Re: [Haskell-cafe] Confused by instances
On Apr 28, 2008, at 16:22 , Fraser Wilson wrote: instance (Num a) = ValueClass a where fromValue (Value n) = fromInteger n What I'm really confused by is the response to instance (Num a) = ValueClass a -- what I am trying to say is if a is an instance of Num, then can be an instance of ValueClass too, and here's how. The format is instance [context =] classname instance. Your classname is ValueClass. Your instance is a. a is not of the form (T a1 ... an). (How to fix it? Not sure, and am trying to get myself out of here and on the road :) -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED] system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED] electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Confused by instances
On Mon, Apr 28, 2008 at 10:50 PM, Brandon S. Allbery KF8NH [EMAIL PROTECTED] wrote: The format is instance [context =] classname instance. Your classname is ValueClass. Your instance is a. a is not of the form (T a1 ... an). But neither is instance (Show a) = Show [a] ... *sound of penny dropping * Ah. Wood, trees, all that. Thanks! cheers, Fraser. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Confused by instances
2008/4/28 Fraser Wilson [EMAIL PROTECTED]:
On Mon, Apr 28, 2008 at 10:50 PM, Brandon S. Allbery KF8NH
[EMAIL PROTECTED] wrote:
The format is instance [context =] classname instance.
Your classname is ValueClass.
Your instance is a. a is not of the form (T a1 ... an).
But neither is
instance (Show a) = Show [a] ...
Yes it is, it's just a weird looking T, (namely []). This works just as well:
instance (Show a) = Show ([] a) ...
(Unless that's not H98, but I think it is)
Instances have to have concrete constructors at their heads is for
technical reasons, namely you can't in general do type inference with
unrestricted instances. It is a pattern-matching algorithm as you'd
expect, but it goes *backwards*, from right to left; i.e it sees the
pattern [a] and generates a new constraint Show a. This is contrary to
the intuition that it has a big set of instances, and when it sees,
say, Show Int it adds Show [Int]. The direction of the arrow can be
misleading :-)
To answer your other question, no, there is no list show hack. What
is being complained about is that you're using a type synonym as an
instance. If you just expand the synonym everything works fine. This
is not specific to lists, nor show. You're just not allowed to use
synonyms in instances (and as the compiler suggests, this restriction
can be lifted with {-# LANGUAGE FlexibleInstances #-}).
Luke
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Re: [Haskell-cafe] Confused by instances
2008/4/28 Fraser Wilson [EMAIL PROTECTED]:
what I am trying to say is if a is an instance of Num, then
can be an instance of ValueClass too, and here's how.
Oh, didn't answer this one. This is almost canned response, questions
like this get asked weekly on this list.
Short answer: you can't.
Longer answer: you can, but you have to wrap it in a newtype, which is
irritating.
newtype NumValue a = NumV a
instance (Num a) = ValueClass (NumV a) where
fromValue (Value n) = NumV (fromInteger n)
Essentially you have to tell the compiler when you use this
instance. So you still get all the power, but with less convenience
(than the impossible thing you want).
Abridged longest answer: you can, and you don't need a newtype, but
only if you're The Devil. Here's an explanation, but *please do not
do this*! It's unpredictable, poor style, a bad habit, nonmodular,
etc. etc. etc.
First enable undecidable instances:
{-# LANGUAGE UndecidableInstances #-}
With this pragma you are forfeiting your right to a terminating
compiler. The compiler may instance stack overflow or run forever
for no discernible reason.
Now you are allowed exactly one instance of the form you desire:
instance (Num a) = ValueClass a where
fromValue (Value n) = fromInteger n
If you're lucky, you might be able to define some well-formed
instances in addition and have everything behave. It *will* break if
you add another such instance, for example:
instance (Read a) = ValueClass a where ...
Because when the compiler sees fromValue, it will try to match it
against the head of an instance. Both the Num and the Read forms
match every type, so it will *pick one arbitrarily*, without
backtracking. So if you wanted the Read one and it picked the Num
one, you are permanently out of luck and you basically have to scrap
everything.
So, yeah, there's a little excursion into the dirty corners of the
typeclass system. If you don't want to get spontaneously eaten by a
bear, use a newtype as above :-).
And now it's time to go make/edit a wiki page on the subject.
Luke
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Re: [Haskell-cafe] Confused by instances
On Mon, Apr 28, 2008 at 11:33 PM, Luke Palmer [EMAIL PROTECTED] wrote: To answer your other question, no, there is no list show hack. Perhaps hack was a strong word. I'm not referring to type synonyms, but to the fact that Prelude's show class happens to have a special show function for lists, which happens to be handy when writing an instance for Show Char. I find the coupling here (between Show and a particular instance of Show, namely Show Char) to be disturbing -- Show should not (if you ask me) contain special machinery for specific instances. Not that I'll lose sleep over it, and of course the benefit (having pretty strings) far outweighs the philosophical cost. The showList function has always struck me as a bit of a wart though. cheers, Fraser. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Confused by instances
On Mon, Apr 28, 2008 at 3:47 PM, Fraser Wilson [EMAIL PROTECTED] wrote: On Mon, Apr 28, 2008 at 11:33 PM, Luke Palmer [EMAIL PROTECTED] wrote: To answer your other question, no, there is no list show hack. Perhaps hack was a strong word. I'm not referring to type synonyms, but to the fact that Prelude's show class happens to have a special show function for lists, which happens to be handy when writing an instance for Show Char. Oh, showList, that hack! Right, I had forgotten about that :-) Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Confused by instances
On Mon, Apr 28, 2008 at 11:46 PM, Luke Palmer [EMAIL PROTECTED] wrote: 2008/4/28 Fraser Wilson [EMAIL PROTECTED]: what I am trying to say is if a is an instance of Num, then can be an instance of ValueClass too, and here's how. Oh, didn't answer this one. This is almost canned response, questions like this get asked weekly on this list. Thanks for the detailed response. In the end, I've just selected the types I'm really interested in, and added instances for those. It works a treat. cheers, Fraser. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Confused by instances
On Apr 28, 2008, at 17:47 , Fraser Wilson wrote: Perhaps hack was a strong word. I'm not referring to type synonyms, but to the fact that Prelude's show class happens to have a special show function for lists, which happens to be handy when writing an instance for Show Char. I find the coupling here (between Show and a particular instance of Show, namely Show Char) to be disturbing -- Show should not (if you ask me) contain special machinery for specific instances. Not that I'll lose sleep over it, and of course the benefit (having pretty strings) far outweighs the philosophical cost. This is what comes of overloading lists as strings. (And what goes along with it, namely slowness. See Data.ByteString for a solution that solves both problems but sadly is unlikely to replace String) -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED] system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED] electrical and computer engineering, carnegie mellon universityKF8NH ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
