Posts tagged ‘hdbc’

Haskell and Time

16th September 2008, 11:31 pm

I’ve found myself getting confused over the Data.Time modules a few times now. Today I wanted to do something that I ought to be simple both to do (it was) and find out how to do (it wasn’t, at least not for me). I thought I’d write it down, because I’m bound to forget about it otherwise, and it might be helpful for some other poor soul out there.

I wanted to store a date in a database (SQLite) using Haskell (HDBC). Since I’m only interested in the date and not the time I chose to represent it in Haskell as a Date.Time.Calendar.Day. For Day 0 means 1858-11-17. HDBC on the other hand needs the date in seconds since epoch (1970-01-01 00:00 UTC), which conveniently can be represented with a single Integer. Time.Data.Clock.POSIX contains a few handy functions to convert between POSIXTime (NominalDiffTime) and UTCTime. However, there are no special functions for converting between an Integer and NominalDiffTime. The way I found was to do it via the class types that NominalDiffTime implements.

First I thought that Enum might offer the solution, but my experiments with that was a bit confusing:

> pt <- getPOSIXTime
1221603766.526661s
> fromEnum pt
4118657661830593344
> toEnum 4118657661830593344 :: POSIXTime 
4118657.661830593344s

Not really the result I had hoped for. No, instead the function to use to get the Integer is floor, and the only way I found to convert a NominalDiffTime from an Integer is to go via a Ratio:

> fromRational (1221603766 % 1) :: POSIXTime
1221603766s

Have I missed something or is this the best way to achieve this?

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Confusion with HaskellDB

6th September 2008, 12:59 am

Dear LazyWeb,

I’ve recently made some comparisons between HaskellDB and HDBC. My gut feeling is that I’d like to use HaskellDB due to its type safety and that it allows me to abstract away from SQL—something in me twitches every time I see strings containing SQL mixed in with other code, no matter what language it is. However there are some things that confuse me with HaskellDB. After overcoming the initial puzzlement that I seem to hit whenever I look at a Haskell API I stumbled on the apparent lack of support for primary keys. This surprised me and to check if I overlooked something I wrote the following code:

module Main where
 
import System.Environment
import System.IO
import Database.HaskellDB.HSQL.SQLite3
import Database.HaskellDB.DBSpec
 
testDBOpts = DBOptions { useBString=False }
 
sqliteOptions= SQLiteOptions { filepath="replace me", mode=ReadMode }
 
main :: IO ()
main = do
    dbfn <- fmap (!! 0) getArgs
    let options = sqliteOptions { filepath=dbfn }
    sqliteConnect options (dbToDBSpec False "extracted.db") >>= print

This is the surprising output when I point this to two different databases. The first database has no primary key:

% sqlite3 test1.db .dump
BEGIN TRANSACTION;
CREATE TABLE person (name text  not null,
                     age int);
COMMIT;

and the DBInfo looks the way I expected:

% ./extract test1.db 
DBInfo {dbname = "extracted.db", opts = DBOptions {useBString = False},
tbls = [TInfo {tname = "person", cols = [CInfo {cname = "name",
descr = (StringT,False)},CInfo {cname = "age", descr = (StringT,True)}]}]}

The second database does have a primary key:

% sqlite3 test2.db .dump
BEGIN TRANSACTION;
CREATE TABLE person (name text primary key not null, age int);
COMMIT;

but not only does the DBInfo lack any mention of it, it also contains the table info twice:

% ./extract test2.db 
DBInfo {dbname = "extracted.db", opts = DBOptions {useBString = False},
tbls = [TInfo {tname = "person", cols = [CInfo {cname = "name",
descr = (StringT,False)},CInfo {cname = "age", descr = (StringT,True)}]},
TInfo {tname = "person", cols = [CInfo {cname = "name",
descr = (StringT,False)},CInfo {cname = "age", descr = (StringT,True)}]}]}

Am I doing something fundamentally wrong or does HaskellDB not support the notion of a primary key?

Is it a bug in HaskellDB that the second extracted DBInfo contains two instances of TInfo when the database clearly only has one table?

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