>
> But you can't just put a float 42.0 on the heap or even stack when the
> GC might get called. That needs to be boxed in some way to avoid it
> getting misread as pointer.
>
>
It wouldn't be too hard to add a word of metadata to the stack to tell the
GC what's a pointer and what isn't. Haskell does this for function calls
(in fact, if the metadata doesn't fit in a word, it allocates a separate
metadata structure), and F# has this from the .NET runtime which has full
type metadata for everything.

Problem is that a value is a fixed size and fits in a register. A
> tuple does not. (int * float) even takes the space of 3 values on
> 32bit. You can unbox that in the optimizer for local use but in memory
> and in function calls you need to pass this as box. Otherwise
> polymorphic functions break.
>
> Putting larger structures into an array without boxing also only works
> for immutable objects and by copying them every time they are passed
> around. You can't copy a mutable and you can't pass a pointer to the
> middle of an array to another function or the GC might freak out.
>

Leaving it to the optimizer is problematic because it might cause a lot of
unneeded boxing and unboxing.
Haskell has the {- #UNPACK -} pragma to unbox types. You have to be really
careful in haskell with this, because you're also changing the evaluation
semantics to be strict. This makes for really ugly optimized haskell code,
but maybe we can do something similar (but not as ugly). F# inherits .NET's
struct types, which are similarly limited, but also useful. Of course, once
you unbox, all parametric polymorphism is lost, but because you have
control over it, you can decide where it's worthwhile.

An example of unpack usage in haskell:

data T = T {-# UNPACK #-} !(Int,Int)

which would be equivalent to something like
type t = (int * int) [@u]

Note that the whole tuple is unboxed.

In haskell, you can now do

f :: T -> Int
f (T(i1,i2)) = i1 + i2

and in ocaml you'd do

let f : t -> int = fun (i1,i2) = i1 + i2

Of course, this would require more metadata for the stack. For the heap,
you'd probably want to just use a new tag or the custom tag. For ocaml
you'd also probably have to stipulate that polymorphic marshalling cannot
be performed on this type, and neither can polymorphic comparison -- you'd
have to have specific marshalling/comparison functions, which aren't too
difficult to generate automatically.

-Yotam



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