[ghc] kind of the function arrow

[Dominic Orchard]

I was just playing around and noticed that the kind of the function
arrow in GHC is (?? -> ? -> *) when I (naively) expected it to be (* ->
* -> *).
After looking at
(http://hackage.haskell.org/packages/archive/ghc/6.10.2/doc/html/Type.html#5)
I see that the kind of (->) means that the parameter type cannot be an
unboxed tuple, whilst the result type can be anything. Why is this?
After reading this documentation I would expect the kind (? -> ? -> *).

I'm now wondering if the kind of (->) could cause problems if the
following style of declaration is requried:

 > type FunArg a b = (a -> b, a)

*Main> :k FunArg
FunArg :: * -> * -> *

By using type variables, whose default kind is *, the function type is
fixed to use only boxed types. But if one wanted to allow unboxed type
parameters the kind would be wrong, and an explicit kind signature of #
or ? can't be given as they are not part of Haskell's source language.

I guess my question is why the (?? -> ? -> *) kind on (->) and what to
do if synonyms or data types over (->) are required such as the example
just stated. I'm just curious.

Thanks,
Dominic
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[wren ng thornton]

Dominic Orchard wrote:
> I was just playing around and noticed that the kind of the function
> arrow in GHC is (?? -> ? -> *) when I (naively) expected it to be (* ->
> * -> *).
> After looking at
> (http://hackage.haskell.org/packages/archive/ghc/6.10.2/doc/html/Type.html#5)
> I see that the kind of (->) means that the parameter type cannot be an
> unboxed tuple, whilst the result type can be anything. Why is this?
> After reading this documentation I would expect the kind (? -> ? -> *).

Unboxed tuples don't exist. That is, they are in evidence, but they
don't actually have any physical representation.

The elements of an unboxed tuple are stored in registers when returning
from a function, so that the caller can access them immediately (rather
than needing to indirect through a pointer to a tuple). ISTR that
because of this strategy, there are restrictions on what types of tuples
can be unboxed. Theoretically GHC could also allow passing certain
arguments in registers when invoking a function, but this isn't
supported as yet.

--
Live well,
~wren
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