[Haskell-beginners] Defining an instance: Syntax that works exactly sometimes

[Jeffrey Brown]

Thank you, everybody. I had thought "deriving" and "instance" were just
different words for the same thing. Your answers were deeper than I am
currently capable of understanding.

I found instance declarations specified in 7.6.3 of The Glorious Glasgow
Haskell Compilation System User's Guide, Version 7.0.2:
https://downloads.haskell.org/~ghc/7.0.2/docs/html/users_guide/type-class-extensions.html

I have not found the "deriving" keyword's specification anywhere.

On Thu, Jan 22, 2015 at 8:57 AM, Kim-Ee Yeoh <ky3 at atamo.com> wrote:

> Jeffrey,
>
> You didn't explain what you're trying to accomplish, and therefore folks
> can only address the symptoms.
>
> Here's what I'm seeing:
>
> Suppose you have:
>
>     data A = A | B | C | D | E
>
> You'd like a function that
> given A returns E,
> given B, returns D
> given C, returns er, C
> given D, returns B,
> given E, returns A.
>
> So you're hoping to write a Rev type class with singleton member
>     rev :: Rev a => a -> a
> that would do the trick.
>
> Thing is, you can already write a very generic
>
>     rev :: (Enum a, Bounded a) => a -> a
>
> that would do the job.
>
> Why is that sweet?
>
> Because Enum + Bounded are auto-derivable for huge swathes of data types.
> So you write the rev function (not type class instance!) once, and it'll
> work for lots of your structures.
>
> The best type class is often no type class. Especially when the compiler
> isn't smart enough to read your mind. But I'm sure it's flattered you
> thought so highly of it  ;)
>
>
> -- Kim-Ee
>
> On Thu, Jan 22, 2015 at 7:23 AM, Jeffrey Brown <jeffbrown.the at gmail.com>
> wrote:
>
>> Dear Haskellers,
>>
>> The following compiles. (Rev stands for Reversible, and Dirn for
>> Direction.)
>>
>>     class Rev a where
>>         rev :: a -> a
>>
>>     data Dirn = Succ | Pred
>>         deriving (Eq, Show, Ord)
>>
>>     -- implement Ord
>>     (<=) Succ Pred = False
>>     (<=) _ _ = True
>>
>>     -- implement Rev
>>     instance Rev Dirn where
>>         rev Succ = Pred
>>         rev Pred = Succ
>>
>> But if I try to define the Rev instance the same way the Ord instance is
>> being defined, it does not compile:
>>
>>     class Rev a where
>>         rev :: a -> a
>>
>>     data Dirn = Succ | Pred
>>         deriving (Eq, Show, Ord, Rev)
>>
>>     -- implement Ord, because Dirn is used as a key in a Map
>>     (<=) Succ Pred = False
>>     (<=) _ _ = True
>>
>>     -- implement Rev
>>     rev Succ = Pred
>>     rev Pred = Succ
>>
>> What's going on?
>>
>> Many thanks,
>> Jeff
>>
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://www.haskell.org/mailman/listinfo/beginners
>>
>>
>
> _______________________________________________
> Beginners mailing list
> Beginners at haskell.org
> http://www.haskell.org/mailman/listinfo/beginners
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/beginners/attachments/20150122/51ca811d/attachment.html>