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Today's Topics:
1. Re: Real world example of Typeclasses > Interfaces (Ozgur Akgun)
2. Re: Real world example of Typeclasses > Interfaces (Alec Benzer)
3. Re: Real world example of Typeclasses > Interfaces (Brent Yorgey)
4. Re: Real world example of Typeclasses > Interfaces (Alec Benzer)
5. Re: Re: Motivation to Learn Haskell (A Smith)
----------------------------------------------------------------------
Message: 1
Date: Sat, 4 Sep 2010 18:26:25 +0100
From: Ozgur Akgun <ozgurak...@gmail.com>
Subject: Re: [Haskell-beginners] Real world example of Typeclasses >
Interfaces
To: Alec Benzer <alecben...@gmail.com>
Cc: beginners@haskell.org
Message-ID:
<aanlktimpyscxo1y038tgey0j2x-8+rgz+rko-czhe...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
On 4 September 2010 18:04, Alec Benzer <alecben...@gmail.com> wrote:
> What I mean is, if you had a function: something n =
> [1..n], and your type was a -> [a], a needs to be declared as (Num a,
> Enum a), even though this is sort of redundant, since you can't really
> have a number that isn't also enumerable.
>
Well, you can actually have a Num that isn't Enum.
ghci> :info Num
class (Eq a, Show a) => Num a where
(+) :: a -> a -> a
(*) :: a -> a -> a
(-) :: a -> a -> a
negate :: a -> a
abs :: a -> a
signum :: a -> a
fromInteger :: Integer -> a
--
Ozgur Akgun
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Message: 2
Date: Sat, 4 Sep 2010 13:41:44 -0400
From: Alec Benzer <alecben...@gmail.com>
Subject: Re: [Haskell-beginners] Real world example of Typeclasses >
Interfaces
To: Ozgur Akgun <ozgurak...@gmail.com>
Cc: beginners@haskell.org
Message-ID:
<aanlktik4f7-=4yob_tn=jxezn-0hriynrcroy4h9p...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
I was speaking more generally, not specifically about the Haskell
typeclasses Num and Enum (although actually irrational numbers aren't
enumerable now that I think of it, but I also guess that's a
relatively moot point since you can't really represent irrational
numbers in a programming language)
On Sat, Sep 4, 2010 at 1:26 PM, Ozgur Akgun <ozgurak...@gmail.com> wrote:
>
> On 4 September 2010 18:04, Alec Benzer <alecben...@gmail.com> wrote:
>>
>> What I mean is, if you had a function: something n =
>> [1..n], and your type was a -> [a], a needs to be declared as (Num a,
>> Enum a), even though this is sort of redundant, since you can't really
>> have a number that isn't also enumerable.
>
> Well, you can actually have a Num that isn't Enum.
>
> ghci> :info Num
> class (Eq a, Show a) => Num a where
> (+) :: a -> a -> a
> (*) :: a -> a -> a
> (-) :: a -> a -> a
> negate :: a -> a
> abs :: a -> a
> signum :: a -> a
> fromInteger :: Integer -> a
>
>
> --
> Ozgur Akgun
>
------------------------------
Message: 3
Date: Sat, 4 Sep 2010 13:54:02 -0400
From: Brent Yorgey <byor...@seas.upenn.edu>
Subject: Re: [Haskell-beginners] Real world example of Typeclasses >
Interfaces
To: Alec Benzer <alecben...@gmail.com>
Cc: beginners@haskell.org
Message-ID: <20100904175402.ga10...@seas.upenn.edu>
Content-Type: text/plain; charset=us-ascii
On Sat, Sep 04, 2010 at 01:04:43PM -0400, Alec Benzer wrote:
> On Sat, Sep 4, 2010 at 12:29 PM, Brent Yorgey <byor...@seas.upenn.edu> wrote:
>
> I know there are cases where this comes up in Haskell, but in certain
> cases the needs arises only because of the way Haskell handles
> typeclasses. What I mean is, if you had a function: something n =
> [1..n], and your type was a -> [a], a needs to be declared as (Num a,
> Enum a), even though this is sort of redundant, since you can't really
> have a number that isn't also enumerable. I'm talking specifically
> about cases where you'd have (TypeClass1 a, TypeClass2 a), and
> TypeClass1 doesn't imply TypeClass2 or visa versa (Brandon's example
> might have served that purpose, though I'm very new to Haskell and
> haven't gotten a full grasp on monads yet).
