[Haskell-cafe] what is the magic hash?
wren ng thornton
wren at freegeek.org
Wed Sep 24 00:24:37 EDT 2008
Jason Dusek wrote:
> Derek Elkins <derek.a.elkins at gmail.com> wrote:
>> Jason Dusek wrote:
>>> It is not much covered in the docs. It has something to do
>>> with magic triggered by a postfix octothorpe?
>> All it does is allow them in identifiers.
> That's it? So it's for use in conjunction with primitive
> types, I guess (those seem to be denoted like 'Int#')?
> With the fancy name and the absence of documentation, I
> assumed it must be really special.
The octothrorp is for the unboxed/unlifted kind in GHC. In addition to
the postfix usage it's also used for unboxed tuples (# , #) (# ,, #)...
which don't actually exist, and also as a kinding symbol for these
native types (as opposed to the * symbol for the usual boxed/lifted kind).
There are various limitations on where you can use types in the # kind,
which should be in the GHC docs. Unless you're trying to hammer the last
fraction of performance out of GHC, it's best to leave them alone and
trust the optimizer to reorganize code to use them. These types of
performance gains are often more portably gained by using bang patterns
or strict fields, FWIW.
 Each field of the unboxed tuple is stored in a register (or
otherwise fed directly to its destination).
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