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Today's Topics:
1. Re: Compiler can't deduce Bool as instance of ToField
(Oliver Charles)
2. Re: Heterogeneous Lists (Giacomo Tesio)
3. Trouble with layout in wxHaskell (Nathan H?sken)
4. Re: print [] (Michael Peternell)
5. Re: How Haskell Fits Into an Operating System / API
Environment (Kelleher, Kevin)
6. Re: Trouble with layout in wxHaskell (Nathan H?sken)
----------------------------------------------------------------------
Message: 1
Date: Wed, 14 Aug 2013 13:16:49 +0100
From: Oliver Charles <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Compiler can't deduce Bool as
instance of ToField
Message-ID:
<CAGRp5RkKNk+8PfgESppM2cvM9E_7Zr8+MhXX=wobboldxke...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
You have declared that foo is *any* type that has a ToField instance,
allowing the caller of foo to determine the type at their will. However,
your implementation of foo is more specific and requires a is actually Bool
and nothing else.
On 14 Aug 2013 10:24, "Bryan Vicknair" <[email protected]> wrote:
> postgresql-simple declares Bool as an instance of the ToField class. The
> compiler can't deduce that given this simple code however:
>
> import Database.PostgreSQL.Simple.ToField (ToField(..))
>
> foo :: (ToField a) => a
> foo = True
>
>
> It fails with this error:
>
> Db.hs:64:7:
> Could not deduce (a ~ Bool)
> from the context (ToField a)
> bound by the type signature for foo :: ToField a => a
> at Db.hs:63:8-23
> `a' is a rigid type variable bound by
> the type signature for foo :: ToField a => a at Db.hs:63:8
> In the expression: True
> In an equation for `foo': foo = True
> Failed, modules loaded: none.
>
> What am I missing?
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
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Message: 2
Date: Wed, 14 Aug 2013 15:21:13 +0200
From: Giacomo Tesio <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>,
[email protected]
Subject: Re: [Haskell-beginners] Heterogeneous Lists
Message-ID:
<cahl7psex9yst6ueglay_9f8jyltxi1d8ud0ujpsug9u43ps...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
> > I recently ran into this with a GUI application, where I needed to
> > process a list of widgets that were members of the same typeclass, but
> > I had to wrap them all because they were different types.
>
> Your misconception is likely that you need to /store/ widgets, because
> you think in terms of memory. What you really need is /definitions/ of
> widgets, which can very well have different types. Then you can combine
> them using the usual monadic or applicative interface.
>
>
Ertugrul, can you please expand on this?
Giacomo
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Message: 3
Date: Wed, 14 Aug 2013 17:03:48 +0200
From: Nathan H?sken <[email protected]>
To: [email protected]
Subject: [Haskell-beginners] Trouble with layout in wxHaskell
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Hey,
I am trying to understand how do a layout in wxHaskell. I am trying to
make a frame with 2 text entries. One single lined at the top and one
multiline below it. Both should expand with the window size horizontal.
When the window is expanded vertical, only the multiline text entry
should expand.
Her is what I wrote:
main = start mainFrame
mainFrame = do
f <- frame [text := "BiVision"]
buffer <- textEntry f []
mainArea <- textCtrl f []
set f [layout := expand $ column 2 [hstretch $ widget buffer,
stretch $ widget mainArea]]
This gives me a window with the desired controls. But they are not
expanding.
Why?
I tried different combinations of expand and stretch (I do not really
understand the difference?). But nothing works.
I am guessing there is something simple I do not yet understand :).
Thanks for any help!
Nathan
------------------------------
Message: 4
Date: Wed, 14 Aug 2013 20:30:01 +0200
From: Michael Peternell <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] print []
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
There is a way. It's
print "[]"
if the compiler cannot infer the type of an empty list, it's because you wrote
a literal empty list [].
Sorry for giving such a practical (non-theoretical) answer ;)
Michael
------------------------------
Message: 5
Date: Wed, 14 Aug 2013 19:04:43 +0000
From: "Kelleher, Kevin" <[email protected]>
To: "The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell" <[email protected]>
Subject: Re: [Haskell-beginners] How Haskell Fits Into an Operating
System / API Environment
Message-ID:
<644E83C9A0C2C54A9F40341B5F63393716B8E184@CAMBOSEXC06.camelot.local>
Content-Type: text/plain; charset="us-ascii"
I have had the same frustration. It feels like you're learning a foreign
language
that doesn't have any vocabulary for walking directions, prices, or ordering
food.
So, while you could discuss philosophy, say, you wouldn't be able to ask for a
drink of water, or how to find the bathroom.
I do believe there is a payoff to studying Haskell, and I do keep plugging away,
rather inconstantly. It's difficult, because I don't really have the time now,
but hopefully I can devote some of an upcoming vacation to make what I
know of Haskell finally touch the ground.
Kevin
From: Beginners [mailto:[email protected]] On Behalf Of Stephen
Tetley
Sent: Sunday, August 11, 2013 11:45 AM
To: The Haskell-Beginners Mailing List - Discussion of primarily beginner-level
topics related to Haskell
Subject: Re: [Haskell-beginners] How Haskell Fits Into an Operating System /
API Environment
I tend to see OO subtyping and heterogeneous collections as more an impediment
to (pure) functional integration with the "rest-of-the-world" than state. There
are a lot of quality wrappers to stateful C libraries from Haskell, but few to
object oriented C++ or Objective C libraries.
On 11 August 2013 16:10, Philippe Sismondi
<[email protected]<mailto:[email protected]>> wrote:
As a result of this little thread I have come to another conclusion, and this
is just my subjective view. Most of the software that I am interested in seems
to live most comfortably with a stateful conception of the world. (The native
libraries I find most useful certainly are stateful.) I am reasonably competent
with monads and monad transformers in Haskell. But, to be honest, after three
years of pluggin away at Haskell, I am not the least convinced that the problem
of handling a changing external world in a pure functional language has been
successfully solved by those techniques. I always feel as though I am using the
robot arm on a space shuttle when a screwdriver would do. (Again, no need to
rebut this - I may be wrong or just to stupid to use Haskell effectively - so
be it.)
Perhaps in the end I do not really believe that functional programming is the
panacea that its devotees claim it to be.
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Message: 6
Date: Thu, 15 Aug 2013 13:56:25 +0200
From: Nathan H?sken <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] Trouble with layout in wxHaskell
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Ok, I need to use fill und hfill.
I personally think the documentation is little confusing, but I will
take this to the developers.
Regards,
Nathan
On 08/14/2013 05:03 PM, Nathan H?sken wrote:
> Hey,
>
> I am trying to understand how do a layout in wxHaskell. I am trying to
> make a frame with 2 text entries. One single lined at the top and one
> multiline below it. Both should expand with the window size
> horizontal. When the window is expanded vertical, only the multiline
> text entry should expand.
>
> Her is what I wrote:
>
> main = start mainFrame
>
> mainFrame = do
> f <- frame [text := "BiVision"]
> buffer <- textEntry f []
> mainArea <- textCtrl f []
> set f [layout := expand $ column 2 [hstretch $ widget buffer,
> stretch $ widget mainArea]]
>
> This gives me a window with the desired controls. But they are not
> expanding.
> Why?
> I tried different combinations of expand and stretch (I do not really
> understand the difference?). But nothing works.
>
> I am guessing there is something simple I do not yet understand :).
>
> Thanks for any help!
> Nathan
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
------------------------------
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