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

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-ex... I have not found the "deriving" keyword's specification anywhere. On Thu, Jan 22, 2015 at 8:57 AM, Kim-Ee Yeoh 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 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

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