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Today's Topics:
1. Lifting (Either SqlError a) in a monad stack (Bryan Vicknair)
2. Re: Lifting (Either SqlError a) in a monad stack (Karl Voelker)
3. Re: print [] (AntC)
----------------------------------------------------------------------
Message: 1
Date: Mon, 26 Aug 2013 14:05:00 -0700
From: Bryan Vicknair <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] Lifting (Either SqlError a) in a monad
stack
Message-ID: <20130826210500.GA4710@bry-m6300>
Content-Type: text/plain; charset=us-ascii
I'm at the beginning of the monad transformers journey, and this is stumping
me. I've read "Monad Transformers Step by Step" which made me confident enough
to play with simple stacks, but I'm a bit lost in the following example from
work. Please excuse any incorrect transformer terminology.
In a WAI web app, I want to chain these two actions inside a monad stack:
> safeConnect :: ConnectInfo -> IO (Either SqlError Connection)
> safeInsert :: Thing -> Connection -> IO (Either SqlError Id)
Here is an example where I would like 'result' to be (Right Id) only if the DB
connection *and* the insert were successful, (Left SqlError) otherwise.
> add :: Request -> ResourceT IO Response
> add _ = do
> result <- liftIO $ safeConnect devConnInfo >>= safeInsert thing
> case result of
> (Left e) -> return $ dbErr e
> (Right _) -> return $ postRedirect
> where thing = exampleThing
The compiler tells me (edited for brevity):
Expected type: Either SqlError Connection -> GHC.Types.IO a0
Actual type: Connection -> GHC.Types.IO (Either SqlError Id)
In the return type of a call of `safeInsert'
In the second argument of `(>>=)', namely
`safeInsert thing'
How can I chain safeConnect and safeInsert using the (Either SqlError a) monad
inside WAI's (ResourceT IO Response) monad stack?
------------------------------
Message: 2
Date: Mon, 26 Aug 2013 16:18:16 -0700
From: Karl Voelker <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Lifting (Either SqlError a) in a
monad stack
Message-ID:
<CAFfow0x=qtktos2wtqkjnoozhstnfjrz7fgdrotrhjfndl0...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
The problem is that the Either SqlError monad is not actually a part of
your "monad stack". It's just something that appears in the result type of
safeConnect and safeInsert. In order to put Either SqlError into your
stack, you'll need a monad transformer which exhibits Either-like
semantics, such as EitherT or ErrorT. Then your safeConnect would have type
"ConnectInfo -> EitherT SqlError IO Connection".
http://hackage.haskell.org/packages/archive/either/3.4.1/doc/html/Control-Monad-Trans-Either.html
http://hackage.haskell.org/packages/archive/transformers/0.3.0.0/doc/html/Control-Monad-Trans-Error.html
-Karl
On Mon, Aug 26, 2013 at 2:05 PM, Bryan Vicknair <[email protected]> wrote:
> I'm at the beginning of the monad transformers journey, and this is
> stumping
> me. I've read "Monad Transformers Step by Step" which made me confident
> enough
> to play with simple stacks, but I'm a bit lost in the following example
> from
> work. Please excuse any incorrect transformer terminology.
>
> In a WAI web app, I want to chain these two actions inside a monad stack:
>
> > safeConnect :: ConnectInfo -> IO (Either SqlError Connection)
> > safeInsert :: Thing -> Connection -> IO (Either SqlError Id)
>
> Here is an example where I would like 'result' to be (Right Id) only if
> the DB
> connection *and* the insert were successful, (Left SqlError) otherwise.
>
> > add :: Request -> ResourceT IO Response
> > add _ = do
> > result <- liftIO $ safeConnect devConnInfo >>= safeInsert thing
> > case result of
> > (Left e) -> return $ dbErr e
> > (Right _) -> return $ postRedirect
> > where thing = exampleThing
>
> The compiler tells me (edited for brevity):
>
> Expected type: Either SqlError Connection -> GHC.Types.IO a0
> Actual type: Connection -> GHC.Types.IO (Either SqlError Id)
> In the return type of a call of `safeInsert'
> In the second argument of `(>>=)', namely
> `safeInsert thing'
>
> How can I chain safeConnect and safeInsert using the (Either SqlError a)
> monad
> inside WAI's (ResourceT IO Response) monad stack?
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
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------------------------------
Message: 3
Date: Tue, 27 Aug 2013 09:46:59 +0000 (UTC)
From: AntC <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] print []
Message-ID: <[email protected]>
Content-Type: text/plain; charset=utf-8
> Lukas Lehner <lehner.lukas <at> gmail.com> writes:
>
> ... My question is that I have to enforce the type that specific way
(don't want to use?ExtendedDefaultRules) or is there some more generic way?
>
print flatten (List ([] :: Show a => [a])
Even for an empty list, we have to provide evidence that the list is
showable. That's what the type signature is doing: I am a list of
something/anything showable.
(If you want all of your NestedLists's to be showable, it's better to put
a Show constraint on the data decl. But that would take us into
existential types or GADT's.)
AntC
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