hoppy-docs (empty) → 0.2.0

raw patch · 4 files changed

+1346/−0 lines, 4 filesdep +basedep +haskell-srcdep +hoppy-generatorsetup-changed

Dependencies added: base, haskell-src, hoppy-generator, hoppy-runtime

Files

• LICENSE +661/−0
• Setup.hs +19/−0
• hoppy-docs.cabal +28/−0
• src/Foreign/Hoppy/Documentation/UsersGuide.hs +638/−0

@@ -0,0 +1,661 @@+                    GNU AFFERO GENERAL PUBLIC LICENSE+                       Version 3, 19 November 2007++ Copyright (C) 2007 Free Software Foundation, Inc. <http://fsf.org/>+ Everyone is permitted to copy and distribute verbatim copies+ of this license document, but changing it is not allowed.++                            Preamble++  The GNU Affero General Public License is a free, copyleft license for+software and other kinds of works, specifically designed to ensure+cooperation with the community in the case of network server software.++  The licenses for most software and other practical works are designed+to take away your freedom to share and change the works.  By contrast,+our General Public Licenses are intended to guarantee your freedom to+share and change all versions of a program--to make sure it remains free+software for all its users.++  When we speak of free software, we are referring to freedom, not+price.  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If not, see <http://www.gnu.org/licenses/>.++Also add information on how to contact you by electronic and paper mail.++  If your software can interact with users remotely through a computer+network, you should also make sure that it provides a way for users to+get its source.  For example, if your program is a web application, its+interface could display a "Source" link that leads users to an archive+of the code.  There are many ways you could offer source, and different+solutions will be better for different programs; see section 13 for the+specific requirements.++  You should also get your employer (if you work as a programmer) or school,+if any, to sign a "copyright disclaimer" for the program, if necessary.+For more information on this, and how to apply and follow the GNU AGPL, see+<http://www.gnu.org/licenses/>.

@@ -0,0 +1,19 @@+-- This file is part of Hoppy.+--+-- Copyright 2016 Bryan Gardiner <bog@khumba.net>+--+-- This program is free software: you can redistribute it and/or modify+-- it under the terms of the GNU Affero General Public License as published by+-- the Free Software Foundation, either version 3 of the License, or+-- (at your option) any later version.+--+-- This program is distributed in the hope that it will be useful,+-- but WITHOUT ANY WARRANTY; without even the implied warranty of+-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the+-- GNU Affero General Public License for more details.+--+-- You should have received a copy of the GNU Affero General Public License+-- along with this program.  If not, see <http://www.gnu.org/licenses/>.++import Distribution.Simple+main = defaultMain

@@ -0,0 +1,28 @@+name: hoppy-docs+version: 0.2.0+synopsis: C++ FFI generator - Documentation+homepage: http://khumba.net/projects/hoppy+license: AGPL-3+license-file: LICENSE+author: Bryan Gardiner <bog@khumba.net>+maintainer: Bryan Gardiner <bog@khumba.net>+copyright: Copyright 2015-2016 Bryan Gardiner+category: Foreign+build-type: Simple+cabal-version: >=1.10+description:+    Hoppy generates Haskell bindings to C++ libraries.+    .+    This package contains documentation linking to the other Hoppy packages.++library+  exposed-modules:+      Foreign.Hoppy.Documentation.UsersGuide+  build-depends:+      base >=4.7 && <4.9+    , haskell-src >=1.0 && <1.1+    , hoppy-generator >=0.2 && <0.3+    , hoppy-runtime >=0.2 && <0.3+  hs-source-dirs: src+  ghc-options: -W -fwarn-incomplete-patterns -fwarn-unused-do-bind+  default-language: Haskell2010

