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Today's Topics:
1. Re: Can fields in a record be optional? (Yitzchak Gale)
2. Re: cabal install reactive-banana-wx (Heinrich Apfelmus)
3. Re: Can fields in a record be optional? (David Virebayre)
4. Re: Can fields in a record be optional? (Yitzchak Gale)
5. Re: Can fields in a record be optional? (Yitzchak Gale)
6. Impossible lib definition? (Obscaenvs 74)
7. about parser question < Programming in haskell _ chapter 8 >
( anyzhen )
----------------------------------------------------------------------
Message: 1
Date: Mon, 18 Jul 2011 09:57:34 +0300
From: Yitzchak Gale <g...@sefer.org>
Subject: Re: [Haskell-beginners] Can fields in a record be optional?
To: David Virebayre <dav.vire+hask...@gmail.com>
Cc: "beginners@haskell.org" <beginners@haskell.org>
Message-ID:
<CAOrUaLa-_GvK58Jw8HjRt6izkg7y400emnZO5bpSf7tcHoD=e...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Roger L. Costello wrote:
>> data Contract =
>> Contract {
>> currency :: Currency
>> , payments :: Double
>> , contracts :: [Contract]
>> }
>> deriving (Show)
David Virebayre wrote:
> I'm not sure I would model your datatype this way, I don't like the
> idea to put unnecessary undefined values in the case of subcontracts.
>
> data Contract = Contract ?{ currency :: Currency, payments :: Double }
> ? ? ? ? ? ? ?| SubContract { contracts :: [Contract] }
I think Roger's original design is just fine.
There is nothing "undefined" about the empty list. Roger is
saying that every contract has a list of subcontracts. Lists can
be empty, and that is legitimate.
In effect, Roger has defined his collection of contracts to be
a rose tree, and this is the classic way to do that in Haskell.
Another option would be to be more explicit about that and
use Data.Tree from the containers[1] package, which is
included in the Haskell Platform.
Tree algorithms are often quite elegant.
We haven't seen the rest of Roger's program, but I'll
bet it looks really nice with his original definition.
Regards,
Yitz
------------------------------
Message: 2
Date: Mon, 18 Jul 2011 09:08:07 +0200
From: Heinrich Apfelmus <apfel...@quantentunnel.de>
Subject: Re: [Haskell-beginners] cabal install reactive-banana-wx
To: beginners@haskell.org
Message-ID: <j00m4o$ui8$1...@dough.gmane.org>
Content-Type: text/plain; charset=UTF-8; format=flowed
Stephen Tetley wrote:
> Try installing either installing wxDirect 0.12.1.4; or wxDirect
> 012.1.3 with the build option "splitBase" first.
>
> There's a difference in the cabal files between 0.12.1.3 and 0.12.1.4
> where the file defaults to container Base 3.0 and Containers 0.3
> (wxDirect 0.12.1.3) or Base 4.0 and Containers 0.4 (wxDirect
> 0.12.1.4).
>
> Latest GHC versions have Base 4.0 and Containers 0.4, wxDirect
> 0.12.1.4 should work by default for latest GHC versions. wxDirect
> 0.12.1.3 needs building with the "splitBase" flag passed to work with
> latest GHCs.
Use
cabal update
to fetch the latest package definitions.
Since wxdirect only contains an executable, I think you can also do
cabal install wxdirect-0.12.1.4
before installing reactive-banana-wx .
Best regards,
Heinrich Apfelmus
--
http://apfelmus.nfshost.com
------------------------------
Message: 3
Date: Mon, 18 Jul 2011 09:15:45 +0200
From: David Virebayre <dav.vire+hask...@gmail.com>
Subject: Re: [Haskell-beginners] Can fields in a record be optional?
To: g...@sefer.org
Cc: "beginners@haskell.org" <beginners@haskell.org>
Message-ID:
<cam_wfvvakqxgnbu4yvt03euthhdkg0xabjsc9jf4g4xg1yx...@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
2011/7/18 Yitzchak Gale <g...@sefer.org>:
> Roger L. Costello wrote:
>>> data Contract =
>>> ? Contract {
>>> ? ? ? ? ? ? ?currency ?:: Currency
>>> ? ? ? ? ? ?, payments ?:: Double
>>> ? ? ? ? ? ?, contracts :: [Contract]
>>> ? ? ? ? ? ?}
>>> ?deriving (Show)
>
> David Virebayre wrote:
>> I'm not sure I would model your datatype this way, I don't like the
>> idea to put unnecessary undefined values in the case of subcontracts.
>>
>> data Contract = Contract ?{ currency :: Currency, payments :: Double }
>> ? ? ? ? ? ? ?| SubContract { contracts :: [Contract] }
>
> I think Roger's original design is just fine.
>
> There is nothing "undefined" about the empty list. Roger is
> saying that every contract has a list of subcontracts. Lists can
> be empty, and that is legitimate.
I'm not sure you read his post entirely.
Here's his and function :
> (and) c1 c2 = Contract { currency = undefined, payments = undefined,
> contracts = [c1, c2] }
Notice how he's setting currency and payments to undefined, since he
needs only the contracts field for subcontracts.
David.
------------------------------
Message: 4
Date: Mon, 18 Jul 2011 10:25:42 +0300
From: Yitzchak Gale <g...@sefer.org>
Subject: Re: [Haskell-beginners] Can fields in a record be optional?
