I'm trying to figure out how to make libraries with Felix.
Roughly speaking you can do this now by:
1. Make some code, and export some types and functions.
This gives the functions dlopen() names (bypassing
C++ typesafe linkage).
The type names are specified are provided as typedefs.
This means they're just aliases for weird Felix mangled names.
Example flib.flx:
fun f(x:int, y:int)=>x + y;
export int as "myint";
export int * int as "myint2";
export fun f of (int * int) as "myf";
2. Write a Felix wrapper for the library, just as
if it were a C library:
type myint = "myint";
type myint2 = "myint2";
fun f: myint2 -> myint = "myf($1)";
Note myint and myint2 are abstract types. You can't
construct myint2, nor can you get one of its components.
In this case, you can also do this:
fun f: int * int -> int = "(int)myf(reinterpret<#t>($1))";
Reinterpret<> is a Felix C++ library template that can do
reinterpret casts on rvalues. #t is the code for "the type
of the argument tuple"
Now, it should be simple enough to make a 'library' declaration
which causes Felix to emit say
flib.flxi
which is the above Felix wrapper, except that there is a problem
with types. Every Felix compilation will name the same algebraic
type differently, including primitives (abstract types) such
as 'int' (actually, int,long etc are special cases and retain
their C names .. but it is a hack for making some code look
cleaner and can't be relied on).
THE PROBLEM is that C++ is a very bad language and templates
do not work the same way as non-template code when it
comes to types: recursive template classes can't be defined.
Ideally, we use the names
template<class T, class U> struct tuple2<T,U>;
template<class T, class Y> struct function<T,U>;
// etc
as canonical type names. Felix would then simply
specialise them, instead of using a synthesised name.
Unfortunately, it isn't possible to do that if the
type is recursive, for example:
typedef function<int,f_t> f_t; // C++ typedef
doesn't work. This is a function type:
typedef r = int -> b as b; // Felix typedef
of which this:
fun f(x:int):r = { println "hi"; return the f; }
is an example (yes, it works fine). The same problem
applies to tuples, unions, etc.
--
John Skaller <skaller at users dot sf dot net>
Felix, successor to C++: http://felix.sf.net
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