[Haskell-cafe] Announce: HDBI-1.3 and friends

[Alexey Uimanov]

Hello haskellers!

HDBI is the fork of HDBC but reworked. It supports SQlite3 and Postgresql
for now. It also supports streaming with coduits.
There is TH deriving mechanism to map database rows to Haskell structures
and back. HDBI is trying to become simple
but still powerfull and flexible database interface. It must be suitable to
become the common RDBMS interface  for higher level interfaces
like persistent or haskelldb.

The documentation is not very good, while I have no enough time to make
some.

In this version changed typeclass signatures of Connection and Statement.
Now methods `run` and `execute` get any instance of `ToRow` and method
`fetch` return an instance of `FromRow`.
Note that [SqlValue] is also an instance of `FromRow` and `ToRow`
typeclasses so you do not loose the control.
Methods `fromRow` and `toRow` for [SqlValue] are just `id`. SQlite and
Postgresql drivers are fixed as well as hdbi-conduit.

There is also new helper functions, like `onei :: Integer -> [SqlValue]`
which helps you to execute queries with one parameter
or execute many queries consistinf of one parameter.

Prelude Database.HDBI Database.HDBI.SQlite> :set -XScopedTypeVariables
Prelude Database.HDBI Database.HDBI.SQlite> :set -XOverloadedStrings
Prelude Database.HDBI Database.HDBI.SQlite> c <- connectSqlite3 ":memory:"
Prelude Database.HDBI Database.HDBI.SQlite> runRaw c "create table test(val
integer)"
Prelude Database.HDBI Database.HDBI.SQlite> withTransaction c $ runMany c
"insert into test(val) values (?)" $ map one [1..1000]

<interactive>:7:76: Warning:
    Defaulting the following constraint(s) to type `Integer'
.........

Prelude Database.HDBI Database.HDBI.SQlite> r :: (Maybe Integer) <-
runFetchOne c "select sum(val) from test" ()
Prelude Database.HDBI Database.HDBI.SQlite> r
Just 500500

Note here that the empty set is used as a parameter of query in
`runFetchOne`. Empty set is an instance of `FromRow` and `ToRow` and return
an empty list of [SqlValue].
Use empty list as a parameters is bad idea, because we could instantiate
some another list of things, suppose the [Integer] as `FromRow` and
`ToRow` instance. So it would lead to ambigous type because [SqlValue] is
also a list instantiating `FromRow` and `ToRow`.
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