[Haskell-cafe] Re: Pickling HList

Keean Schupke k.schupke at imperial.ac.uk
Tue Nov 22 11:51:15 EST 2005

That all depends... In theory all the HList stuff happens at compile 
time, and what you are left with is normal function application... Of 
course compilers arn't that good yet, but as a reasonable idea, consider 
just that value level... Most of the extra work is the packing/unpacking 
of pairs "(,)". I have used HList for database schemas like the "Cow" 
example database (see attached) with no problems. The DB code includes 
code to generate the database from this "Schema" so is doesn't need to 
be entered twice, and it also typechecks the database against the schema 
in a one-way extensional manner on program start. The performance of the 
DB app is good, better than with scripting languages like perl/python, 
and type-safe.
 This code uses records made from HLists (see the paper for examples).


Joel Reymont wrote:

> Keean,
> I sort of gave up on HList for the time being since I found easier  
> ways to solve my problem.
> Mainly, I could not estimate the impact it would have on run-time  
> performance of my code and GHC not being able to compile the code was  
> not a good indication. Simon PJ fixed that error since.
> My idea was to, basically, create my own record sans labels. I wanted  
> to specify picklers and default values for each field instead. I have  
> over 250 records, though, and some have over 10 fields. There is a  
> lot of sharing of fields between the records but I still think this  
> is too much for GHC to handle.
> Can you venture a guess on runtime performance of such code?
>     Thanks, Joel
> On Nov 22, 2005, at 4:07 PM, Keean Schupke wrote:
>>     hMarkAll Just hlist
>>    class HList l => HMarkAll c l m | c l -> m where
>>       hMarkAll :: (forall a . a -> c a) -> l -> m
>>    instance HMarkAll c HNil HNil where
>>       hMarkAll _ _ = HNil
>>    instance HMarkAll c l m => HMarkAll c (HCons e l) (HCons (c e)  m) 
>> where
>>       hMarkAll c (HCons e l) = HCons (c e) (hMarkAll c l)
> -- 
> http://wagerlabs.com/

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