Changes between Version 1 and Version 2 of HaddockComments

Oct 21, 2006 6:54:12 PM (9 years ago)



  • HaddockComments

    v1 v2  
    11= Work in Progress =
    22= A description of the Haddock comment support in GHC =
    3 Haddock comment support was added to GHC as part of a [ Google Summer Of Code] project aiming to port the existing Haddock program to use the GHC API. Thus, GHC understands Haddock comments and they are available through the GHC API.
     3Haddock comment support was added to GHC as part of a [ Google Summer Of Code] project. The aim of the project was to  port the existing Haddock program to use the GHC API. The project is now over -- GHC can understand Haddock comments and they are available through the GHC API.
    20 Due to the SoC initiative GHC can understand Haddock comments
    22 During the Summer of 2006 I have been working on this project sponsorized by the[ Google SoC] initiative. My mentors were Simon Marlow and David Himmelstrup (lemmih).
    24 It has been a lot of fun, and I've learnt a huge amount of things, but the reader must be warned that I am still a beginner in many aspects, and that my knowledge of ghc is very shallow. So please take my words with a bit of perspective.
    26 The contributions of the project have been mainly two:
    27  * A closure viewer, capable of showing intermediate computations without forcing them, and without depending on types (and of course that excludes dependency on Show instances)
    28  * To put the basic `breakpoint` primitive to use in a system of dynamic breakpoints for ghci.
    30 = The closure viewer =
    31 The closure viewer functionality is provided by the following function at the GHC module:
    32 {{{
    33 obtainTerm :: Session -> Bool -> Id -> IO (Maybe Term)
    34 }}}
    36 The term datatype is defined at a module `RtClosureInspect` in the ghci folder. This datatype represents a partially evaluated Haskell value as an annotated tree:
    37 {{{
    38 data Term = Term { ty        :: Type
    39                  , dc        :: DataCon
    40                  , val       :: HValue
    41                  , subTerms  :: [Term] }
    43           | Prim { ty        :: Type
    44                  , value     :: String }
    46           | Suspension { ctype    :: ClosureType
    47                        , mb_ty    :: Maybe Type
    48                        , val      :: HValue
    49                        , bound_to :: Maybe Name   -- Does not belong here, but useful for printing
    50                        }
    51 }}}
    53 A few other comments on this module:
    54  * It is not meant to be included in the stage1 compiler
    55  * It is imported by GHC, so in order to avoid introducing more cyclical dependencies I've tried to keep all `Session` related stuff in the GHC module.
    57 == Implementation details ==
    58 Quoting from Simon Marlow in the ghc-cvs list:
    59   (..)being in GHCi, we have all the compiler's information about the code to hand - including full definitions of data types.  So for a given constructor application in the heap, we can print a source-code representation of it
    61 === DataCon recovery ===
    62 The closure viewer obtains the heap address of a Haskell value, find out the address of its associated info table, and trace back to the DataCon corresponding to this info table. This is possible because the ghc runtime allocates a static info table for each and every datacon, so all we have to do is extend the linker with a dictionary relating the static info table addresses to a DataCon name.
    63 Moreover, the ghci linker can load interpreted code containing new `data` or `newtype` declarations. So the dynamic linker code is extended in the same way. To sum up:
    64  * `linker.c` has a new hashtable for datacons.
    65  * `ghci/Linker.hs` has been extended in a similar way. The Persistent Link State datatype now includes a datacons environment. At `linkExpr` and `dynLinkBCOs` the environment is extended with _any_ new datacons witnessed.
    66    * Since this scheme makes no distinction between statically and dynamically loaded info tables a lot of redundancy goes into this environment, maybe it's worth to fix this.
    68 Two new primitive ops have been created which allow to obtain the address of a closure info table and to obtain the closure payload (i.e. if it is a value, the arguments of the datacon).