No, I really do mean that this comes up all the time in a nontrivial
way. Here's an example I just pulled randomly from a library of mine:
-- | Create a scale transformation.
scaling :: (HasLinearMap v, HasLinearMap (Scalar v),
Scalar (Scalar v) ~ Scalar v, Fractional (Scalar v))
=> Scalar v -> Transformation v
scaling s = fromLinear $ (s *^) <-> (^/ s)
None of those class constraints implies any of the others (not even in
theory), and they are all necessary.
> Ya but isn't that a pretty big hassle, especially assuming there are
> many useful cases where you require a type to belong to multiple
> typeclasses?
Sure, it's a hassle, but you were looking for examples of why type
classes are better than interfaces, and I'm not sure that simple
matters of convenience really count.
> Read through some of that (will probably read through more later), but
> as I did it also occurred to me that I guess the typeclass system
> might simply be a necessity to work with the rest of Haskell's type
> system? Ie, the point of them isn't their benefits over interfaces,
> it's that it's just the way things need to work in order to
> effectively connect with algebraic types?
No, you don't need type classes to use algebraic data types; see
OCaml. Type classes were a major innovation at the time they were
added to Haskell, but algebraic data types had been around for a
while.
-Brent
------------------------------
Message: 4
Date: Sat, 4 Sep 2010 14:42:50 -0400
From: Alec Benzer <alecben...@gmail.com>
Subject: Re: [Haskell-beginners] Real world example of Typeclasses >
Interfaces
To: Brent Yorgey <byor...@seas.upenn.edu>
Cc: beginners@haskell.org
Message-ID:
<aanlktik4pey0z9mbxzclvdz8awxnqsslgbnzcbjpz...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
On Sat, Sep 4, 2010 at 1:54 PM, Brent Yorgey <byor...@seas.upenn.edu> wrote:
> On Sat, Sep 04, 2010 at 01:04:43PM -0400, Alec Benzer wrote:
>> On Sat, Sep 4, 2010 at 12:29 PM, Brent Yorgey <byor...@seas.upenn.edu> wrote:
>>
>> I know there are cases where this comes up in Haskell, but in certain
>> cases the needs arises only because of the way Haskell handles
>> typeclasses. What I mean is, if you had a function: something n =
>> [1..n], and your type was a -> [a], a needs to be declared as (Num a,
>> Enum a), even though this is sort of redundant, since you can't really
>> have a number that isn't also enumerable. I'm talking specifically
>> about cases where you'd have (TypeClass1 a, TypeClass2 a), and
>> TypeClass1 doesn't imply TypeClass2 or visa versa (Brandon's example
>> might have served that purpose, though I'm very new to Haskell and
>> haven't gotten a full grasp on monads yet).
>
> No, I really do mean that this comes up all the time in a nontrivial
> way. Here's an example I just pulled randomly from a library of mine:
>
> -- | Create a scale transformation.
> scaling :: (HasLinearMap v, HasLinearMap (Scalar v),
> Scalar (Scalar v) ~ Scalar v, Fractional (Scalar v))
> => Scalar v -> Transformation v
> scaling s = fromLinear $ (s *^) <-> (^/ s)
>
> None of those class constraints implies any of the others (not even in
> theory), and they are all necessary.
>
>> Ya but isn't that a pretty big hassle, especially assuming there are
>> many useful cases where you require a type to belong to multiple
>> typeclasses?
>
> Sure, it's a hassle, but you were looking for examples of why type
> classes are better than interfaces, and I'm not sure that simple
> matters of convenience really count.
For what I meant I'd classify that sort of convenience as a benefit. I
mostly was curious about Learn You a Haskell for Great Good's remark
that "You can think of [typeclasses] kind of as Java interfaces, only
better." Not having to create a new interface every time you want to
check for a different collection of typeclasses would qualify as
"better" for me.
>> Read through some of that (will probably read through more later), but
>> as I did it also occurred to me that I guess the typeclass system
>> might simply be a necessity to work with the rest of Haskell's type
>> system? Ie, the point of them isn't their benefits over interfaces,
>> it's that it's just the way things need to work in order to
>> effectively connect with algebraic types?
>
> No, you don't need type classes to use algebraic data types; see
> OCaml. Type classes were a major innovation at the time they were
> added to Haskell, but algebraic data types had been around for a
> while.
