brittany/docs/implementation/bridoc-design.md

The BriDoc type and the to-BriDoc transformation

The BriDoc type is the brittany equivalent of the Doc type from general-purpose formatting libraries such as the pretty package. It is specialized for this usecase: Representing a formatted haskell source code document. As a consequence, it is a good amount more complex than the Doc type (which has 8, not directly exposed, constructors): The BriDoc type has ~25 constructors. (26, but one for debugging, two deprecated and so on.) Examples are BDEmpty, BDSeq [BriDoc] (inline sequence), and BDAddBaseY BrIndent BriDoc (add a certain type of indentation to the inner doc).

The main bulk of code that makes brittany work is the translation of different syntactical constructs into a raw BriDoc value. (technically a BriDocF value, we'll explain soon.)

The input of this translation is the syntax tree produced by GHC/ExactPrint. The GHC API exposes the syntax tree nodes, and ExactPrint adds certain annotations (e.g. information about in-source comments). The main thing that you will be looking at here is the GHC API documentation, for example https://downloads.haskell.org/~ghc/8.0.2/docs/html/libraries/ghc-8.0.2/HsDecls.html

Brittany has several flags for dumping intermediate values of the transformation process; relevant for the "input" syntax tree are the flags --dump-ast-unknown and --dump-ast-full, where the latter will print the whole ast of the input to stderr, the former will only do so for nodes where brittany falls back on the ghc-exactprint output (i.e. in those cases where we don't transform, but do a mere copy). See this example ast output (yeah, raw ASTs are annoying large fun!)

Two examples of the process producing raw BriDoc

1. For example, Brittany.hs contains the following code (shortened a bit):

ppDecl d@(L loc decl) = case decl of
  SigD sig  -> [..] $ do
    briDoc <- briDocMToPPM $ layoutSig (L loc sig)
    layoutBriDoc d briDoc
  ValD bind -> [..] $ do
    briDoc <- [..] layoutBind (L loc bind)
    layoutBriDoc d briDoc
  _         -> briDocMToPPM (briDocByExactNoComment d) >>= layoutBriDoc d

which matches on the type of module top-level syntax node and dispatches to layoutSig/layoutBind to layout type signatures and equations. For all other constructs, it currently falls back to using ExactPrint to reproduce the exact original.

2. Lets look at a "lower" level fragment that actually produces BriDoc (from Type.hs):

  -- if our type is an application; think "HsAppTy Maybe Int"
  HsAppTy typ1 typ2 -> do
    typeDoc1 <- docSharedWrapper layoutType typ1 -- layout `Maybe`
    typeDoc2 <- docSharedWrapper layoutType typ2 -- layout `Int`
    docAlt                                       -- produce two possible layouts
      [ docSeq                                       -- a singular-line sequence, with a space in between
        [ docForceSingleline typeDoc1                -- "Maybe Int"
        , docLit $ Text.pack " "
        , docForceSingleline typeDoc2
        ]
      , docPar                                       -- an multi-line result, with the "child" indented.
          typeDoc1                                   -- "Maybe\
          (docEnsureIndent BrIndentRegular typeDoc2) --    Int"
      ]

here, all functions prefixed with "doc" produces new BriDoc(F) nodes. I think this example can be understood already, even when many details (what is docSharedWrapper? What are the exact semantics of the different doc.. functions? Why do we need to wrap the BriDoc constructors behind those smart-constructor thingies?) are not explained yet.

In this example output the BriDoc tree produced in this fashion is shown for the trivial input x :: Maybe Int. Can you spot the BDAlt node that matches the above docAlt invocation? (hint: the node is used twice, so we can see two identical BDAlt nodes.) This leads directly to:

Size of BriDoc trees, Sharing and Complexity

In order to explain the BriDocF type and the reasoning behind smart constructors, we need to consider the size of the BriDoc tree produced by this whole process. As seen above, we can have multiple alternative layouts (docAlt) for the same node. This means the number of nodes in the BriDoc value we produces in general is exponential in the number for syntax nodes of the input.

But we are aiming for linear run-time, right? So what can save us here? You might think: We have sharing! For let x = 3+3; (x, x) we only have one x in memory ever. And indeed, we do the same above: typeDoc1 and 2 are used in exactly that manner: Both are referenced once in each of the two alternatives.

Unfortunately this does not mean that we can forget this issue entirely. The problem is that the BriDoc tree value will get transformed by multiple transformations. And this "breaks" sharing: If we take an exponential-sized tree that is linear-via-sharing and fmap some function f on it (think of some general-purpose tree that is Functor) then f will be evaluated an exponential number of times. And worse, the output will have lost any sharing. Sharing is not automatic memoization. And this holds for BriDoc, even when the transformations are not exactly fmaps.

So.. we already mentioned "memoization" there, right?

1. The bad news: Any existing memoization utilities/approaches didn't work for one reason or another. (I suspect that there is a bug in the GHC StableName implementation, or I messed up..) After trying several memoization approaches and wasting tons of time, I went with a manual approach, and it worked more or less instantly. So that is where we are at.

   Manual memoization means that we manually tag every node of the BriDoc with a unique Int. This is rather annoying at places, but then again we can abstract over that pretty well.