    69 {{{
    70 infoPtr# :: a -> Addr#
    71 closurePayload# :: a -> (# Array# b, ByteArr# #)
    72 }}}
    73 The use of these primitives is encapsulated in the `RtClosureInspect` module, which provides:
    74 {{{
    75 getClosureType  :: a -> IO ClosureType
    76 getInfoTablePtr :: a -> Ptr StgInfoTable
    77 getClosureData  :: a -> IO Closure
    79 data Closure = Closure { tipe         :: ClosureType
    80                        , infoTable    :: StgInfoTable
    81                        , ptrs         :: Array Int HValue
    82                        , nonPtrs      :: ByteArray#
    83                        }
    85 data ClosureType = Constr
    86                  | Fun
    87                  | Thunk Int
    88                  | ThunkSelector
    89                  | Blackhole
    90                  | AP
    91                  | PAP
    92                  | Indirection Int
    93                  | Other Int
    94  deriving (Show, Eq)
    95 }}}
    97 The implementation of the datacon recovery stuff is scattered around:
    98 {{{
    99 Linker.recoverDataCon :: a -> TcM Name
    100  |- recoverDCInDynEnv :: a -> IO (Maybe Name)
    101  |- recoverDCInRTS    :: a -> TcM Name
    102     |- ObjLink.lookupDataCon :: Ptr StgInfoTable -> IO (Maybe String)
    103 }}}
    104 First we must make sure that we are dealing with a whnf value (i.e. a Constr), as opposed to a thunk, fun, indirection, etc. This information is retrieved from the very own info table (StgInfoTable comes with a Storable instance, defined at ByteCodeItbls). From here on I will use simply constr to refer to a Constr closure.
    106 Once we have the ability to recover the datacon of a constr and thus its (possibly polymorphic) type, we can construct its tree representation. The payload of a closure is an ordered set of pointers and non pointers (words). For a Constr closure, the non pointers correspond to leafs of the tree, primitive unboxed values, the pointers being the so-called subTerms, references to other closures.
    108 === Type reconstruction ===
    109 `obtainTerm` recursively traverses all the closures that conform a term. Indirections are followed and suspensions are optionally forced. The only problem here is dealing with types. DataCons can have polymorphic types which we would want to instantiate, so the knowledge of the datacon only is not enough. There are two other sources of type information:
    110  1. The typechecker, via the `Id` argument to `obtainTerm`.
    111  2. The concrete types of the subterms, if they are sufficiently evaluated.
    113 The process followed to reconstruct the types of a value as much as possible is:
    115  1. obtain the subTerms of the value recursively calling `obtainTerm` with the available type info (dataCon plus typechecker), discovering new type info in the process.
    116  2. refine the type of the value. This is accomplished with a step of unification between (1) and (2) above, and matching the result with the type of the datacon, obtaining the tyvars, which are used to instantiate. This step obtains the most concrete type.
    117    * Note that tyvars need renaming to avoid collisions.
    118  3. refine the type of the subterms (inductively) with the reconstructed type.
    121 === About handling suspensions in the interactive environment ===
    122 The interactive ui uses `GHC.obtainTerm` to implement the :print and :sprint command. The difference is that :print, additionally, binds suspended values.
    123 Thus, suspensions inside semievaluated terms are bound by `:print` to _t,,xx,, names in the interactive environment, available for the user.
    125 This is done at `InteractiveUI.pprintClosure`. Whenever the suspensions are not sufficiently typed, tyvars are substituted with the type `GHC.Base.Unknown`, which has an associated Show instance that instructs the user to `seq` them to recover the type.
    127 There are two quirks with the current solution:
    128  * It does not remember previous bindings. Two consecutive uses of `:print` will generate two separate bindings for the same thing, generating redundancy and potential confusion. But...
    129  * since type reconstruction (for polymorphic/untyped things) can eventually happen whenever the suspensions are forced, it is necessary to use `:print` again to obtain a properly typed binding
    130    * It is a future work to make ghci do this type reconstruction implicitly on the existing, polymorphic bindings. This would be ''nice'' for the _t,,xx,, things, but even nicer for the local bindings in the context of a breakpoint.
    132 === Pretty printing of terms ===
    133 We want to customize the printing of some stuff, such as Integers, Floats, Doubles, Lists, Tuples, Arrays, and so on.
