Re: [Haskell-cafe] What's wrong with the classes/insances?
Pieter Laeremans wrote: HI, What 's wrong with this: [...] class Item a where getCatalog :: Catalog catalog => a -> catalog This is a shorthand for > class Item a where > getCatalog :: forall c. (Catalog c) => a -> c That is, the class provides the contract that given some value of type a it will be able to return *any* type c which adheres to the contract of the class Catalog. This function is polymorphic in the return type. instance Catalog c => Item (Content c) where getCatalog (Content _ _ c) = c The problem is, here you're returning a *specific* type c which adheres to Catalog. What happens if the caller of getCatalog is expecting some other type (Catalog c') => c' or (Catalog c'') => c'' etc? There are a few different solutions you could take. The easiest one is to use multi-parameter type classes and functional dependencies to define Item like so: > class Item a c | a -> c where > getCatalog :: (Catalog c) => a -> c This says that for any given type a there is one particular type c which getCatalog returns. Depending on your goals this may be enough, but if you really want getCatalog to be polymorphic in c then it won't work. (If you only want to be, er, multimorphic in c then you can leave out the fundep and define instances for each a*c type pair. This can lead to needing to declare excessively many type signatures however.) If you really want to be able to return any c then there are a couple of approaches you could take. First is to add a conversion function to the Catalog class: > class Catalog c where > ... > convertCatalog:: forall c'. (Catalog c') => c -> c' Given the rest of the definition for Catalog, this looks to be eminently doable-- at least in as far as people don't try to access any other fiddly bits inside the value c'. Of course this gives no way of preventing them from trying to do that fiddling, which leads to... The other common approach is to use existential types to wrap values up with their class dictionaries like so: > data CatalogWrapper = forall c. (Catalog c) => CatalogWrapper c In this case you'd have getCatalog return a CatalogWrapper instead of a (Catalog c) => c. If you're not familiar with existential types they can be harder to think about since they loose information about what type you actually have inside; once wrapped they're only ever accessible by the methods of type classes restricting what we can wrap. But if you want to restrict people to only ever using the Catalog interface to manipulate them, then this is exactly what you want. -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] What's wrong with the classes/insances?
Thanks all!
I have to admit, I have an OO mndset :-).
I think I have found a more "functional design" :
import Text.StringTemplate.Classes
import Text.StringTemplate
data (ToSElem a) => Item a b = Item {cargo::a , catalog :: (Catalog a b)}
instance (ToSElem a) => ToSElem (Item a b) where
toSElem = (toSElem . cargo)
type Id = String
type ItemToSelem a b = Item a b -> SElem b
type ItemParser a b = String -> Item a b
type AllItemReader a b = IO [Item a b]
type SingleItemReader a b = Id -> IO (Item a b)
data Catalog a b = Catalog {reader:: (AllItemReader a b),
singleReader ::( SingleItemReader a b)}
data Content = Content {authort :: String, text:: String}
type ContentItem = Item Content String
What I want to express is that there exists differnet kinds of catalogs which,
depending on how they are configured can read from a file system or a database.
And each catalog can contain a specific type of Item.
For each Item I have to be able to produce the toSELem representation
that subsequently can be used by HStringTemplate
I thik that means I could declare
On Sat, Jun 21, 2008 at 12:26 AM, Dan Doel <[EMAIL PROTECTED]> wrote:
> On Friday 20 June 2008, Pieter Laeremans wrote:
>> type Id = String
>>
>> class Catalog a where
>> listItems :: a -> IO [String]
>> getItem :: a -> Id -> IO (Maybe String)
>>
>> class Item a where
>> getCatalog :: Catalog catalog => a -> catalog
>>
>> data Catalog c => Content c = Content {auteur :: String, inhoud::
>> String, catalog::c}
>>
>> instance Catalog c => Item (Content c) where
>>getCatalog (Content _ _ c) = c
>>
>> I get this as error from ghci:
>>
>> Couldn't match expected type `catalog' against inferred type `c'
>> `catalog' is a rigid type variable bound by
>> the type signature for `getCatalog'
>> at
>> ../Sites/liberaleswebsite/www.liberales.be/cgi-bin/Test.hs:16:26
>> `c' is a rigid type variable bound by
>> the instance declaration
>> at
>> ../Sites/liberaleswebsite/www.liberales.be/cgi-bin/Test.hs:20:17 In the
>> expression: c
>> In the definition of `getCatalog': getCatalog (Content _ _ c) = c
>> In the definition for method `getCatalog'
>> Failed, modules loaded: none.
>
> The problem is in the type of getCatalog:
>
> (Item a, Catalog catalog) => a -> catalog
>
> That type says that given the a, you can produce a value of any type 'catalog'
> so long as that type is an instance of Catalog.
