A discussion of binding modules, the principles behind the tool, and a
discussion of related work can be found in a research paper located at
http://www.cse.unsw.edu.au/~chak/papers/papers.html#c2hs. All
features described in the paper, except enum define
hooks are implemented
in the tool, but since the publication of the paper, the tool has been
extended further.
Furthermore, the distribution contains examples that illustrate the use of
C->Haskell. In the source distribution, these examples are located below
the directories tests
and examples
. The latter contains a binding
for the
Gnome HTTP 1.1 library
ghttp
. The sources of the marshalling library C2HS
are in the
directory lib
and contain a fair amount of comments, which should help
getting you started.
Since version 0.8.1 the interface of the marshalling library C2HS
changed. The new interface essentially consists of the new Haskell FFI
Marshalling Library. More details about this library are provided in the next
section. For backward compatibilitym the old interface (i.e., the pre-0.8.1
interface) can still be used by importing C2HSDeprecated
instead of
C2HS
.
The remainder of this section describes the hooks that are available in binding modules.
{#import [qualified] modid#}
Is translated into the same syntactic form in Haskell, which implies that it
may be followed by an explicit import list. Moreover, it implies that
the module modid is also generated by C->Haskell and instructs the
tool to read the file modid.chi
.
If an explicit output file name is given (--output
option), this name
determines the basename for the .chi
file of the currently translated
module.
Currently, only pointer hooks generate information that is stored in a
.chi
file and needs to be incorporated into any client module that makes
use of these pointer types. It is, however, regarded as good style to use
import hooks for any module generated by C->Haskell.
{#context [header = header] [lib = lib] [prefix = prefix]#}
Context hooks define a set of global configuration options. Currently, there are three parameters all of which are strings:
gtk_
, as a form of poor man's
name spaces. Any occurrence of underline characters between a prefix and the
main part of an identifier must also be dropped. Case is not relevant in a
prefix. In case of a conflict of the abbreviation with an explicitly defined
identifier, the explicit definition takes preference.All three parameters are optional. An example of a context hook is the following:
{#context header = "gtkwidget.h" prefix = "gtk"#}
If a binding module contains a binding hook, it must be the first hook in the module.
{#type ident#}
A type hooks maps a C type to a Haskell type. As an example, consider
type GInt = {#type gint#}
The type must be a defined type, primitive types, such as int
, are not
admissible.
{#sizeof ident#}
A sizeof hooks maps a C type to its size in bytes. As an example, consider
gIntSize :: IntgIntSize = {#sizeof gint#}
The type must be a defined type, primitive types, such as int
, are not
admissible. The size of primitive types can always be obtained using
Storable.sizeOf
.
{#enum cid [as hsid] {alias1 , ... , aliasn}
[with prefix = pref] [deriving (clid1 , ... , clidn)]#}
Rewrite the C enumeration called cid into a Haskell data type
declaration, which is made an instance of Enum
such that the ordinals
match those of the enumeration values in C. This takes explicit enumeration
values in the C definitions into account. If hsid is given, this is
the name of the Haskell data type. The identifiers clid1 to clidn
are added to the deriving clause of the Haskell type.
By default, the names of the C enumeration are used for the constructors in
Haskell. If alias1 is underscoreToCase
, the original C names are
capitalised and the use of underscores is rewritten to caps. Moreover,
alias1 to aliasn may be aliases of the form cid as
hsid, which map individual C names to Haskell names. Instead of the
global prefix introduced by a context hook, a local prefix pref can
optionally be specified.
As an example, consider
{#enum WindowType {underscoreToCase} deriving (Eq)#}
Note: The enum define
hooks described in the C->Haskell are not
implemented yet.
{#call [fun] [unsafe] cid [as hsid]#}
A call hook rewrites to a call to the C function cid and also ensures
that the appropriate foreign import declaration is generated. The tags
fun
and unsafe
specify that the external function is purely
functional and cannot re-enter the Haskell runtime, respectively. If
hsid is present, it is used as the identifier for the foreign
declaration, which otherwise defaults to the cid.
