[Haskell-cafe] Access to Oracle database from Haskell

[Henning Thielemann]

On Thu, 26 Jun 2008, Alistair Bayley wrote:

> Yes, puzzling. I don't know why you need to say -lclntsh, because
> that's the point of all of this Setup shenanigans: to get things set
> up so that ghci works nicely.
>
> That's why we have flags that expose/hide modules in the API: ghci has
> a custom linker, and this tries to link everything in the library, and
> of course this fails if you don't have the .ddl/.so installed for a
> particular backend. ghc uses gnu ld to link, and this does not try to
> link unused modules, so this works fine with the entire library API
> compiled.
>
> Do you need to say -lclntsh when you use ghc to compile?

Ah, I see, I must run both GHCi and GHC with -package Takusen and 
everything is fine.

>> Next step is to fetch real data from the database. But now I encounter new
>> problems. If I write a real 'select' statement I get at best:
>>
>> Unexpected null in row 1, column 1.
>> *** Exception: (unknown)
>
> This looks like you are fetching a null value back with an iteratee
> that does not expect nulls. We use Maybe to specify that a column in
> the result-set can be null e.g.
>
>  iter :: Int -> Maybe Int -> Maybe String -> IterAct [(Int, Maybe Int, Maybe String)]

Indeed, using Maybe as type solves that problem.

>> and at worst:
>>
>> 50001752
>> *** glibc detected *** /usr/lib/ghc-6.8.2/ghc-6.8.2: free(): invalid
>> pointer: 0x08d10065 ***
>> ======= Backtrace: =========
>
> If this continues, can you provide a test case for me?

I cannot reliably reproduce this. Sometimes it seems that the first run of 
'main' succeeds, and the second one crashes in GHCi.


So, many thanks for the help!