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Today's Topics:
1. Re: exception, not in IO (Kees Bleijenberg)
2. Re: Lens (Brent Yorgey)
----------------------------------------------------------------------
Message: 1
Date: Tue, 16 Jul 2013 10:04:41 +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] exception, not in IO
Message-ID: <000001ce81fb$20308c30$6091a490$@[email protected]>
Content-Type: text/plain; charset="utf-8"
Jeda?
......
We discover that this function is pretty simple in fact and combine some pure
code that does provide pure exceptions, so defining a new function may bring a
solution :
tryDecodingJSON :: (Data a) => String -> Result a tryDecodingJSON s =
case runGetJSON readJSValue s of
Left msg -> Error msg
Right j -> fromJSON j
You may need to import Text.JSON.String for runGetJSON.
---
This is a nice solution for the problem.
Thanks!
Kees
------------------------------
Message: 2
Date: Tue, 16 Jul 2013 07:24:52 -0400
From: Brent Yorgey <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Lens
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Tue, Jul 16, 2013 at 07:09:43AM +0200, Emmanuel Surleau wrote:
> Hi Brent,
>
> Thanks for the link to the video. I watched the first half yesterday, and
> it's definitely content rich. But the overall design of the library is
> starting to make sense now. The reason it doesn't work for Sets (and I
> assume any similar structure like hashmaps) is because, depending on the
> implementation of Eq on the item the Set is parametrized on and the field
> being modified, you could accidentally remove another item - and this would
> be a BAD THING, since applying a setter on a traversal is basically
> "fmap++" and fmap doesn't let you remove things. Is this correct?
Yes, that's a good way to think of it.
> Also, thanks for the traversal example with the list, that's certainly
> going to be useful.
Glad I could be of help!
-Brent
>
> Cheers,
>
> Emm
>
>
> On Mon, Jul 15, 2013 at 3:16 PM, Brent Yorgey <[email protected]>wrote:
>
> > Hi Emmanuel,
> >
> > On Sun, Jul 14, 2013 at 10:47:16PM +0200, Emmanuel Surleau wrote:
> > > Hello there,
> > >
> > > The first (concrete) problem I ran into is how to update the members of a
> > > set with the result of an IO action. I have managed to do this with a
> > pure
> > > function (prefixName) but I'm not sure of how to do this with
> > > promptName.
> >
> > Unfortunately you cannot do this using a Set. The reason is that
> > modifying the contents of a Set might result in a smaller Set and that
> > cannot possibly satisfy the lens laws. However, if we change the Set
> > of dogs to be a list, we can do this using the (%%~) operator:
> >
> > > ... same as before ...
> > >
> > > data Dogs = Dogs { _dogs :: [Dog] }
> > > deriving Show
> > > makeLenses ''Dogs
> > >
> > > main :: IO ()
> > > main = do
> > > ... as before ...
> > >
> > > -- change dog names by prompting the user
> > > doggies' <- doggies & (dogs.traverse.name) %%~ prefixName
> > > print doggies'
> > >
> > > return ()
> >
> > But astoundingly, if you look at the implementation of (%%~), it
> > is... (%%~) = id! So this code also works:
> >
> > doggies' <- (dogs.traverse.name) prefixName doggies
> >
> > That's right, the magic solution is to just treat the
> > (dogs.traverse.name) lens as a *function* and apply it to prefixName!
> >
> > To understand why this works we have to look a bit at the
> > implementation. I am not surprised that you were baffled by it
> > because it looks quite magical if you don't understand where it comes
> > from.
> >
> > > type Lens s t a b = forall f. Functor f => (a -> f b) -> s -> f t
> >
> > Instead of trying to understand why this is the definition, let's just
> > see what happens if we take the right-hand side and set f = IO:
> >
> > (a -> IO b) -> s -> IO t
> >
> > In the above example, a = b = String, and s = t = Dogs, that is,
> > (dogs.traverse.name) has type
> >
> > Lens Dogs Dogs String String
> >
> > which expands to
> >
> > (String -> IO String) -> Dogs -> IO Dogs
> >
> > (actually this is a lie, it is really just a Traversal and not a Lens,
> > but it's the same idea). So if we apply it to 'prefixName :: String
> > -> IO String' we get a
> > function of type Dogs -> IO Dogs! Nifty!
> >
> > Now, to answer your specific questions:
> >
> > > a) I'm not sure why the explicit forall is needed here (isn't this
> > > equivalent to just Functor f => ...)?
> >
> > Yes, it is equivalent; the explicit forall is not strictly necessary.
> > The forall is there to emphasize that the 'f' does not show up on the
> > left-hand side: something of type Lens s t a b is a function which
> > works for *any* specific Functor f that a user might choose. (E.g., we
> > specifically chose IO in the example above). So e.g. a lens cannot do
> > IO operations, because then it would only work for IO and not for
> > other Functors. So a lens may only interact with the f via the fmap
> > function. (Similarly a Traversal must work with all Applicatives, and
> > so on.)
> >
> > > b) My understanding is that a lens packs both getter and setters, but I
> > > don't know which is supposed to be which here...
> >
> > You can think of lenses as generalizations of getters+setters, but
> > that is NOT how they are implemented! Nothing in the type (a -> f b)
> > -> s -> f t corresponds directly to getters or setters.
> >
> > For understanding more about what this type means and why it
> > corresponds to the idea of lenses, I highly recommend the video of
> > Edward's presentation to the NY Haskell user's group:
> > http://youtu.be/cefnmjtAolY?hd=1 .
> >
> > > c) Is there any kind of in-depth guide to Control.Lens somewhere? I have
> > > found some examples and tutorials but nothing that seemed to do more than
> > > scratch the surface.
> >
> > You can find pretty much everything there is on lens here:
> >
> > http://lens.github.io/
> >
> > -Brent
> >
> > _______________________________________________
> > Beginners mailing list
> > [email protected]
> > http://www.haskell.org/mailman/listinfo/beginners
> >
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