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Today's Topics:
1. Re: Functional analysis and design (Martin Drautzburg)
2. Confused by the qualified imports (Libor Wagner)
3. Re: Confused by the qualified imports (David McBride)
4. Re: Confused by the qualified imports (Brandon Allbery)
5. Natural functions versus existential types (Darren Grant)
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Message: 1
Date: Sun, 6 Jan 2013 12:17:44 +0100
From: Martin Drautzburg <martin.drautzb...@web.de>
Subject: Re: [Haskell-beginners] Functional analysis and design
To: beginners@haskell.org
Message-ID: <201301061217.45041.martin.drautzb...@web.de>
Content-Type: Text/Plain; charset="iso-8859-15"
On Sunday, 6. January 2013 08:13:14 Rustom Mody wrote:
> A favourite example of mine:
> Today J S Bach is regarded as the pinnacle of western classical music.
> When he was a young man he spent much of his time just copying out 'the
> masters'
Perfect analogy! You see, with music there is a set of rules which can be
learned, because they are written down. But mastering these does not make you
a good composer. When given the task "write a tooth-paste jingle" you would
still not know where to start. But when you talk to a jingle writer, he could
probably tell you his train of thoughts and you would be able to learn from
him.
Incidently I am interested in writing music too and I ran into the exact same
problem as with haskell. Try to find information about how to write 3-4voice
vocals, so it sounds like the Beach Boys, the BGs or Abba. Even though this
wisdom is known to someone, it is impossible to find something which resembles
a tutorial. Either this wisdom cannot be written down at all, or the masters
prefer to keep their wisdom concealed.
Copying the masters will certainly help, but it is a long and time-consuming
process. I was hoping for ways to shorten it.
--
Martin
------------------------------
Message: 2
Date: Sun, 6 Jan 2013 17:30:50 +0100
From: Libor Wagner <liborwag...@gmail.com>
Subject: [Haskell-beginners] Confused by the qualified imports
To: beginners@haskell.org
Message-ID: <88f0283390c24324a53acf918d0e3...@gmail.com>
Content-Type: text/plain; charset="utf-8"
Hi,
I was looking into Gloss examples for usage of Repa arrays. For example the
Snow example
(http://code.ouroborus.net/gloss/gloss-stable/gloss-examples/raster/Snow/Main.hs).
What I do not get is that there is qualified import of the Repa package, but
the "Z" and ":." are used without the prefix. When I some times accidentally
omit the prefix the ghc always yell at me. What em I missing?
Thanks,
Libor
------------------------------
Message: 3
Date: Sun, 6 Jan 2013 12:00:03 -0500
From: David McBride <toa...@gmail.com>
Subject: Re: [Haskell-beginners] Confused by the qualified imports
To: Libor Wagner <liborwag...@gmail.com>
Cc: beginners@haskell.org
Message-ID:
<CAN+Tr43q_S0Fi_=d3ncroq-mlt+1ddfta7xbnypcwmgp3cy...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
If you use "import qualified Data.Array.Repa as R" then you have to specify
R in front of every function, operator, or type.
But if you use "import Data.Array.Repa as R" without the qualified keyword,
then you only have to use it when it would be ambiguous which module your
function or type comes from (ie. you had multiple modules imported without
qualification).
On Sun, Jan 6, 2013 at 11:30 AM, Libor Wagner <liborwag...@gmail.com> wrote:
> Hi,
>
> I was looking into Gloss examples for usage of Repa arrays. For example
> the Snow example (
> http://code.ouroborus.net/gloss/gloss-stable/gloss-examples/raster/Snow/Main.hs).
> What I do not get is that there is qualified import of the Repa package,
> but the "Z" and ":." are used without the prefix. When I some times
> accidentally omit the prefix the ghc always yell at me. What em I missing?
>
> Thanks,
> Libor
>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
>
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Message: 4
Date: Sun, 6 Jan 2013 12:01:53 -0500
From: Brandon Allbery <allber...@gmail.com>
Subject: Re: [Haskell-beginners] Confused by the qualified imports
To: Libor Wagner <liborwag...@gmail.com>
Cc: beginners@haskell.org
Message-ID:
<CAKFCL4X=c3vbo+etay-vhqm7nzaudm51gza2mrj7p9vfe5f...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
On Sun, Jan 6, 2013 at 11:30 AM, Libor Wagner <liborwag...@gmail.com> wrote:
> I was looking into Gloss examples for usage of Repa arrays. For example
> the Snow example (
> http://code.ouroborus.net/gloss/gloss-stable/gloss-examples/raster/Snow/Main.hs).
> What I do not get is that there is qualified import of the Repa package,
> but the "Z" and ":." are used without the prefix. When I some times
> accidentally omit the prefix the ghc always yell at me. What em I missing?
>
It's not a fully qualified import (no "qualified" keyword), so names are
available both with and without the prefix. This means that you can use
names that don't collide with other local or imported bindings without the
prefix, but you need to use the prefix if you do need to disambiguate names
that colide.
For one example, Data.Array.Repa exports a binding "zipWith" --- but so
does the Prelude, so you can't use that name unqualified because the
compiler doesn't know which one you mean. (:.), on the other hand, is
specific to Repa (at least with the imports in your example) so does not
need qualification. (You could also get around this by explicitly
importing the Prelude with a "hiding (zipWith)" clause.)
--
brandon s allbery kf8nh sine nomine associates
allber...@gmail.com ballb...@sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net
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Message: 5
Date: Sun, 6 Jan 2013 23:22:51 -0800
From: Darren Grant <therealklu...@gmail.com>
Subject: [Haskell-beginners] Natural functions versus existential
types
To: Haskell Beginners <beginners@haskell.org>
Message-ID:
<ca+jd6sjux1pdctmeakbqvgdqlh97ojpjtaxkazjrt1-1_yx...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
I've seen some cases where Haskell examples dive into existential types
instead of using natural higher order functions. For instance, the
"Existential type" wiki page [1] section 2.2 proposes existentials as the
solution to a heterogeneous list problem where a ray-tracer must evaluate a
trace function for different types of objects. Is there a good reason for
this, or is it just a case of prior language assumptions leading to
unnatural code? Why could I not just make the list homogeneous by supplying
a list of partially evaluated trace functions instead?
Sometimes I read an argument that existentials are required due to unknown
constraints at compile time, but do not have an intuition for this
situation yet. For instance, I read that IO requires existentials. Still
working on that one. :)
Cheers,
Darren
[1] http://www.haskell.org/haskellwiki/Existential_type
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