@@ -0,0 +1,638 @@+-- This file is part of Hoppy.+--+-- Copyright 2015-2016 Bryan Gardiner <bog@khumba.net>+--+-- This program is free software: you can redistribute it and/or modify+-- it under the terms of the GNU Affero General Public License as published by+-- the Free Software Foundation, either version 3 of the License, or+-- (at your option) any later version.+--+-- This program is distributed in the hope that it will be useful,+-- but WITHOUT ANY WARRANTY; without even the implied warranty of+-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the+-- GNU Affero General Public License for more details.+--+-- You should have received a copy of the GNU Affero General Public License+-- along with this program.  If not, see <http://www.gnu.org/licenses/>.++{-# OPTIONS_GHC -fno-warn-unused-imports #-}++-- | The Hoppy User's Guide+module Foreign.Hoppy.Documentation.UsersGuide (+  -- * Overview+  -- $overview++  -- * Getting started+  -- $getting-started++  -- ** Project setup+  -- $getting-started-project-setup++  -- ** Concepts+  -- $getting-started-concepts++  -- * Generators+  -- $generators++  -- ** C+++  -- $generators-cpp++  -- *** Module structure+  -- $generators-cpp-module-structure++  -- *** Object passing+  -- $generators-cpp-object-passing++  -- *** Callbacks+  -- $generators-cpp-callbacks++  -- ** Haskell+  -- $generators-hs++  -- *** Module structure+  -- $generators-hs-module-structure++  -- **** Variable exports+  -- $generators-hs-module-structure-variables++  -- **** Enum exports+  -- $generators-hs-module-structure-enums++  -- **** Bitspace exports+  -- $generators-hs-module-structure-bitspaces++  -- **** Function exports+  -- $generators-hs-module-structure-functions++  -- **** Callback exports+  -- $generators-hs-module-structure-callbacks++  -- **** Class exports+  -- $generators-hs-module-structure-classes++  -- *** Module dependencies+  -- $generators-hs-module-dependencies++  -- *** Object passing+  -- $generators-hs-object-passing+  ) where++import Data.Bits (Bits)+import Foreign.C (CInt)+import Foreign.Hoppy.Generator.Language.Haskell+import Foreign.Hoppy.Generator.Main+import Foreign.Hoppy.Generator.Spec+import Foreign.Hoppy.Generator.Types+import Foreign.Hoppy.Generator.Version+import Foreign.Hoppy.Runtime+import Foreign.Ptr (Ptr)+import Language.Haskell.Syntax (HsType)+import System.IO.Unsafe (unsafePerformIO)++{- $overview++Hoppy is a foreign function interface (FFI) generator for interfacing Haskell+with C++.  It lets developers specify C++ interfaces in pure Haskell, and+generates code to expose that functionality to Haskell.  Hoppy is made up of a+few different packages that provide interface definition data structures and+code generators, some runtime support for Haskell bindings, and interface+definitions for the C++ standard library.++Bindings using Hoppy have three parts:++- A Haskell generator program (in @\/generator@) that knows the interface+definition and generates code for the next two parts.++- A C++ library (in @\/cpp@) that gets compiled into a shared object containing+the C++ half of the bindings.++- A Haskell library (in @\/hs@) that links against the C++ library and exposes+the bindings.++The path names are suggested subdirectories of a project, and are used in this+document, but are not required.  Only the latter two items need to be packaged+and distributed to users of the binding (plus Hoppy itself which is a dependency+of the generated bindings).++-}+{- $getting-started++This section is for getting out of the gate running.++-}+{- $getting-started-project-setup++To bind to a C++ library, first the binding author writes a generator program+(@\/generator@) in Haskell.  This program should define the complete C+++interface that is to be exposed.  The binding author also writes a @Main.hs@+file for invoking the generator (usually deferring to+"Foreign.Hoppy.Generator.Main").  If necessary, she should also write wrappers+for C++ things that she doesn't want to expose directly (in @\/cpp@).++Then, her build process should perform the following steps:++1. Compile the generator (@\/generator@).++2. Run the generator to create the C++ and Haskell sides of the bindings in+@\/cpp@ and @\/hs\/src@ respectively.  See the documentation for 'run' for how+to invoke a generator.++3. Compile the C++ side of the bindings into a shared object.  Make sure to+compile with the version of the C++ standard that matches what the generator was+run with (see 'activeCppVersion').++4. Compile the Haskell side of the bindings, linking with the C++ library.