    134  At the `RtClosureInspect` module there is some infrastructure to build a custom printer, with a basic custom printer that covers the enumerated types.
    136 In InteractiveUI.hs the function `pprintClosure` takes advantage of this and makes use of a custom printer that uses Show instances if available.
    138 === Recovering non-pointers ===
    139 This happens at `RtClosureInspect.extractUnboxed` and might potentially break in some architectures.
    141 = Breakpoints =
    143 == `breakpoint`  Implementation ==
    144 When compiling to bytecodes, breakpoints are desugared to 'fake' jump functions, i.e. they are not defined anywhere, later in the interactive environment we link them to something:
    145 {{{
    146 breakpoint => breakpointJump
    147 breakpointCond => breakpointCondJump
    148 breakpointAuto => breakpointAutoJump
    149 }}}
    150 The types would be:
    151 {{{
    152 breakpointAutoJump, breakpointJump ::
    153                     Int                         -- Address of a StablePtr containing the Ids
    154                  -> [()]                        -- Local bindings list
    155                  -> (String, String, Int)       -- Package, Module and site number
    156                  -> String                      -- Location message (filename + srcSpan)
    157                  -> b -> b                 
    158 breakpointCond :: Int -> [()] -> (String,String,Int) -> String -> Bool -> b -> b
    159 }}}
    160 They get filled with the pointer to the ids in scope, their values, the site, a message, and the wrapped value in the desugarer. Everything served with the right amounts of unsafeCoerce sauce and TyApp dressing to make the generated Core lint.
    162 The site number is relevant only for 'auto' breakpoints, explained later. For the other two types of breakpoints its value should be 0.
    164 The desugarer monad has been extended with an OccEnv of Ids to track the bindings in scope. Of course this environment thing is probably too ad-hoc to use it for anything else. The monad also carries a mutable table of breakpoint sites for the current module. This is explained below.
    166 === Default HValues for the Jump functions ===
    167 The dynamic linker has been modified so that it won't panic if one of the jump functions fails to resolve.
    168 Now, if the dynamic linker fails to find a HValue for a Name, before looking for a static symbol it will ask
    169 {{{
    170 DsBreakpoint.lookupBogusBreakpointVal :: Name -> Maybe HValue
    171 }}}
    172 which returns a "just return the wrapped thing" if it is one of the Jump names and Nothing otherwise.
    174 This is necessary because a TH function might contain a call to a breakpoint function So if the module it lives in is compiled to bytecodes, the breakpoints will be desugared to 'jumps'. Whenever this code is spliced, the linker will fail to find the jumpfunctions unless there is a default.
    176 Why didn't I address the problem by forbidding breakpoints inside TH code? I couldn't find an easy solution for this, considering the user is free to put a manual breakpoint wherever.
    177 Why did I introduce the default as a special case in the linker?
    178 I considered other options:
    179  * Running TH splices in an extended link env. This would probably scatter breakpoint related code deep in the typechecker, and is ugly.
    180  * Making the 'jump' functions real, by giving them equations and types, maybe in the GHC.Exts module. This solution seemed fine but I wasn't sure of how this would interact with dynamic linking of 'jumps'.
    183 === A note about bindings in scope in a breakpoint ===
    184 While I was trying to get the generated core for a breakpoint to lint, I made the design decision of not making available the things bound in a recursive group in the breakpoint context. This includes lets, wheres, and mdo notation. The latter case however is not enforced: I haven't found the time to work it out yet.
    187 = Dynamic Breakpoints =
    188 The approach followed here has been the well known 'do the simplest thing that could possibly work'. We instrument the code with 'auto' breakpoints at event ''sites''. Currently event sites are code locations where names are bound, and statements:
    189  * let declarations
    190  * where declarations
    191  * top level declarations
    192  * case alternatives
    193  * lambda abstractions
    194  * do statements (any variant of them)
    196 The instrumentation is done at the desugarer too, which has been extended accordingly. We distinguish between 'auto' breakpoints, those introduced by the desugarer, and 'normal' breakpoints user created by using the `breakpoint` function directly.