>
> What you probably meant it to say is that you can produce *some particular*
> type that belongs to catalog. There are a couple ways you could express this.
> For instance, using functional dependencies:
>
>class Catalog cat => HasCatalog a cat | a -> cat where
> getCatalog :: a -> cat
>
> or the new type families:
>
>class (Catalog (Cat a)) => Item a where
> type Cat a :: *
> getCatalog :: a -> Cat a
>
> Or you could wrap catalogues in an existential type:
>
>data SomeCatalog = forall c. Catalog c => Cat c
>
>class Item a where
> getCatalog :: a -> SomeCatalog
>
> However, as just a word of warning, I'd say that when you run into something
> like this, it's probably an indication that you're structuring your program
> from an object oriented mindset, and that may not be the best fit for
> programming in Haskell (of course, it's possible an existential type or some
> such is the appropriate way to do things).
>
> Cheers,
> -- Dan
>
--
Pieter Laeremans <[EMAIL PROTECTED]>
"The future is here. It's just not evenly distributed yet." W. Gibson
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Re: [Haskell-cafe] What's wrong with the classes/insances?
On Sat, 2008-06-21 at 00:04 +0200, Pieter Laeremans wrote:
> HI,
>
> What 's wrong with this:
It looks like you think Haskell is an OO language or want it to be. It
is not.
>
> type Id = String
>
> class Catalog a where
> listItems :: a -> IO [String]
> getItem :: a -> Id -> IO (Maybe String)
>
> class Item a where
> getCatalog :: Catalog catalog => a -> catalog
This means
class Item a where
getCatalog :: forall catalog. Catalog catalog => a -> catalog
That means, given some Item a, I can create a value of -any- type that
is an instance of Catalog. There is no well-defined function that could
do that.
>
> data Catalog c => Content c = Content {auteur :: String, inhoud::
> String, catalog::c}
>
> instance Catalog c => Item (Content c) where
>getCatalog (Content _ _ c) = c
>
> I get this as error from ghci:
>
> Couldn't match expected type `catalog' against inferred type `c'
> `catalog' is a rigid type variable bound by
> the type signature for `getCatalog'
> at
> ../Sites/liberaleswebsite/www.liberales.be/cgi-bin/Test.hs:16:26
> `c' is a rigid type variable bound by
> the instance declaration
> at
> ../Sites/liberaleswebsite/www.liberales.be/cgi-bin/Test.hs:20:17
> In the expression: c
> In the definition of `getCatalog': getCatalog (Content _ _ c) = c
> In the definition for method `getCatalog'
> Failed, modules loaded: none.
>
> thanks in advance,
>
> P
>
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Re: [Haskell-cafe] What's wrong with the classes/insances?
On Friday 20 June 2008, Pieter Laeremans wrote:
> type Id = String
>
> class Catalog a where
> listItems :: a -> IO [String]
> getItem :: a -> Id -> IO (Maybe String)
>
> class Item a where
> getCatalog :: Catalog catalog => a -> catalog
>
> data Catalog c => Content c = Content {auteur :: String, inhoud::
> String, catalog::c}
>
> instance Catalog c => Item (Content c) where
>getCatalog (Content _ _ c) = c
>
> I get this as error from ghci:
>
> Couldn't match expected type `catalog' against inferred type `c'
> `catalog' is a rigid type variable bound by
> the type signature for `getCatalog'
> at
> ../Sites/liberaleswebsite/www.liberales.be/cgi-bin/Test.hs:16:26
> `c' is a rigid type variable bound by
> the instance declaration
> at
> ../Sites/liberaleswebsite/www.liberales.be/cgi-bin/Test.hs:20:17 In the
> expression: c
> In the definition of `getCatalog': getCatalog (Content _ _ c) = c
> In the definition for method `getCatalog'
> Failed, modules loaded: none.
The problem is in the type of getCatalog:
(Item a, Catalog catalog) => a -> catalog
That type says that given the a, you can produce a value of any type 'catalog'
so long as that type is an instance of Catalog.
What you probably meant it to say is that you can produce *some particular*
type that belongs to catalog. There are a couple ways you could express this.
For instance, using functional dependencies:
class Catalog cat => HasCatalog a cat | a -> cat where
getCatalog :: a -> cat
or the new type families:
class (Catalog (Cat a)) => Item a where
type Cat a :: *
getCatalog :: a -> Cat a
Or you could wrap catalogues in an existential type:
data SomeCatalog = forall c. Catalog c => Cat c
class Item a where
getCatalog :: a -> SomeCatalog
However, as just a word of warning, I'd say that when you run into something
like this, it's probably an indication that you're structuring your program
from an object oriented mindset, and that may not be the best fit for
programming in Haskell (of course, it's possible an existential type or some
such is the appropriate way to do things).