To: David Virebayre <dav.vire+hask...@gmail.com>
Cc: "beginners@haskell.org" <beginners@haskell.org>
Message-ID:
<CAOrUaLYbBRfDoNap4EyqPOfR2eC8H6MW=niLDXVvbAV=xxs...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
David Virebayre wrote:
> I'm not sure you read his post entirely.
>
> Here's his and function :
>> (and) c1 c2 ?= ?Contract { currency = undefined, payments = undefined,
>> contracts = [c1, c2] }
>
> Notice how he's setting currency and payments to undefined, since he
> needs only the contracts field for subcontracts.
Ah, right, I didn't notice that. Not a good idea. Your design
is better then.
Another possibility would be Maybe Currency and Maybe Double.
That often comes out simpler - but not always. If the case
with Currency and Payments is a totally separate case than
when there are subcontracts, with different logic, then David's
design is best. If the cases are usually handled pretty much
in the same way with special handling for when one or more
fields is missing, then Maybe is best.
-Yitz
------------------------------
Message: 5
Date: Mon, 18 Jul 2011 10:54:28 +0300
From: Yitzchak Gale <g...@sefer.org>
Subject: Re: [Haskell-beginners] Can fields in a record be optional?
To: Michael Snoyman <mich...@snoyman.com>
Cc: "beginners@haskell.org" <beginners@haskell.org>
Message-ID:
<CAOrUaLZLjragLEmUokup6KLpogoBFJeuPWxaN7TbbEQFW=3...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
I wrote:
>> You don't need the added complexity of a type class for
>> this - just define the default value.
Michael Snoyman wrote:
> I don't think it's a case of necessity. It just happens to be very
> convenient to be able to use the three letters "def" instead of
> "defaultFrobnicatorSettings" or something like that.
You can use whatever name you'd like for a default value,
short or descriptive.
Type classes are a beautiful and powerful technique
in Haskell, but they do add some underlying complexity
to your types. In my experience, when building and
maintaining large code bases in Haskell, things are
much easier when you limit the use of type classes
to when they are really needed.
I create type classes when I need non-trivial polymorphism.
Even then, I often prefer simpler techniques such
as data types with parameters or passing functions as
parameters. There are also advanced techniques
using type classes which are needed for building some
kinds of libraries.
In particular, I have come to the conclusion that type classes
are not a good tool for simple namespace control in a typical
Haskell program.
It is a common mistake for beginners who come to Haskell
with a background in OOP to massively overuse type
classes.
Style is a matter of taste, of course. But much of the power
and beauty of Haskell lies away from type classes, and it's
a shame to miss out on it. So I think it is important for beginners,
especially when coming from OOP, to start out by avoiding
defining type classes whenever there is a good alternative
(i.e. almost always).
Regards,
Yitz
------------------------------
Message: 6
Date: Mon, 18 Jul 2011 10:27:39 +0200
From: Obscaenvs 74 <obscae...@gmail.com>
Subject: [Haskell-beginners] Impossible lib definition?
To: beginners@haskell.org
Message-ID:
<ca+jttenmcmwudjxmoqavmvgmcmttarxpdezcjrihdy-1t5x...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
>From the source of the package Text.ParserCombinators.ReadP, available
at
http://hackage.haskell.org/packages/archive/base/latest/doc/html/src/Text-ParserCombinators-ReadP.html#many
:
many :: ReadP a -> ReadP [a]
-- ^ Parses zero or more occurrences of the given parser.
many p = return [] +++ many1 p
many1 :: ReadP a -> ReadP [a]
-- ^ Parses one or more occurrences of the given parser.
many1 p = liftM2 (:) p (many p)
So `many` is defined in terms of `many1` and vice versa - completely
understandable, but here the argument (`p`) does not change. This is
above me right now. Anyone care to explain? I'd be grateful. Not that
I have to understand it, but it keeps pestering me during my lonely
hours.
/Fredrik
------------------------------
Message: 7
Date: Mon, 18 Jul 2011 17:20:28 +0800
From: " anyzhen " <jiangzhe...@qq.com>
Subject: [Haskell-beginners] about parser question < Programming in
haskell _ chapter 8 >
To: " Beginners " <Beginners@haskell.org>
Message-ID: <tencent_531cb7c84b1631106b874...@qq.com>
Content-Type: text/plain; charset="iso-8859-1"
well,i am reading the book :<Programming in haskell >
chapter 8 functional parsers
this chapter try teaching an parser technology,but i have some problem about do
keyword
--some brief info of types :
type Parser a = String -> [(a,String)]
item :: Parser Char --item "abc" = [('a',"bc")]
return' :: a -> Parser a --return' 'a' "foo" = [('a',"foo")]
--below code complier notice... sorry format
p2 :: Parser (Char,Char)
p2 = do x <- item
item
y <- item
return' (x,y)
"No instance for (Monad ((-)) String))
arising from a execute statement
Possible fix : add an instance declaration for (Monad ((-)) String))
In a stmt of a 'do' expression : y <- item
In the expression:
do { x <- item;
item;
y <- item;
return' (x,y)}
In an equation for 'p2':
p2
= do { x <- item;
item;
y <- item;
....}"
thanks for any help
jiangzhen <mail:jiangzhe...@qq.com>
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