[Haskell-beginners] Equivalent of inheritance in Haskell

[Richard Mittel]

I'm afraid that apologies and humor did not make you seem less "crude," as
you put it, and your language is too apt to be misunderstood by homosexuals
and women as unwelcoming.  So please exercise more care when posting.

On Mon, Dec 20, 2010 at 8:18 AM, Paul Sargent <psarge at gmail.com> wrote:

>
> On 14 Dec 2010, at 03:10, C K Kashyap wrote:
>
> > If I understand you right .. you'd build a 'Man' type and 'Woman' type
> > by using a 'Person' type. Lets say, there is a function called getName
> > that is Person -> String
>
> In this case you can just make a new data type:
>
>    data Person = Man {name :: String, age :: Int} | Woman {name :: String}
>
>    marry :: Person -> Person -> String
>    marry (Man m _) (Woman w)     = m ++ ", " ++ w ++ ", I know pronounce
> you Man and Wife"
>    marry a@(Woman _) b@(Man _ _) = (name b) ++ ", " ++ (name a) ++ ", I
> know pronounce you Man and Wife"
>    marry _       _               = error "Not in these parts!"
>
> The clumsy second line is just to show the function <name :: Person ->
> String> which accesses the name of both men and women.
>
> The function "age" also takes a Person as it's argument, but in this case,
> if you ask a Woman her age things won't end well.
>
> (..and forgive the example. I was finding it hard to come up with something
> that would be different for men and women without getting crude.)
>
> This isn't the same as the classes you're describing in OO land though.
> Here, Man and Woman are data constructor functions which both return the
> type Person. There's no inheritance here, so it's *not* possible to have a
> Mammal data type of which Person's are a member, and so by extension Man &
> Woman.
>
> Man and Woman are *not* data types themselves, so I can't write a function
> <giveBirth :: Woman -> Person>. I'd have to write a <giveBirth :: Person ->
> Person> and fail if it was passed a Man.
>
> I personally find this type of construct very useful, but you just have to
> be careful to make sure you check to make sure an item can support the
> functions you want before calling them (e.g. 'age' above). It's not caught
> by the type checker at compile time, so it'll only be caught at run time and
> it causes an exception.
>
>
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