As an example, consider
sin :: Float -> Float
sin = {#call fun sin as "_sin"#}
{#get apath#}
A get hook supports accessing a member value of a C structure. The hook itself yields a function that, when given the address of a structure of the right type, performs the structure access. The member that is to be extracted is specified by the access path apath. Access paths are formed as follows (following a subset of the C expression syntax):
struct
tag.*
apath denotes dereferencing of
the pointer yielded by accessing the access path apath..
cid specifies that the
value of the struct
member called cid should be accessed.->
cid, as in
C, specifies a combination of dereferencing and member selection.For example, we may have
visualGetType :: Visual -> IO VisualType
visualGetType (Visual vis) = liftM cToEnum $ {#get Visual->type#} vis
{#get apath#}
Set hooks are formed in the same way as get hooks, but yield a function that assigns a value to a member of a C structure. These functions expect a pointer to the structure as the first and the value to be assigned as the second argument. For example, we may have
{#set sockaddr_in.sin_family#} addr_in (cFromEnum AF_NET)
{#pointer [*] cid [as hsid] [foreign | stable] [newtype | -> hsid2]#}
A pointer hook facilitates the mapping of C to Haskell pointer types. In
particular, it enables the use of ForeignPtr
and StablePtr
types and
defines type name translations for pointers to non-basic types. In general,
such a hook establishes an association between the C type cid or
*
cid and the Haskell type hsid, where the latter defaults to
cid if not explicitly given. The identifier cid will usually be a
type name, but in the case of *
cid may also be a struct, union, or
enum tag. If both a type name and a tag of the same name are available, the
type name takes precedence. Optionally, the Haskell representation of
the pointer can be by a ForeignPtr
or StablePtr
instead of a plain
Ptr
. If the newtype
tag is given, the Haskell type hsid is
defined as a newtype
rather than a transparent type synonym. In case of
a newtype
, the type argument to the Haskell pointer type will be
hsid, which gives a cyclic definition, but the type argument is here
really only used as a unique type tag. Without newtype
, the default
type argument is ()
, but another type can be specified after the symbol
->
.
For example, we may have
{#pointer *GtkObject as Object foreign newtype#}
This will generate a new type Object
as follows:
newtype Object = Object (ForeignPtr Object)
which allows to export Object
as an abstract type and facilitates type
checking at call sites of imported functions using the encapsulated foreign
pointer. The latter is achieved by C->Haskell as follows. The tool
remembers the association of the C type *GtkObject
with the Haskell type
Object
, and so, it generates for the C function
void gtk_unref_object (GtkObject *obj);
the import declaration
foreign import gtk_unref_object :: Object -> IO ()
This function can obviously only be applied to pointers of the right type, and thus, protects against the common mistake of confusing the order of pointer arguments in function calls.
However, as the Haskell FFI does not allow to return ForeignPtr
s from
function calls, the tool will use the type Ptr HsName
in this case, where
HsName
is the Haskell name of the type. In the above example, that would
be Ptr Object
.
As an example that does not represent the pointer as an abstract type, consider the C type declaration:
typedef struct {int x, y;} *point;
We can represent it in Haskell as
data Point = Point {x :: Int, y :: Int}
{#pointer point as PointPtr -> Point#}
which will translate to
data Point = Point {x :: Int, y :: Int}
type PointPtr = Ptr Point
and establish a type association between point
and PointPtr
.
Restriction: The name cid cannot be a basic C type (such as
int
), it must be a defined name.
The following grammar rules define the syntax of binding hooks:
hook -> `{#' inner `#}'
inner -> `import' ['qualified'] ident
| `context' ctxt
| `type' ident
| `sizeof' ident
| `enum' idalias trans [`with' prefix] [deriving]
| `call' [`fun'] [`unsafe'] idalias
| `get' apath
| `set' apath
| `pointer' ['*'] idalias ptrkind
ctxt -> [`header' `=' string] [`lib' `=' string] [prefix]
idalias -> ident [`as' ident]
prefix -> `prefix' `=' string
deriving -> `deriving' `(' ident_1 `,' ... `,' ident_n `)'
apath -> ident
| `*' apath
| apath `.' ident
| apath `->' ident
trans -> `{' alias_1 `,' ... `,' alias_n `}'
alias -> `underscoreToCase'
| ident `as' ident
ptrkind -> [`foreign' | `stable'] ['newtype' | '->' ident]