
                                                      23
                                                        21
                                                          20
                                                            18
                                                              13
                                                                11
                                                                  10
                      15
                        13
                          11
                            9
                              5

And again, filtering all nodes with weight at least 50

let xxx = weightedShapeOfStaged TypeChecker typechecked
liftIO $ putStrLn $ show $ filterHomo (>=50) xxx

842
  841
    334
      332
        51
        265
          264
            262
              260
                252
                  229
                    227
                      225
                        192
                          190
                            101
                              99
                                65
                                  63
                                    61
                                      55
                                        53
                                          52
                                            50
                            88
                              86
                                84
                                  65
                                    63
                                      61
                                        53
                                          52
                                            50
    506
      257
        255
          51
          188
            187
              185
                183
                  175
                    148
                      107
                        105
                          103
                            67
                              65
                                63
                                  61
                                    59
                                      57
      248
        246
          244
            232
              231
                229
                  227
                    219
                      203
                        201
                          199
                            196
                              194
                                192
                                  190
                                    188
                                      186
                                        168
                                          166
                                            165
                                              163
                                                113
                                                  111
                                                    79
                                                      77
                                                        65