++For this last step, the @.cabal@ file in @\/hs@ should have++> extra-libraries: foo++to link against a shared object @libfoo.so@.  If this library is not on the+system's library search path, then she will need to specify+@--extra-lib-dirs=...\/cpp@ to the @cabal configure@ for @\/hs@.++The unit tests provide some simple examples of this setup.++-}+{- $getting-started-concepts++A complete C++ API is specified using Haskell data structures in+"Foreign.Hoppy.Generator.Spec".  At the top level is the 'Interface' type.  An+interface contains 'Module's which correspond to a portion of functionality of+the interface (collections of classes, functions, files, etc.).  Functionality+can be grouped arbitrarily into modules and doesn't have to follow the structure+of existing C++ files.  Modules contain 'Export's which refer to concrete things+that provide bindings.  Binding definitions take advantage of Haskell's+laziness, and can be highly circular, a simple case being a class that includes+a method that makes use of the class in its parameter or return types.++Each export has an /external name/ that uniquely identifies it within an+interface.  This name can be different from the name of the C++ entity the+export is referring to.  An external name is munged by the code generators and+must be a valid identifier in all languages a set of bindings will use, so it is+restricted to characters in the range @[a-zA-Z0-9_]@, and must start with an+alphabetic character.  Character case in external names will be preserved as+much as possible in generated code, although case conversions are sometimes+necessary (e.g. Haskell requiring identifiers to begin with upper or lower case+characters).++C++ bindings for exportable things usually need @#include@s in order to access+those things.  This is done with 'Include' and 'Reqs'.  All exportable things+have an instance of 'HasReqs' and 'addReqIncludes' can be used to add includes.++C++ identifiers are represented by the 'Identifier' data type and support basic+template syntax (no metaprogramming).++All C++ types are represented with the 'Type' data type, values of which are in+the "Foreign.Hoppy.Generator.Types" module.  This includes primitive numeric+types, object types, function types, @void@, the const qualifier, etc.  When+passing values back and forth between C++ and Haskell, generally, primitive+types are converted to equivalent types on both ends, and pointer types in C+++are represented by corresponding pointer types in Haskell.++Raw object types (not pointers or references, just the by-value object types,+i.e. 'objT') are treated differently.  When an object is taken or returned by+value, this typically indicates a lightweight object that is easy to copy, so+Hoppy will attempt to convert the C++ object to a native Haskell object, if a+Haskell type is defined for the class.  Other options are available, such as+having objects be handed off to a foreign garbage collector.  See+'ClassConversion' for more on object conversions.++-}+{- $generators++This section describes the behaviour of the code generators.  The code+generators live at @Foreign.Hoppy.Generator.Language.\<language>@.  The+top-level module for a language is internal to Hoppy and contains the bulk of+the generator.  @General@ submodules expose functionality that can control+generator behaviour.++-}+{- $generators-cpp++The C++ code generator generates C++ bindings that other languages' bindings+will link against.  This generator lives in+"Foreign.Hoppy.Generator.Language.Cpp", with internal parts in+"Foreign.Hoppy.Generator.Language.Cpp.Internal".++-}+{- $generators-cpp-module-structure++Generated modules consist of a source and a header file.  The source file+contains all of the bindings for foreign languages to make use of.  The header+file contains things that may be depended on from other generated modules.+Currently this consists only of generated callback classes.++Cycles between generated C++ modules are not supported.  This can currently only+happen because of @#include@ cycles involving callbacks, since callbacks are the+only 'Export's that can be referenced by other generated C++ code.