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Today's Topics:
1. phantom type (PICCA Frederic-Emmanuel)
2. Re: phantom type (Francesco Ariis)
----------------------------------------------------------------------
Message: 1
Date: Sat, 31 Mar 2018 07:54:50 +0000
From: PICCA Frederic-Emmanuel
<[email protected]>
To: "The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell" <[email protected]>
Subject: [Haskell-beginners] phantom type
Message-ID:
<a2a20ec3b8560d408356cac2fc148e530107dd5...@sun-dag3.synchrotron-soleil.fr>
Content-Type: text/plain; charset="us-ascii"
Hello,
Here my problem.
I extract some information from a database and put them into a type with 50
parameters
Type MyData = MyData Int String ...
But I need to be able to differenciate two different case. depending on the
String content.
something like if the String start with "ref-" it is a Caracterization and if
not it is a Collect
So I end up with this solution
data Unchecked
data Caracterization
data Collect
data MyData t = MyData Int String ...
now I defined a function
getMyDataFromDatabase :: ... -> Mydata Unchecked
and isCollect in order to verify that I have a Collect and not something Else
isACollect :: MyData Unchecked -> Either Text (MyData Collect).
which use coerc in order to avoid copying all the members. It is great.
now Here is my problem.
I create a method
myMethod :: MyData Collect -> IO ()
but I want my method to work for MyData Collect and Mydata Caracterization but
not Unchecked.
So It seems to me that I can not just do
myMethod :: MyData t -> IO ()
what is the right way to solve this problem ?
thanks for your help.
Frederic
------------------------------
Message: 2
Date: Sat, 31 Mar 2018 10:40:11 +0200
From: Francesco Ariis <[email protected]>
To: [email protected]
Subject: Re: [Haskell-beginners] phantom type
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Sat, Mar 31, 2018 at 07:54:50AM +0000, PICCA Frederic-Emmanuel wrote:
> I create a method
>
> myMethod :: MyData Collect -> IO ()
>
> but I want my method to work for MyData Collect and Mydata Caracterization
> but not Unchecked.
>
> So It seems to me that I can not just do
>
> myMethod :: MyData t -> IO ()
>
>
> what is the right way to solve this problem ?
Hello Frederic,
I am in a rush so I cant write a minimal example, but wouldn't
a typeclass + make Collect and Cara instances of that typeclass do?
-F
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