[Haskell-cafe] HDBC, postgresql, bytestrings and embedded NULLs
jgoerzen at complete.org
Mon Jan 17 22:38:50 CET 2011
On 01/17/2011 03:16 PM, Michael Snoyman wrote:
> I've brought up before my problem with the convertible package: it
> encourages usage of partial functions. I would prefer two typeclasses,
> one for guaranteed conversions and one for conversions which may fail.
> In fact, that is precisely why convertible-text exists.
I would be open to making that change in convertible. The unfortunate
reality with databases, however, is that many times we put things into
strings for sending to the DB engine, and get things back from it in the
form of strings, which must then be parsed into numeric types and the
like. We can't, as a matter of type system principles, guarantee that a
String can be converted to an Integer. How were you thinking the
separation into these typeclasses would be applied in the context of
> As a related issue, there are a large number of data constructors in
> HDBC for SqlValue. I would not argue with the presence of any of them:
> for your purposes, every one of them is necessary. But for someone
> writing a cross-backend package with a more limited set of datatypes,
> it gets to be a problem. I know I can use convertible for this, but
> see my previous paragraph ;).
How about using an import...hiding statement? Perhaps even your own
module that only re-exports the constructors you like?
> I also don't like using the lazy result functions. I'm sure for many
> people, they are precisely what is needed. However, in my
> applications, I try to avoid it whenever possible. I've had bugs crop
> up because I accidently used the lazy instead of strict version of a
> function. I would prefer using an interface that uses enumerators.
It would be pretty simple to add an option to the API to force the use
of the strict versions of functions in all cases (or perhaps to generate
an exception if a lazy version is attempted.) Would that address the
concern? Or perhaps separating them into separate modules?
I took a quick look at the enumerators library, but it doesn't seem to
have the necessary support for handling data that comes from arbitrary C
API function calls rather than handles or sockets.
> For none of these do I actually think that HDBC should change. I think
> it is a great library with a well-thought-out API. All I'm saying is
> that I doubt there will ever be a single high-level API that will suit
> everyone's need, and I see a huge amount of value in splitting out the
> low-level code into a separate package. That way, *everyone* can share
> that code together, *everyone* can find the bugs in it, and *everyone*
> can benefit from improvements.
Splitting out the backend code is quite reasonable, and actually that
was one of the goals with the HDBC v2 API. I would have no objection if
people take, say, HDBC-postgresql and add a bunch of non-HDBC stuff to
it, or even break off the C bindings to a separate package and then make
HDBC-postgresql an interface atop that.
I hope that we can, however, agree upon one low-level database API. The
Java, Python, and Perl communities, at least, have. Failing to do so
produces unnecessary incompatibility.
I would also hope that this database API would be good enough that there
is rarely call to bypass it and use a database backend directly.
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