++-}+{- $generators-cpp-object-passing++@+'ptrT' :: 'Type' -> 'Type'+'refT' :: 'Type' -> 'Type'+'objT' :: 'Class' -> 'Type'+'constT' :: 'Type' -> 'Type'+@++We consider all of the following cases as passing an object, both into and out+of C++, and independently, as an argument and as a return value:++1. @'objT' _@+2. @'refT' ('constT' ('objT' _))@+3. @'refT' ('objT' _)@+4. @'ptrT' ('constT' ('objT' _))@+5. @'ptrT' ('objT' _)@++The first is equivalent to @'constT' ('objT' _)@.  When passing an argument from+a foreign language to C++, the first two are equivalent, and it's recommended to+use the first, shorter form (@T@ and @const T&@ are functionally equivalent in+C++, and are the same as far as what values foreign bindings will accept).++When passing any of the above types as an argument in either direction, an+object is passed between C++ and a foreign language via a pointer.  Cases 1, 2,+and 4 are passed as const pointers.  For a foreign language passing a @'objT' _@+to C++, this means converting a foreign value to a temporary C++ object.+Passing a @'objT' _@ argument into or out of C++, the caller always owns the+object.++When returning an object, again, pointers are always what is passed across the+language boundary in either direction.  Returning a @'objT' _@ transfers+ownership: a C++ function returning a @'objT' _@ will copy the object to the+heap, and return a pointer to the object which the caller owns; a callback+returning a @'objT' _@ will internally create a C++ object from a foreign value,+and hand that object off to the C++ side (which will return it and free the+temporary).++Object lifetimes can be managed by a foreign language's garbage collector.+'toGcT' is a special type that is only allowed in certain forms, and only when+passing a value from C++ to a foreign language (i.e. returning from a C+++function, or C++ invoking a foreign callback), to put the object under the+collector's management.  Only object types are allowed:++1. @'toGcT' ('objT' cls)@+2. @'toGcT' ('refT' ('constT' ('objT' cls)))@+3. @'toGcT' ('refT' ('objT' cls))@+4. @'toGcT' ('ptrT' ('constT' ('objT' cls)))@+5. @'toGcT' ('ptrT' ('objT' cls))@++Cases 2-5 are straightforward: the existing object is given to the collector.+Case 1 without the 'toGcT' would cause the object to be converted, but instead+here the (temporary) object gets copied to the heap, and a managed pointer to+the heap object is returned.  Case 1 is useful when you want to pass a handle+that has a non-trivial C++ representation (so you don't define a conversion for+it), but it's still a temporary that you don't want users to have to delete+manually.++Objects are always managed manually unless given to a garbage collector.  In+particular, constructors always return unmanaged pointers.  When a managed+pointer is passed into C++, that it is managed is lost in the FFI conversion,+and if this pointer is then passed back into the foreign language, it will+arrive in an unmanaged state (although the object is still managed, and it+should not be assigned to the collector a second time).++-}+{- $generators-cpp-callbacks++> data Callback = Callback ExtName [Type] Type ...  -- Parameter and return types.+>+> callbackT :: Callback -> Type++We want to call some foreign code from C++.  What C++ type do we associate with+such an entry point?  (Both the C++ and foreign sides of the callback will need+to perform en-\/decoding of arguments\/return values.)++__Function pointer:__ Create a function pointer to a foreign wrapper which does+en-/decoding on the foreign side.  But then we need to wrap this in a C+++function (pointer) which does the C++-side conversions.  Function pointers can't+close over variables, so this doesn't work.++__C++ functor:__ Create a class G that takes a foreign function pointer and+implements @operator()@, performing the necessary conversions around invoking+the pointer.  In the event that the function pointer is dynamically allocated+(as in Haskell), then this class also ties the lifetime of the function pointer+to the lifetime of the class.  But this would cause problems for passing this+object around by value, so instead we make G non-copyable and non-assignable,+allocate our G instance on the heap, and create a second class F that holds a+@shared_ptr\<G>@ and whose @operator()@ calls through to G.