>
> -Brent
>
------------------------------
Message: 5
Date: Sat, 4 Sep 2010 22:42:06 +0100
From: A Smith <asmith9...@gmail.com>
Subject: Re: [Haskell-beginners] Re: Motivation to Learn Haskell
To: "Benjamin L. Russell" <dekudekup...@yahoo.com>
Cc: beginners@haskell.org
Message-ID:
<aanlkti=y989un58hgefm2zd9n=zeaadr4ukdeqmat...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
My tuppence worth is the wonderful conciseness of programs, the way
abstraction works, it almost forces you into creating simple very testable
functions. These functions will have a well defined uncomplicated
transformation, and as such may be re-used to build more complex functions.
As a beginner, the templtation is to resist the temptation to implemrnt
imperative code, but to link functions together in my thinking like a shell
script. With a shell script programs are stated left to right with the
output of the leftmost being fed into the next one to the right. I think
with Haskell on a similar pattern with a pipeline of functions feed from the
right-most one. The next thing is to learn associativity rules and the use
of parenthesis, and $ operators.
Read everything on www.haskell.org.
Confuse yourself by trying to read Prelude source code!
--
Andrew Edinburgh,Scotland
On 4 September 2010 01:30, Benjamin L. Russell <dekudekup...@yahoo.com>wrote:
> Tim Perry <perry2...@yahoo.com> writes:
>
> > If you do jump in, I'd recommend the Real World Haskell book or the The
> Haskell
> > School of Expression book.
>
> Another interesting title is _Programming in Haskell,_ by Graham Hutton
> (see
> http://www.cs.nott.ac.uk/~gmh/book.html<http://www.cs.nott.ac.uk/%7Egmh/book.html>).
> Duncan Coutts has
> written a review on the title (see
> http://www.cs.nott.ac.uk/~gmh/book-review.pdf<http://www.cs.nott.ac.uk/%7Egmh/book-review.pdf>
> ).
>
> As for motivation for learning Haskell, one motivator is the purely
> functional nature of the language, which is referentially transparent
> and therefore facilitates reasoning about programs. Haskell has roots
> in category theory, and therefore, it is frequently possible to use
> category-theoretical reasoning to reason about the correctness of
> programs; this cannot be said of most other programming languages.
>
> -- Benjamin L. Russell
>
> >
> > Good luck,
> > Tim
> >
> >
> >
> >
> >
> > ----- Original Message ----
> > From: Lorenzo Isella <lorenzo.ise...@gmail.com>
> > To: beginners@haskell.org
> > Sent: Fri, September 3, 2010 3:57:26 PM
> > Subject: [Haskell-beginners] Motivation to Learn Haskell
> >
> > Dear All,
> > It is my first post to this list and please do not take it as an attempt
> to
> > start any flamewar.
> >>From time to time, I try to find the motivation to learn at least the
> > fundamentals of another programming language.
> > I normally use R and Python on a daily basis (but I am not that much into
> OO
> > programming) and have a good knowledge of Fortran and a rather
> superficial one
> > of C.
> > Beside learning a new language as a sort of mind expanding exercise, I
> try to
> > figure out how and if it can save me some time in my work and how it
> measures up
> > against other languages.
> > These days I tend to rely on R for data analysis and visualization
> whereas I use
> > Python (in particular Numpy+SciPy) for number crunching (it is very
> convenient
> > to use scipy/numpy to solve ODE's, manipulate arrays and so on).
> > Now, I wonder what benefit I would gain from learning Haskell since I
> mainly
> > write codes for numerical simulations/data analysis.
> > I know Haskell is gaining momentum e.g. in the financial environment (I
> happened
> > to see Haskell knowledge as a specification in some quant jobs) hence it
> must be
> > more than suitable for numerical work and, by the little I have
> understood so
> > far, it allows one to write code really resembling mathematical
> expressions (I
> > was impressed by guards and curried functions).
> > However, it also looks to me (correct me if I am mistaken) that Haskell
> is a far
> > cry from the wealth of standard and contributed scientific modules you
> have in
> > Python or R and thanks to which you do not re-implement the wheel
> yourself.
> > Any thoughts/suggestions are really appreciated.
> > Cheers
> >
> > Lorenzo
> > _______________________________________________
> > Beginners mailing list
> > Beginners@haskell.org
> > http://www.haskell.org/mailman/listinfo/beginners
> >
>
> --
> Benjamin L. Russell / DekuDekuplex at Yahoo dot com
> http://dekudekuplex.wordpress.com/
> Translator/Interpreter / Mobile: +011 81 80-3603-6725
> "Furuike ya, kawazu tobikomu mizu no oto." -- Matsuo Basho^
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
>
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