[Haskell-beginners] hiding members of a data, separate accessors instead

Gabriel Gonzalez gabriel439 at gmail.com
Sun Mar 24 18:34:29 CET 2013

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 (
     ) where

That's perfectly legal, too.

Normally, when you write something like:

module MyModule (
     ) 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/
>  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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