Optimisation of unpackCString#

Neil Mitchell ndmitchell at gmail.com
Mon Apr 28 17:02:45 EDT 2008


> 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.



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