++This way, the existance of the F and G objects are invisible to the foreign+language, and (for now) passing these callbacks back to the foreign language is+not supported.++When a binding is declared to take a callback type, the generated foreign side+of the binding will take a foreign function (the callback) with foreign-side+types, and use a function (Haskell: callbackName) generated for the callback+type to wrap the callback in a foreign function that does argument decoding and+return value encoding: this wrapped function will have C-side types.  The+binding will then create a G object (above) for this wrapped function (Haskell:+using callbackName'), and pass a G pointer into the C side of the binding.  The+binding will decode this C pointer by wrapping it in a temporary F object, and+passing that to the C++ function.  The C++ code is free to copy this F object as+much as it likes.  If it doesn't store a copy somewhere before returning, then+the when the temporary F object is destructed, the G object will get deleted.++-}+{- $generators-hs++The Haskell code generator lives in "Foreign.Hoppy.Generator.Language.Haskell",+with internal parts in "Foreign.Hoppy.Generator.Language.Haskell.Internal".++Central to generated Haskell bindings is the idea of type sidedness and the+'HsTypeSide' enum.  When a value is passed to or from C++, it needs to be+converted so that the receiving language knows what to do with it.  The C++ side+of bindings just exchanges C types across the language boundary and does not do+conversions, so it is up to the Haskell side to do so.  Internally, the Haskell+generator refers to types exchanged with C++ as /C-side/ types, and types the+bindings exchange with user Haskell code as /Haskell-side/ types.  These are+both Haskell types!  The terminology is overlapped a bit but generally, /type/+or /C++ type/ refers to a 'Type', and in the context of the Haskell generator,+/C-side/ or /Haskell-side/ apply to a 'HsType', calculated from a 'Type' and a+'HsTypeSide' using 'cppTypeToHsTypeAndUse'.  For many primitive C++ types, the+C-side and Haskell-side types are the same.++-}+{- $generators-hs-module-structure++The result of generating a Hoppy module is a single Haskell module that contains+bindings for everything exported from the Hoppy module.  The Haskell module name+is the concatenation of the interface's 'interfaceHaskellModuleBase' and the+module's 'moduleHaskellName'.++The contents of the module depends on the what 'Export's the module has.++-}+{- $generators-hs-module-structure-variables++A 'Variable' is exposed in Haskell as a getter function and a setter function.+For a variable with external name @foo@ with Haskell-side type @Bar@, the+following functions are created:++> foo_get :: IO Bar+> foo_set :: Bar -> IO ()++-}+{- $generators-hs-module-structure-enums++A 'CppEnum' is exposed in Haskell as an enumerable data type.  For an enum+defined as follows:++@+alignment :: 'CppEnum'+alignment =+  'makeEnum' ('ident' \"Alignment\") Nothing+  [ (0, [\"left\", \"align\"])+  , (1, [\"center\", \"align\"])+  , (2, [\"right\", \"align\"])+  ]+@++the following data type will be generated:++@+data Alignment =+    Alignment_LeftAlign+  | Alignment_CenterAlign+  | Alignment_RightAlign+@++with instances for 'Bounded', 'Enum', 'Eq', 'Ord', and 'Show'.++-}+{- $generators-hs-module-structure-bitspaces++'Bitspace's, unlike enums, materialize in Haskell using a single data+constructor and bindings for values, rather than multiple data constructors.  A+bitspace declaration such as++@+formatFlags :: 'Bitspace'+formatFlags =+  'makeBitspace' ('toExtName' \"Format\") 'intT'+  [ (1, [\"format\", \"letter\"])+  , (2, [\"format\", \"jpeg\"])+  , (4, [\"format\", \"c\"])+  ]+@++will generate the following:++@+newtype Format++instance 'Bits' Format+instance 'Bounded' Format+instance 'Eq' Format+instance 'Ord' Format+instance 'Show' Format++fromFormat :: Format -> 'CInt'++class IsFormat a where+  toFormat :: a -> Format++instance IsFormat 'CInt'++format_FormatLetter :: Format+format_FormatJpeg :: Format+format_FormatC :: Format+@++-}+{- $generators-hs-module-structure-functions++For a 'Function' export, a single Haskell function will be generated named after+the external name of the export.  