Cheers,
-- Dan
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Re: [Haskell-cafe] What's wrong with the classes/insances?
I think the problem is here:
> getCatalog :: Catalog catalog => a -> catalog
This wants to constrain the result of getCatalog to be an instance of
Catalog, but this only works for function arguments, not results. The
following code does typecheck, though I have no idea what is does or if
it is what you want:
type Id = String
class Catalog a where
listItems :: a -> IO [String]
getItem :: a -> Id -> IO (Maybe String)
class Catalog q => Item q a where
getCatalog :: a -> q
data Content d = MkContent {auteur :: String,
inhoud :: String,
catalog :: d}
instance Catalog c => Item c (Content c) where
getCatalog (MkContent _ _ e) = e
Pieter Laeremans wrote:
HI,
What 's wrong with this:
type Id = String
class Catalog a where
listItems :: a -> IO [String]
getItem :: a -> Id -> IO (Maybe String)
class Item a where
getCatalog :: Catalog catalog => a -> catalog
data Catalog c => Content c = Content {auteur :: String, inhoud::
String, catalog::c}
instance Catalog c => Item (Content c) where
getCatalog (Content _ _ c) = c
I get this as error from ghci:
Couldn't match expected type `catalog' against inferred type `c'
`catalog' is a rigid type variable bound by
the type signature for `getCatalog'
at
../Sites/liberaleswebsite/www.liberales.be/cgi-bin/Test.hs:16:26
`c' is a rigid type variable bound by
the instance declaration
at ../Sites/liberaleswebsite/www.liberales.be/cgi-bin/Test.hs:20:17
In the expression: c
In the definition of `getCatalog': getCatalog (Content _ _ c) = c
In the definition for method `getCatalog'
Failed, modules loaded: none.
thanks in advance,
P
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Re: [Haskell-cafe] What's wrong with the classes/insances?
Hello Pieter,
Saturday, June 21, 2008, 2:04:10 AM, you wrote:
for me, it seems just like you directly translated OOP classes into
Haskell that is the wrong way. you may look into
http://haskell.org/haskellwiki/OOP_vs_type_classes and
ghc user manual which discuss functional dependencies on the example
of collection classes
> HI,
> What 's wrong with this:
> type Id = String
> class Catalog a where
> listItems :: a -> IO [String]
> getItem :: a -> Id -> IO (Maybe String)
> class Item a where
> getCatalog :: Catalog catalog => a -> catalog
data Catalog c =>> Content c = Content {auteur :: String, inhoud::
> String, catalog::c}
> instance Catalog c => Item (Content c) where
>getCatalog (Content _ _ c) = c
> I get this as error from ghci:
> Couldn't match expected type `catalog' against inferred type `c'
> `catalog' is a rigid type variable bound by
> the type signature for `getCatalog'
> at
> ../Sites/liberaleswebsite/www.liberales.be/cgi-bin/Test.hs:16:26
> `c' is a rigid type variable bound by
> the instance declaration
> at
> ../Sites/liberaleswebsite/www.liberales.be/cgi-bin/Test.hs:20:17
> In the expression: c
> In the definition of `getCatalog': getCatalog (Content _ _ c) = c
> In the definition for method `getCatalog'
> Failed, modules loaded: none.
> thanks in advance,
> P
--
Best regards,
Bulatmailto:[EMAIL PROTECTED]
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[Haskell-cafe] What's wrong with the classes/insances?
HI,
What 's wrong with this:
type Id = String
class Catalog a where
listItems :: a -> IO [String]
getItem :: a -> Id -> IO (Maybe String)
class Item a where
getCatalog :: Catalog catalog => a -> catalog
data Catalog c => Content c = Content {auteur :: String, inhoud::
String, catalog::c}
instance Catalog c => Item (Content c) where
getCatalog (Content _ _ c) = c
I get this as error from ghci:
Couldn't match expected type `catalog' against inferred type `c'
`catalog' is a rigid type variable bound by
the type signature for `getCatalog'
at
../Sites/liberaleswebsite/www.liberales.be/cgi-bin/Test.hs:16:26
`c' is a rigid type variable bound by
the instance declaration
at ../Sites/liberaleswebsite/www.liberales.be/cgi-bin/Test.hs:20:17
In the expression: c
In the definition of `getCatalog': getCatalog (Content _ _ c) = c
In the definition for method `getCatalog'
Failed, modules loaded: none.
thanks in advance,
P
--
Pieter Laeremans <[EMAIL PROTECTED]>
"The future is here. It's just not evenly distributed yet." W. Gibson
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