[Haskell-beginners] hiding members of a data, separate accessors instead
Emmanuel Touzery
etouzery at gmail.com
Sun Mar 24 21:03:54 CET 2013
Yes, exactly I just was typing a mail on that. I just realized that myself.
And the Network.Http.Types module is not available because the author has
not exported it through cabal.
exposed-modules: Network.Http.Client
other-modules: Network.Http.Types,
Network.Http.Connection,
Network.Http.RequestBuilder,
Network.Http.ResponseParser,
Network.Http.Inconvenience
It's the first time I realise that this pattern exists.
So he has effectively returned the data types, but in an opaque way, and
users must use his accessors and pattern matching is impossible.
Thank you!
Emmanuel
On Sun, Mar 24, 2013 at 9:00 PM, Gabriel Gonzalez <gabriel439 at gmail.com>wrote:
> **
> On 03/24/2013 12:24 PM, Emmanuel Touzery wrote:
>
> But then since the library is using (..) that would mean everything is
> exported?
>
> It only means that those fields are exported from that specific
> module. Downstream modules that use Network.Http.Types internally may or
> may not re-export everything.
>
> Your example below doesn't import Network.Http.Types; it imports
> Network.Http.Client. If you look at the source for Network.Http.Client you
> will see that it does not re-export everything it imported from
> Network.Http.Types:
>
>
> http://hackage.haskell.org/packages/archive/http-streams/0.4.0.0/doc/html/src/Network-Http-Client.html
>
> When you import Network.Http.Client, `ghc` only uses whatever is in the
> export list of Network.Http.Client.
>
> For instance testing on the Request data:
>
>
> http://hackage.haskell.org/packages/archive/http-streams/0.4.0.0/doc/html/src/Network-Http-Types.html#Request
>
> module Network.Http.Types ( Request(..),
>
> data Request = Request { qMethod :: !Method, qHost :: Maybe ByteString, qPath :: !ByteString, qBody :: !EntityBody, qExpect :: !ExpectMode, qHeaders :: !Headers }
>
>
> ----
> {-# LANGUAGE OverloadedStrings #-}
>
> import Network.Http.Client
>
> main = do
> q <- buildRequest $ do
> http GET "/"
> setAccept "text/html"
>
> print q
> print $ qMethod q
>
> ---
>
> test-hs.hs:11:17: Not in scope: `qMethod'
>
> With regards to what Daniel wrote, I realize my email was confusing. When
> I was talking about warnings I was talking of another problem entirely,
> that i probably should not have mentioned in this context.
> In that other context I had data declarations for types that I would
> instanciate only from Data.Aeson parsing from JSON. I would then only use
> pattern matching on the instances, never call the "accessor functions" by
> themselves, then I get a warning that they're unused which annoys me. But
> it's quite unrelated to this mail...
>
> Emmanuel
>
>
>
> On Sun, Mar 24, 2013 at 6:34 PM, Gabriel Gonzalez <gabriel439 at gmail.com>wrote:
>
>> Assume you have the following type:
>>
>> data Type = T { field1 :: String, field2 :: Double }
>>
>> ... and you want to export the type `Type` and the acessors `field1` and
>> `field2`, but not the constructor `T`, then you would write:
>>
>> module MyModule (
>> Type(field1, field2)
>> ) where
>>
>> Another way to do this is like so:
>>
>> module MyModule (
>> Type,
>> field1,
>> field2
>> ) where
>>
>> That's perfectly legal, too.
>>
>> Normally, when you write something like:
>>
>> module MyModule (
>> Type(..)
>> ) where
>>
>> the ".." expands out to:
>>
>> module MyModule (
>> Type(T, field1, field2)
>> ) where
>>
>> All the first solution does is just leave out the T constructor from
>> those exports.
>>
>>
>> On 03/24/2013 09:14 AM, Emmanuel Touzery wrote:
>>
>> hi,
>>
>> i was looking at the response type in http-streams:
>>
>> http://hackage.haskell.org/packages/archive/http-streams/0.4.0.0/doc/html/Network-Http-Client.html#t:Response
>>
>> I'm used that simply the data type and all its "members" are visible --
>> the functions to access its contents. But in this case on the HTML
>> documentation the response type looks like it has no members. And the
>> author has defined like "public accessors" later in the code:
>>
>> getStatusCode :: Response -> StatusCode
>> getStatusCode = pStatusCode
>>
>> So I'm not even sure how he achieved that the members are not visible,
>> the data are exported with (..) as is usually done... And the other thing
>> is why
>> would you do that.. You could name the member getStatusCode in the first
>> place, but then it might increase encapsulation to hide it (depending on
>> how he
>> managed to hide the members).. But did you then make
>> it impossible to deconstruct a Response through pattern matching? That
>> sounds like a minus... Although pattern matching on a data with 6 fields
>> is always going to be a pain and decreasing the chances for modifying
>> the data type without breaking compatibility.
>>
>> These "members" are also causing me problems in other situations, for
>> instance I have some cases when I use a data type only a few times and with
>> -Wall the compiler tells me I don't use the accessor; in fact I read that
>> value from the data, but through pattern matching/deconstruction only, not
>> through that particular function. I'm thinking to try to hide the warning
>> as I think my code is correct.
>>
>> Anyway I'm curious on the mechanism used by that library... I've already
>> noticed a few nice tricks in this library, like a small state monad to take
>> optional parameters, much more elegant than any other mechanism i've seen
>> so far to achieve the same effect.
>>
>> Thank you!
>>
>> Emmanuel
>>
>>
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>>
>
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