How would you write this in Haskell?

struct Point {int x; int y;}
distance = sqrt(p.x*p.x + p.y*p.y);

Oh, right, Andrew doesn't know any C-like languages. Hmmm...

A "point" structure with an X and a Y element, and a distance function 
that takes a point and calculates its distance from the origin.

Then, is it possible to add a Z element to the "Point" type without 
having to rewrite the "distance" part? (Sure, it'll give the wrong 
answer, but that's the point I'm trying to make with someone elsewhere 
claiming strong typing avoids this sort of error.)

-- 
   Darren New / San Diego, CA, USA (PST)
     On what day did God create the body thetans?

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Darren New wrote:
> How would you write this in Haskell?
> 
> struct Point {int x; int y;}

   data Point = Point {x, y :: Int}

Caution: Note that the names "x" and "y" must be unique within the 
entire module. For this reason, you'd probably use longer names.

For something like this, a "typical" Haskell approach would "probably" 
use unnamed fields:

   data Point = Point Int Int

In either case, the following are all valid:

   let p = Point 3 6 ...
   my_function (Point x y) = ...

If the fields are named, you can also write

   let p = Point {x=3, y=6} ...
   let p = Point {y=6, x=3} ...
   my_function p = ... x p ...

The expression "x p" is like the Pascal-style "p.x". (That is, "x" is a 
function that takes a Point and yields its X coordinate.)

> distance = sqrt(p.x*p.x + p.y*p.y);

   distance (Point x y) = sqrt (x*x + y*y)

> Then, is it possible to add a Z element to the "Point" type without 
> having to rewrite the "distance" part? (Sure, it'll give the wrong 
> answer, but that's the point I'm trying to make with someone elsewhere 
> claiming strong typing avoids this sort of error.)

If you change the function to

   distance p = sqrt (x p * x p + y p * y p)

then it will work with either of

   data Point = Point {x, y :: Int}
   data Point = Point {x, y, z :: Int}

If you're asking about having two seperate Point types in the same 
program... that's another story.

Clear?

-- 
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http://www.zazzle.com/MathematicalOrchid*

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Wouldn't povray.off-topic.fan.haskell.bork.bork.bork be a more 
appropriate group? :)

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Nicolas Alvarez <nic### [at] gmailisthebestcom> wrote:
> Wouldn't povray.off-topic.fan.haskell.bork.bork.bork be a more
> appropriate group? :)

huhuhuhuhu...

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Orchid XP v7 <voi### [at] devnull> wrote:
>    data Point = Point {x, y :: Int}

> Caution: Note that the names "x" and "y" must be unique within the 
> entire module. For this reason, you'd probably use longer names.

  Doesn't Haskell support encapsulation?

-- 
                                                          - Warp

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Warp wrote:
> Orchid XP v7 <voi### [at] devnull> wrote:
>>    data Point = Point {x, y :: Int}
> 
>> Caution: Note that the names "x" and "y" must be unique within the 
>> entire module. For this reason, you'd probably use longer names.
> 
>   Doesn't Haskell support encapsulation?

Define "encapsulation".

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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Warp wrote:

>   Doesn't Haskell support encapsulation?

Last time I checked, Java has the same limitation.

In a single Java class, all methods must have unique names. (Ignoring 
for a moment the fact that Java supports overloading which Haskell 
doesn't.) Similarly, in a Haskell module all function names must be unique.

Either way, you can't use the same name for multiple things in a single 
source code file, in either language. And yet I don't see people 
claiming that Java doesn't "support encapsulation". So I'm not sure what 
you're getting at...

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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> Warp wrote:
> 
>>   Doesn't Haskell support encapsulation?
> 
> Last time I checked, Java has the same limitation.
> 
> In a single Java class, all methods must have unique names. (Ignoring 
> for a moment the fact that Java supports overloading which Haskell 
> doesn't.) Similarly, in a Haskell module all function names must be unique.
> 
> Either way, you can't use the same name for multiple things in a single 
> source code file, in either language. And yet I don't see people 
> claiming that Java doesn't "support encapsulation". So I'm not sure what 
> you're getting at...
> 

Without knowing a single thing about haskell, "module" *sounds* to me 
like a Java package, not a class...

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Nicolas Alvarez wrote:

> Without knowing a single thing about haskell, "module" *sounds* to me 
> like a Java package, not a class...

Each Java class has to go into a separate source code file. Each Haskell 
module has to go into a seperate source code file. Both languages 
require unique names within a single source file file. ;-)

(And a Java "package" is really just a bunch of classes that have the 
same prefix in their name, nothing more...)

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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Invisible <voi### [at] devnull> wrote:
> Warp wrote:

> >   Doesn't Haskell support encapsulation?

> Last time I checked, Java has the same limitation.

  You said:

> > struct Point {int x; int y;}
> 
>    data Point = Point {x, y :: Int}
> 
> Caution: Note that the names "x" and "y" must be unique within the
> entire module. For this reason, you'd probably use longer names.

  I understand to mean that the names 'x' and 'y' in your code above
actually garbage the global namespace.

  In the original C code they are local to 'Point' and do not affect
anything outside it.

-- 
                                                          - Warp

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