Hi, I have a situation where I need to define a data type T such that data T = C1 Int | C2 Char | C3 T However, I want to enforce a constraint that C3 only allows (C2 Char) and not (C1 Int). That is x = C3 (C1 10) -- should not compile - but my above definition will let it compile I was thinking of this - data C1 = C1 Int data C2 = C2 Char data T = TC1 C1 | TC1 C2 | TC3 C2 Is there a better way to do it? Regards, Kashyap
You could use a GADT:
{-# LANGUAGE GADTs #-}
data T a where
C1 :: Int -> T Int
C2 :: Char -> T Char
C3 :: T Char -> T Char
This will allow you to put a C3 in a C3. If you want to prevent that,
just invent some other index, something like:
{-# LANGUAGE GADTs, EmptyDataDecls #-}
data Yes
data No
data T a where
C1 :: Int -> T No
C2 :: Char -> T Yes
C3 :: T Yes -> T No
Not sure if this is a *better* way though. Your initial solution is
also ok, I guess.
Regards,
Erik
On Thu, Mar 21, 2013 at 1:48 PM, C K Kashyap
Hi,
I have a situation where I need to define a data type T such that
data T = C1 Int | C2 Char | C3 T
However, I want to enforce a constraint that C3 only allows (C2 Char) and not (C1 Int). That is
x = C3 (C1 10) -- should not compile - but my above definition will let it compile
I was thinking of this -
data C1 = C1 Int data C2 = C2 Char data T = TC1 C1 | TC1 C2 | TC3 C2
Is there a better way to do it?
Regards, Kashyap
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On Thu, Mar 21, 2013 at 06:18:46PM +0530, C K Kashyap wrote:
Hi,
I have a situation where I need to define a data type T such that
data T = C1 Int | C2 Char | C3 T
However, I want to enforce a constraint that C3 only allows (C2 Char) and not (C1 Int). That is
If C3 should only be able to hold a C2 Char, then why have it hold a T at all? i.e. why not data T = C1 Int | C2 Char | C3 Char but I suppose your real problem is probably more complicated, in which case I would recommend using a GADT as others have suggested. -Brent
Thanks Eric and Brent,
Even with GADT, it appears that I'd need extra data definitions. I'll go
without GADT then ...
Brent, my use case is not particularly complicated. I am trying to model
the pdf spec - which says that pdf contains Objects that could of of types
Number, String, Name, Array and Dictionary - while array is list of
objects, the Disctionary is a list of tuples (Name, Object) not (Object,
Object) - hence my situation.
Regards,
Kashyap
On Thu, Mar 21, 2013 at 8:58 PM, Brent Yorgey
On Thu, Mar 21, 2013 at 06:18:46PM +0530, C K Kashyap wrote:
Hi,
I have a situation where I need to define a data type T such that
data T = C1 Int | C2 Char | C3 T
However, I want to enforce a constraint that C3 only allows (C2 Char) and not (C1 Int). That is
If C3 should only be able to hold a C2 Char, then why have it hold a T at all? i.e. why not
data T = C1 Int | C2 Char | C3 Char
but I suppose your real problem is probably more complicated, in which case I would recommend using a GADT as others have suggested.
-Brent
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
On Mar 22, 2013 2:58 AM, "C K Kashyap"
Thanks Eric and Brent,
Even with GADT, it appears that I'd need extra data definitions. I'll go
without GADT then ...
Brent, my use case is not particularly complicated. I am trying to model
the pdf spec - which says that pdf contains Objects that could of of types Number, String, Name, Array and Dictionary - while array is list of objects, the Disctionary is a list of tuples (Name, Object) not (Object, Object) - hence my situation.
Regards, Kashyap
You could have a look at how the aeson package represents JavaScript values, which have a similar structure to your requirement, e.g. http://hackage.haskell.org/packages/archive/aeson/0.6.1.0/doc/html/Data-Aeso... Doug
Brent, my use case is not particularly complicated. I am trying to model the pdf spec - which says that pdf contains Objects that could of of types Number, String, Name, Array and Dictionary - while array is list of objects, the Disctionary is a list of tuples (Name, Object) not (Object, Object) - hence my situation.
The WASH web framework used a trick to type HTML. If I recall correctly, they had a couple of phantom type parameters and then wrote MPTC instances for only the allowed combinations. So you'd have "HTML InXContext OrderedList" and "HTML InXContext UnorderedList", etc. Then the constructor for an X would have a type signature that required 'HTML InXContext a' of its argument. Or something like that. See if you can dig up any old WASH docs for more details.
participants (5)
-
Brent Yorgey -
C K Kashyap -
Doug Burke -
Erik Hesselink -
Evan Laforge