Optimisation of unpackCString#

[Don Stewart]

ndmitchell:
> Hi
> 
> > The first case makes sense, and is just a RULE. Though it seems GHC already
> >  does this?
> >
> >     g = head ""
> >
> >  goes to:
> >
> >     M.g = badHead @ Char
> >
> >  without prompting.
> 
> Nope, as far as I can tell "" gets translated to [], not
> unpackCString# "" - hence the unpack never gets in the way. If you had
> the second rule, you'd soon want the first.
> 
> >  So I wondered if I could write a rule for head/unpackCString
> >
> >  Urgh. not what we want though. GHC won't index the string at compile time,
> >  and I can't write a RULE for it.
> 
> Yeah, I tried that too...
> 
> >  Perhaps you can newtype Char and use string overloading to work around the
> >  packing issue?
> 
> That would work on GHC, but not on Hugs.

Optimisation and Hugs don't go together anyway.

-- Don