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If not, see . + +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 +. diff --git a/Setup.hs b/Setup.hs new file mode 100644 --- /dev/null +++ b/Setup.hs @@ -0,0 +1,19 @@ +-- This file is part of Hoppy. +-- +-- Copyright 2016 Bryan Gardiner +-- +-- 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 . + +import Distribution.Simple +main = defaultMain diff --git a/hoppy-docs.cabal b/hoppy-docs.cabal new file mode 100644 --- /dev/null +++ b/hoppy-docs.cabal @@ -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 +maintainer: Bryan Gardiner +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 diff --git a/src/Foreign/Hoppy/Documentation/UsersGuide.hs b/src/Foreign/Hoppy/Documentation/UsersGuide.hs new file mode 100644 --- /dev/null +++ b/src/Foreign/Hoppy/Documentation/UsersGuide.hs @@ -0,0 +1,638 @@ +-- This file is part of Hoppy. +-- +-- Copyright 2015-2016 Bryan Gardiner +-- +-- 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 . + +{-# 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.\@. 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\@ 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. + +-}