The function will take the Haskell-side types+of its arguments, and return the Haskell-side type of its return type.  If the+function is 'Nonpure' then it will return a value in 'IO', otherwise it will+return a pure value using 'unsafePerformIO'.++For most 'Type's, the corresponding Haskell parameter type will be a concrete+type.  This differs for objects (and references and pointers to them), where+typeclass constraints are used to implement C++ parameter type contravariance.+See the section on Haskell object passing for more details.++-}+{- $generators-hs-module-structure-callbacks++Despite needing to be exported as with other 'Export' choices, 'Callback's do+not expose anything to the user.  Instead, they provide machinery for functions+to be able to use 'callbackT'.++-}+{- $generators-hs-module-structure-classes++'Class'es expose quite a few things to the user.  Take a simple class+definition such as:++@+compressor :: 'Class'++zipper :: 'Class'+zipper =+  'makeClass' ('ident' \"Zipper\") Nothing [compressor]+  [ 'mkCtor' \"new\" [] ]+  [ 'mkStaticMethod' \"canZip\" [] 'boolT'+  , 'mkConstMethod' \"hasZipped\" [] 'voidT'+  , 'mkMethod' \"zip\" [] 'voidT'+  ]+@++Let's focus on @zipper@.  Two data types will be generated that represent+const and non-const pointers to @Zipper@ objects:++@+data Zipper+data ZipperConst+@++Internally, these types hold 'Ptr's, and they can be converted to 'Ptr's with+'toPtr' (though this conversion is lossy for pointers managed by the garbage+collector, see the section on object passing).++Several typeclass instances are generated for both types:++- 'Eq', 'Ord', and 'Show' compare and render based on the underlying pointer+address.++- 'CppPtr' and 'Deletable' instances provide object management.++- A single @'Decodable' ('Ptr' Zipper) Zipper@ instance is generated for+converting raw 'Ptr's into object handles.  This is the opposite operation of+'toPtr'.++- If the class -- @Zipper@ in this case -- has an @operator=@ method that takes+either a @'objT' zipper@ or a @'refT' ('constT' ('objT' zipper))@, then an+instance @ZipperValue a => 'Assignable' Zipper a@ is generated to allow+assigning of general zipper-like values to @Zipper@ objects; see below for an+explanation of @ZipperValue@.  This instance is for the non-const @Zipper@ only.++There will also be some typeclasses generated, for types that represent @Zipper@+objects:++@+class ZipperValue a where+  withZipperPtr :: a -> (ZipperConst -> IO b) -> IO b++instance CompressorPtrConst a => ZipperValue a++class CompressorPtrConst a => ZipperPtrConst a where+  toZipperConst :: a -> ZipperConst++class (ZipperPtrConst a, CompressorPtr a) => ZipperPtr a where+  toZipper :: a -> Zipper++instance ZipperPtrConst ZipperConst+instance ZipperPtr Zipper+... instances required by superclasses ...+@++Ignoring the first typeclass and instance for a moment, the two @Ptr@+typeclasses represent const and non-const pointers respectively, and allow+upcasting pointer types.  The const typeclass has as superclasses the const+typeclasses for all of the C++ class's superclasses (or just 'CppPtr' if this+list is empty).  The non-const typeclass has as superclasses the non-const+typeclasses for all of the C++ class's superclasses, plus the current const+typeclass.  Instances will be generated for all of the appropriate typeclasses+for @Zipper@ and @ZipperConst@, all the way up to 'CppPtr'.++The @ZipperValue@ class represents general @Zipper@ values, of which pointers+are one type (hence the first @instance@ above).  Values of these types can be+converted to a temporary const pointer.  If @Zipper@ were to have a native+Haskell type (see 'classHaskellConversion'), then an additional instance would+be generated for that type.  This second instance in this case is overlapping,+and the above instance is overlappable.  These typeclasses allow for mixing+pointer, reference, and object types when calling C++ functions.++For downcasting, separate const and non-const typeclasses are generated with+instances for all direct and indirect superclasses of @Zipper@:++@+-- Enables downcasting from any non-const superclass of Zipper.