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Today's Topics:
1. Re: Good style on "." and "$" pipeline? (Chadda? Fouch?)
2. Re: The Holy Trinity of Functional Programming
(Christopher Howard)
3. Re: The Holy Trinity of Functional Programming
(Costello, Roger L.)
4. Re: The Holy Trinity of Functional Programming (Brandon Allbery)
5. Re: Good style on "." and "$" pipeline? (Brandon Allbery)
6. Re: The Holy Trinity of Functional Programming (Edward Z. Yang)
7. [Real World Haskell] Question (Yang)
8. Re: [Real World Haskell] Question (Lorenzo Bolla)
----------------------------------------------------------------------
Message: 1
Date: Wed, 22 Aug 2012 12:36:39 +0200
From: Chadda? Fouch? <[email protected]>
Subject: Re: [Haskell-beginners] Good style on "." and "$" pipeline?
To: Felipe Almeida Lessa <[email protected]>
Cc: [email protected]
Message-ID:
<canfjzrznpoyr2ompn0p21g8pvcop1fg138uioij7dvwp+7b...@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
On Wed, Aug 22, 2012 at 3:41 AM, Felipe Almeida Lessa
<[email protected]> wrote:
> On Tue, Aug 21, 2012 at 10:34 PM, koomi <[email protected]> wrote:
>> On 21.08.2012 22:43, Brent Yorgey wrote:
>>> Having more than one $, like (f1 $ f2 $ fn $ arg), is frowned upon.
>> Care to explain why this is considered bad? I don't see anything wrong
>> with this.
>
> It's just a matter of taste. Personally, I usually prefer using many
> ($)s rather than combining (.)s and ($)s on the same "phrase".
It is a bit more than a matter of taste : the (.)s then one ($) style
allows easier and clearer refactoring because each part of the
function pipeline can be just copy pasted somewhere else, for instance
you can get from :
> bigFunc = f1 . f2 . f3 . f4 $ arg
to
> bigFunc = f1 . meaningfulFunc . f4 $ arg
>
> meaningfulFunc = f2 . f3
without rewriting a bit of your pipeline to change $ to . or add a
point to meaningfulFunc (note that I ignore the monomorphism
restriction here, I would give those function signatures anyway since
they're top-level).
I find this style makes it easier to reason about in a purely
"pipelinesque" way (focused on functions and their combinations).
--
Jeda?
------------------------------
Message: 2
Date: Wed, 22 Aug 2012 03:19:49 -0800
From: Christopher Howard <[email protected]>
Subject: Re: [Haskell-beginners] The Holy Trinity of Functional
Programming
To: Haskell Beginners <[email protected]>
Message-ID: <[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
On 08/22/2012 01:27 AM, Costello, Roger L. wrote:
> ---------------------------------------------------------
> The Holy Trinity of Functional Programming
> ---------------------------------------------------------
> These three ideas constitute the holy trinity of functional programming:
>
> 1. User-defined recursive data types.
> 2. Recursively defined functions over recursive data types.
> 3. Proof by induction: show that some property P(n) holds for each element
> of a recursive data type.
>
<snip>
>
> Although the infinite Nat value is not of much use, the same is not true of
> the infinite values of other data types.
>
Please bear with us simpletons: But do you think you could provide a
more "real world" example of an application of the "Holy Trinity" ideas?
(One of those mysterious "other data types".)
Not to say that recursive data types and proof by induction aren't
fascinating in and of themselves... but sometimes it is difficult to
make the connection to the practical programming process.
--
frigidcode.com
indicium.us
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Message: 3
Date: Wed, 22 Aug 2012 11:38:01 +0000
From: "Costello, Roger L." <[email protected]>
Subject: Re: [Haskell-beginners] The Holy Trinity of Functional
Programming
To: "[email protected]" <[email protected]>
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="us-ascii"
Hello Christopher,
> But do you think you could provide a
> more "real world" example of an application
> of the "Holy Trinity" ideas?
A commonly cited real-world example that illustrates the first key idea
(recursive data type) is a binary tree:
data Tree = Node String Tree Tree | Leaf
The elements of this data type include:
Leaf
Node "Root" Leaf Leaf
Node "Root" (Node "Left" Leaf Leaf)
(Node "Right" Leaf Leaf)
And so forth.
Common functions on this data type include insertions, deletions, traversal.
I am not sure if there is a use for the infinite value of this data type.
/Roger
------------------------------
Message: 4
Date: Wed, 22 Aug 2012 11:04:48 -0400
From: Brandon Allbery <[email protected]>
Subject: Re: [Haskell-beginners] The Holy Trinity of Functional
Programming
To: "Costello, Roger L." <[email protected]>
Cc: "[email protected]" <[email protected]>
Message-ID:
<CAKFCL4UKWnyaJbo3hsyZDU8zP=qb3aih8muap+znaov05y8...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
On Wed, Aug 22, 2012 at 7:38 AM, Costello, Roger L. <[email protected]>wrote:
> > But do you think you could provide a
> > more "real world" example of an application
> > of the "Holy Trinity" ideas?
