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Today's Topics:
1. Re: Exercise of "Programming with Arrows" (Kim-Ee Yeoh)
2. Re: Exercise of "Programming with Arrows" (Thiago Negri)
3. Re: monads do not fit together? (Kees Bleijenberg)
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Message: 1
Date: Mon, 14 Oct 2013 22:42:51 +0700
From: Kim-Ee Yeoh <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Exercise of "Programming with Arrows"
Message-ID:
<CAPY+ZdSafPAJ_PKumcFNbUb=tyu7mzg07yam8z+d9jp7_-1...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
On Tue, Oct 8, 2013 at 10:43 PM, Thiago Negri <[email protected]> wrote:
> data Queue a = Queue [a]
>
> empty :: Queue a
> empty = Queue []
>
> push :: a -> Queue a -> Queue a
> push a (Queue as) = Queue (a:as)
>
> pop :: Queue a -> Maybe (a, Queue a)
> pop (Queue []) = Nothing
> pop (Queue (a:as)) = Just (a, Queue as)
>
This is not a queue!
This is a stack, a Last In First Out structure.
Queues are First In First Out.
Here, I'll help you with the quickcheck instance:
instance (CoArbitrary a, Arbitrary b) => Arbitrary (SP a b) where
arbitrary = oneof [liftM2 Put arbitrary arbitrary, liftM Get arbitrary]
Also, we need a dummy show:
instance (Show a, Show b) => Show (SP a b) where
show _ = error "SP a b: no show"
The test you need to write is the following:
prop_SP_first :: SP Int Int -> [(Int,Int)] -> Bool
prop_SP_first f xs = x1 == x2 where
-- your name is thiago and you WILL be cool once you've fixed your buggy
code
-- Kim-Ee
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Message: 2
Date: Mon, 14 Oct 2013 13:06:34 -0300
From: Thiago Negri <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Exercise of "Programming with Arrows"
Message-ID:
<CABLneZv65P=XsV==iiDbiEJNc86ErUufyWR7TaQ6Q=wq9+x...@mail.gmail.com>
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Thanks for the new reply.
I've fixed the "queue" not being a real queue (see the last gist I've sent
to the list [1]).
The problem I was having in defining "first" was in my understanding of the
execution of the arrow.
I didn't realize that (>>>) is just the composition function from Category
class.
I tought that only the last output would be received by the second arrow of
(>>>), and it would continue the stream processing from there.
I was really lost.
Now I see that the correct thing to do is to fit a no-op on the second
stream because outputs will be delivered in pairs to the composed arrow.
Sort of a BalanceLine/Merge that I've learned in my COBOL years.
I will run your test cases as soon as I get home.
By the way, I couldn't figure out a way to define an ArrowLoop instance
that "runSP (loop (arr swap))) [1..10]" doesn't evaluate to bottom (loops
forever).
I don't have much time to this, but I'm trying to solve it mentally when I
feel in the mood and typing some code on a 30-min/day basis.
The hackage package that defines a Stream Processor as explained by Hughes
uses a different definition for SP data type and has the same problem when
evaluating "swap" in a loop (it throws an error) [2].
I guess this problem is harder than Hughes expected, as I find it more
difficult to solve than the tricky "first" definition.
Or, I may be missing something huge.
(I still want to solve it by myself, but tips are welcome.)
Thanks,
Thiago.
[1] https://gist.github.com/thiago-negri/2e541a9f9762c727bdd4
[2] http://hackage.haskell.org/package/streamproc
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Message: 3
Date: Tue, 15 Oct 2013 11:00:04 +0200
From: "Kees Bleijenberg" <[email protected]>
To: "'The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell'" <[email protected]>
Subject: Re: [Haskell-beginners] monads do not fit together?
Message-ID: <000601cec984$f09f7090$d1de51b0$@[email protected]>
Content-Type: text/plain; charset="utf-8"
Brandon,
Thanks you for your advice.
I want to add CGI support to a program that writes to a file, but has no
network access. The old code uses a WriterT with IO in it. Adding another monad
to the stack with IO to the network, is a bit too difficult for a beginner like
me.
The old code returns IO [String] ([String] = list of status messages). It is a
bit annoying that I have to adapt the old code. I just want to grab the status
messages and convert them to html and write them to the network.
I wonder, do you always have to create a monadtransformer if your program uses
2 kinds of IO?
Kees
Van: Beginners [mailto:[email protected]] Namens Brandon Allbery
Verzonden: maandag 14 oktober 2013 17:03
Aan: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell
Onderwerp: Re: [Haskell-beginners] monads do not fit together?
On Mon, Oct 14, 2013 at 10:58 AM, Kees Bleijenberg <[email protected]>
wrote:
I get the same message (Couldn't match expected type `CGIT IO a0' with actual
type `IO [String]'). I think I do understand where this message is coming from.
But what can I do to fix it?
For now, you just correct the type of runRefreshFirewall; it should be
runRefreshFirewall :: String -> CGIT IO [String]
If you are intending to run actual IO actions in the real thing, you'll need to
use liftIO to "reach" the IO embedded in the CGIT IO a.
--
brandon s allbery kf8nh sine nomine associates
[email protected] [email protected]
unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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