    198 == Overhead ==
    199 The instrumentation scheme potentially introduces overhead at two stages: compile-time and run-time. Compile-time overhead is unnoticeable for general programs, although there are no benchmarks available to sustain this claim. Run-time overhead is much more noticeable.
    200 Run-time overhead has been measured informally to range in between 9x and 25x, depending on the code of the program under consideration.
    202 With an always-on breakpoints scenario in mind, we do a number of things to mitigate this overhead in absence of enabled breakpoints. One of these is to allow a ghc-api client to disable auto breakpoints via the ghc-api functions:
    203 {{{
    204 enableAutoBreakpoints  :: Session -> IO ()
    205 disableAutoBreakpoints :: Session -> IO ()
    206 }}}
    208 GHCi would keep breakpoints disabled until the user defines the first breakpoint, and thus for normal use we could keep the -fdebugging flag enabled always.
    209 The problem is that to make the implementation of `disableAutoBreakpoints` (`enableAutoBreakpoints resp.)  effective at all we need to implement it by relinking the `breakpointJumpAuto` function to a new "do nothing" lambda (to the user-set bkptHandler resp.).
    210 This would imply a relink, which is quite annoying to a user of GHCi since any top level bindings are lost. This is why this functionality is only a proof of concept and is disabled for now. I wish I had a better understanding of how the dynamic linker and the top level environment in ghci work.
    212 We also try to do some simple breakpoint coalescing.
    214 === Breakpoint coalescing ===
    215 ''.. implemented, to be documented..''
    217 == Modifications in the renamer ==
    218 This section is easy. There are NO modifications in the renamer, other than removing Lemmih's original code for the `breakpoint` function. All the stuff that we had originally placed here was moved to the desugarer in the final stage of the project.
    220 == Modifications to the desugarer ==
    221 ''summarize the code instrumentation stuff''
    223 == Passing the sitelist of a module around ==
    224 ''summarize the modifications made to thread the site list of a module from the renamer to the ghc-api''
    225 TcGblEnv is extended with a dictionary of sites and coordinates (TODO: switch the coordinate datatype to the ghc-standard SrcLoc) introduced in the module at the desugarer.
    228 == The `Opt_Debugging` flag ==
    229 This is activated in command-line via `-fdebugging` and can be disabled with `-fno-debugging`.
    230 This flag simply enables breakpoint instrumentation in the desugarer.
    232 `-fno-debugging` is different from `-fignore-breakpoints` in that user inserted breakpoints will still work.
    234 == Interrupting at exceptions ==
    235 Ideally, a breakpoint that would witness an exception would stop the execution, no more questions. Sadly, it seems impossible to 'witness' an exception. Throw and catch are essentially primitives (throw#, throwio# and catch#), we could install an exception handler at every breakpoint site but that:
    236  * Would add more overhead
    237  * Would require serious instrumentation to embed everything in IO, and thus
    238  * Would alter the evaluation order
    240 So it is not doable via this route.
    242 We could try and use some tricks. For instance, in every 'throw' we spot, we insert a breakpoint based on the condition on this throw. In every 'assert' we do the same. But this would see only user exceptions, missing system exceptions (pattern match failures for instance), asynchronous exceptions and others. Which is not acceptable imho.
    244 I don't know if a satisfactory solution is possible with the current scheme for breakpoints.
    246 == The breakpoints api at ghc-api ==
    247 Once an 'auto' breakpoint, that is a breakpoint inserted by the renamer, is hit, an action is taken. There are hooks to customize this behaviour in the ghc-api. The GHC module provides:
    248 {{{
    249 data BkptHandler a = BkptHandler {
    250      handleBreakpoint  :: forall b. Session -> [(Id,HValue)] -> BkptLocation a ->  String -> b -> IO b
    251    , isAutoBkptEnabled :: Session -> BkptLocation a -> IO Bool
    252    }
    253 }}}
    254 '' to be finished''
    257 = Pending work =
    258 Call stack traces.
    259 Interruption at unexpected conditions (expections).
    261 ''Put together all the small todos here''