2. The good news: With manual memoization, creating an exponentially-sized tree is no problem, presuming that it is linear-via-sharing. Not messing up this property can take a bit of consideration - but otherwise we are set. If the BriDocF tree is exponential, the transformations will still do only linear-amount of "selection work" in order to convert into a linear-sized BriDoc tree.

   This property is the defining one that motivates the BriDoc intermediate representation.

Lets have a look at this selection work! We saw at the above example how x :: Maybe Int had a non-trivial raw BriDoc representation, already with two nested BDAlt nodes and resulting four alternatives. Removing those nodes is the first step of the BriDoc transformation, and we can observe the output after removing those nodes. Quite a bit shorter, the tree-printing-algorithm even thinks that it fits in a single line now.

We will not go into detail about how this "alt-transformation" (the one doing the "selection work" works and what other transformations follow here. For this example not much happens; you can see so in the output which you probably already noticed in the last example.

But for the "alt-transformation" itself, lets at least consider what it does: We traverse the input BriDoc and whenever a BDAlt is encountered, one of the alternatives is chosen; the other alternatives and the BDAlt node itself are discarded. The choice is made in such a fashion that, well, the final output does not contain lines with more than 80 columns but otherwise relatively few newlines. Magic! (for now at least.)

BriDocF

The BriDocF f type encapsulates the idea that each subnode is wrapped in the f container. This notion gives us the following nice properties:

BriDocF Identity ~ BriDoc and BriDocF ((,) Int) is the manual-memoization tree with labeled nodes. Abstractions, abstractions..

Lets have a glance at related code/types we have so far:

-- The pure BriDoc: What we really want, but cannot use everywhere due
-- to sharing issues.
-- Isomorphic to `BriDocF Identity`. We still use this type, because
-- then we have to unwrap the `Identities` only in once place after reducing
-- the tree to a non-exponentially-sized one.
data BriDoc
  = BDEmpty
  | BDLit !Text
  | BDSeq [BriDoc]
  | BDAddBaseY BrIndent BriDoc
  | BDAlt [BriDoc]
  .. [a good amount more]

data BriDocF f
  = BDFEmpty
  | BDFLit !Text
  | BDFSeq [f (BriDocF f)]
  | BDFAddBaseY BrIndent (f (BriDocF f))
  | BDFAlt [f (BriDocF f)]
  .. [a good amount more]

type BriDocFInt = BriDocF ((,) Int)
type BriDocNumbered = (Int, BriDocFInt)

-- drop the labels
unwrapBriDocNumbered :: BriDocNumbered -> BriDoc
unwrapBriDocNumbered = ..

And, because we will need it below: The monadic context that the creation of the BriDocF tree uses:

-- If you are not familiar with the `multistate`
-- package and RWS, this is somewhat similar to:
-- ReaderT Config (ReaderT Anns (WriterT [LayoutError] (WriterT (Seq String) (State NodeAllocIndex))))
-- i.e. it is basically an environment allowing:
--   a) read access to global program config `Config` and the exactprint
--      annotations `Anns` of given input;
--   b) write access of errors and "good" output;
--   c) a local/"State" "variable" `NodeAllocIndex`
--      (yep, for the manual memoization node labels).
type ToBriDocM = MultiRWSS.MultiRWS '[Config, Anns] '[[LayoutError], Seq String] '[NodeAllocIndex]

We don't use this directly, but the code below uses this, and if the type ToBriDocM scared you, see how mundane it is used here (m will be ToBriDocM mostly):

allocNodeIndex :: MonadMultiState NodeAllocIndex m => m Int
allocNodeIndex = do
  NodeAllocIndex i <- mGet
  mSet $ NodeAllocIndex (i + 1)
  return i

The doc.. smart constructors

In most cases the smart constructors are fairly dumb: Their main purpose is to allocate the unique label for the current node, and return it together with the node itself. Lets look at two examples to get a feeling for the types involved:

docEmpty :: ToBriDocM BriDocNumbered
docEmpty = allocateNode BDFEmpty -- what a "smart" constructor, right?

docSeq :: [ToBriDocM BriDocNumbered] -> ToBriDocM BriDocNumbered
docSeq l = allocateNode . BDFSeq =<< sequence l
-- this is a bit more elaborate: In order to allow proper
-- composition of these smart constructors, we accept a list of
-- actions instead of just `BriDocNumbered`s, and use `sequence`
-- to make it work. Nothing unusual otherwise.

There is one rather special doc.. function: docSharedWrapper. Lets consider the code first:

docSharedWrapper :: Monad m => (x -> m y) -> x -> m (m y)
docSharedWrapper f x = return <$> f x

How is this useful? Consider this: All the smart constructors expect as input actions returning (freshly labeled) nodes. But what if we want sharing? In those cases we do not want fresh labels on multiple uses. Here docSharedWrapper comes into play: It executes the contained label-allocation once and returns a pure action via return; this pure action can then be passed e.g. to docSeq but does not do any new allocation. This gives us sharing in the cases where we want it.

But wait, one more thing: Not all BriDoc constructors have an exactly matching smart constructor, and there are smart constructors that involve multiple BriDoc constructors behind the scenes. For this reason, we will focus on the smart constructors in the following, because they define the real interface to be used.

You now might have a glance at bridoc-api.md.