[LLVMdev] Parametric polymorphism

[Jon Harrop]

On Wednesday 18 February 2009 23:36:27 DeLesley Hutchins wrote:
> > Why do you say that people who compile, e.g., functional languages
> > would benefit from type variables in LLVM?
> > I like the level the LLVM is at, and would prefer to deal with
> > instantiating parametric polymorphism at a higher level.
>
> I'm surprised you're happy with a non-polymorphic llvm.  Does
> Cayenne target llvm?  Dependent types take polymorphism to new
> heights -- but perhaps you feel that since llvm cannot hope to
> match Cayenne, you might as well do everything yourself.  :-)
>
> My reasons are:
>
> First, if type variables are handled at the higher-level, then every
> functional language will have its own implementation.

No, they just need to share a HLVM.

> Second, polymorphism can be made more efficient if it has low-level
> codegen support -- see my earlier offset code.

In theory, perhaps.

> Third, any language which has polymorphism is faced with the task
> of ``erasing'' it before it can hand the code to llvm.  Such erasure can be
> done in different ways -- compare Java type erasure (use pointers,
> instantiate nothing), with C++ (instantiate everything), with .Net
> (instantiate some things).  (I don't know how Ocaml does it)  It is very
> difficult to share code between languages if the erasure semantics differ
> so widely.

The same can be said of closures, garbage collection and a dozen other 
features that also cannot feasibly be added to LLVM.

The only logical solution is to build a HLVM on top of LLVM and share that 
between these high-level language implementations.

-- 
Dr Jon Harrop, Flying Frog Consultancy Ltd.
http://www.ffconsultancy.com/?e