+class ZipperSuper a where+  downToZipper :: a -> Zipper++-- Enables downcasting from any const superclass of Zipper.+class ZipperSuperConst a where+  downToZipperConst :: a -> ZipperConst++instance ZipperSuper Compressor+... instances for other non-const superclasses ...+instance ZipperSuperConst CompressorConst+... instances for other const superclasses ...+@++The downcast functions are wrappers around @dynamic_cast@, and will return a+null pointer if the argument is not a supertype of the target type.++Finally, Haskell functions are generated for all of the class's constructors and+methods.  These work much the same as function exports, but non-static methods+take a @this@ object as the first argument.  Const methods take a @ZipperValue@+on the assumption that it's safe to create a temporary C++ object from a Haskell+value if necessary to call a const method.  Non-const methods take a+@ZipperPtr@, since it's potentially a mistake to perform side-effects on a+temporary object that is thrown away immediately.++@+zipper_new :: 'IO' Zipper+zipper_canZip :: 'IO' 'Bool'+zipper_hasZipped :: ZipperValue this => this -> 'IO' 'Bool'+zipper_zip :: ZipperPtr this => this -> 'IO' 'Bool'+@++-}+{- $generators-hs-module-dependencies++While generated C++ modules get their objects from @#include@s of underlying+headers and only depend on each other in the case of callbacks, Haskell modules+depend on each other any time something in one references something in another+(somewhat mirroring the dependency graph of the binding definitions), so cycles+are much more common (for example, when a C++ interface uses a forward class+declaration to break an @#include@ cycle).  Fortunately, GHC supports dependency+cycles, so Hoppy automatically detects and breaks cycles with the use of+@.hs-boot@ files.  The boot files contain everything that could be used from+another generated module, for example class casting functions needed to coerce+pointers to the right type for a foreign call, or enum data declarations.  The+result of this cycle breaking is deterministic: for each non-trivial strongly+connected component in the module dependency graph, @.hs-boot@ files are+generated for all modules, and all @.hs@ files' dependencies within the SCC+import @.hs-boot@ files.++-}+{- $generators-hs-object-passing++All of the comments about argument passing for the C++ generator apply here.+The following types are used for passing arguments from Haskell to C++:++>  C++ type   | Pass over FFI | HsCSide  | HsHsSide+> ------------+---------------+----------+-----------------+>  Foo        | Foo const*    | FooConst | FooValue a => a+>  Foo const& | Foo const*    | FooConst | FooValue a => a+>  Foo&       | Foo*          | Foo      | FooPtr a => a+>  Foo const* | Foo const*    | FooConst | FooValue a => a+>  Foo*       | Foo*          | Foo      | FooPtr a => a++@FooPtr@ contains pointers to nonconst @Foo@ (and all subclasses).  @FooValue@+contains pointers to const and nonconst @Foo@ (and all subclasses), as well as+the convertible Haskell type, if there is one.  The rationale is that @FooValue@+is used where the callee will not modify the argument, so both a const pointer+to an existing object, and a fresh const pointer to a temporary on the case of+passing a @Foo@, are fine.  Because functions taking @Foo&@ and @Foo*@ may+modify their argument, we disallow passing a temporary converted from a Haskell+value implicitly; 'withCppObj' can be used for this.++For values returned from C++, and for arguments and return values in callbacks,+the 'HsCSide' column above is the exposed type; polymorphism as in the+'HsHsSide' column is not provided.++Object pointer types in Haskell hide whether they are managed (garbage+collected) or unmanaged pointers in their runtime representation.  The APIs that+bindings expose to Haskell users should generally not require them to be+concerned about object lifetimes, and also having separate data types for+managed pointers would balloon the size of bindings.  Unmanaged objects can be+converted to managed objects with 'toGc'; after calling this function, the value+it returns should always be used in place of any existing pointers.++-}