>
> A commonly cited real-world example that illustrates the first key idea
> (recursive data type) is a binary tree:
>
I'd have used lists, since in Haskell we make use of infinite lists quite a
bit; a simple example being zipping some list against [0..] (a list
comprising an infinitely ascending sequence of numbers) to pair each item
with its index.
--
brandon s allbery [email protected]
wandering unix systems administrator (available) (412) 475-9364 vm/sms
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Message: 5
Date: Wed, 22 Aug 2012 11:09:32 -0400
From: Brandon Allbery <[email protected]>
Subject: Re: [Haskell-beginners] Good style on "." and "$" pipeline?
To: koomi <[email protected]>
Cc: [email protected]
Message-ID:
<cakfcl4wxf+dvjxz2pmz0pazfwbdzr-gr8wnrnpgna9qeneb...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
On Tue, Aug 21, 2012 at 9:34 PM, koomi <[email protected]> wrote:
> On 21.08.2012 22:43, Brent Yorgey wrote:
> > Having more than one $, like (f1 $ f2 $ fn $ arg), is frowned upon.
> Care to explain why this is considered bad? I don't see anything wrong
> with this.
>
Experientially, we see it a lot in #xmonad from beginners combining stuff
together with a certain amont of cargo-culting (being beginners and usually
quite unfamiliar with Haskell).
I've been moving toward using ($) to separate logical "phrases" and (.)
within the phrases, to make it easier to see which things go with which.
Since layoutHook is rather agglutinative, this helps a lot.
--
brandon s allbery [email protected]
wandering unix systems administrator (available) (412) 475-9364 vm/sms
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Message: 6
Date: Wed, 22 Aug 2012 11:57:11 -0400
From: "Edward Z. Yang" <[email protected]>
Subject: Re: [Haskell-beginners] The Holy Trinity of Functional
Programming
To: Brandon Allbery <[email protected]>
Cc: "[email protected]" <[email protected]>
Message-ID: <1345650945-sup-7654@javelin>
Content-Type: text/plain; charset=UTF-8
Yes, but we can also use a similar style of inductive reasoning
in the face of infinite types, called Scott induction [1].
There is also a perspective that infinite types (or so called 'codata')
should be reasoned with 'coinduction', although the proof principles
there are a little more esoteric.
Cheers,
Edward
[1] http://blog.ezyang.com/2010/12/no-one-expects-the-scott-induction/
Excerpts from Brandon Allbery's message of Wed Aug 22 11:04:48 -0400 2012:
> On Wed, Aug 22, 2012 at 7:38 AM, Costello, Roger L. <[email protected]>wrote:
>
> > > But do you think you could provide a
> > > more "real world" example of an application
> > > of the "Holy Trinity" ideas?
> >
> > A commonly cited real-world example that illustrates the first key idea
> > (recursive data type) is a binary tree:
> >
>
> I'd have used lists, since in Haskell we make use of infinite lists quite a
> bit; a simple example being zipping some list against [0..] (a list
> comprising an infinitely ascending sequence of numbers) to pair each item
> with its index.
>
------------------------------
Message: 7
Date: Wed, 22 Aug 2012 18:15:57 +0200
From: Yang <[email protected]>
Subject: [Haskell-beginners] [Real World Haskell] Question
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Hi all,
In chapter 4 in Real World Haskell comes there the first example of
system I/O, namely `interactWith`. According to author, the right way is
to compile and run program in system shell, though I have loaded source
in GHCi and can't figure out how to invoke `main` interactively.
Therefore the question is simply how can I call `main` that takes
arguments in GHCi? Thanks.
BTW: This is my first message to the group and if there is any
restriction I violate in topics, I'd like to know. The example I
mentioned can be found at the beginning of the web page followed by link
http://book.realworldhaskell.org/read/functional-programming.html
--
Best Regards
Yang Zhao
------------------------------
Message: 8
Date: Wed, 22 Aug 2012 17:18:18 +0100
From: Lorenzo Bolla <[email protected]>
Subject: Re: [Haskell-beginners] [Real World Haskell] Question
Cc: [email protected]
Message-ID:
<cadjgtrz1locfexowxk3wh0ptbk2-35cqd2qgg7vmhwzikot...@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
Try with:
Prelude> :main
L.
On Wed, Aug 22, 2012 at 5:15 PM, Yang
<[email protected]> wrote:
> Hi all,
> In chapter 4 in Real World Haskell comes there the first example of
> system I/O, namely `interactWith`. According to author, the right way is to
> compile and run program in system shell, though I have loaded source in GHCi
> and can't figure out how to invoke `main` interactively.
>
> Therefore the question is simply how can I call `main` that takes
> arguments in GHCi? Thanks.
>
> BTW: This is my first message to the group and if there is any restriction I
> violate in topics, I'd like to know. The example I mentioned can be found at
> the beginning of the web page followed by link
> http://book.realworldhaskell.org/read/functional-programming.html
> --
> Best Regards
> Yang Zhao
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
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