[Haskell-beginners] Structural restrictions in type constructor

[Matt Williams]

Dear All,

This has reminded me that perhaps there is an easier way.

I have a Map, whose elements are indexed by a subset of their structure.

I.e. if we have MyType = MyType I T T O E

Where I T O and E are types defined elsewhere.

The Map the indexes elements of type MyType by a pair, (T, T).

I want to be able to index by a pair, independent of order. I had thought
about indexing by a pair of pairs, where the elemtns could be the same but
reversed.

However, the alternative might be to index by a pair, but define that pair
as a type, and alter its Eq => definition:

MyPair = (T, T)
where (t, t') == (t', t) -- I know this syntax is wrong

I could then use that as the index to the Map.

Does that approach make some sense?

Thanks,
Matt

On Tue, 23 Jun 2015 15:33 Kostiantyn Rybnikov <k-bx at k-bx.com> wrote:

> Imants,
>
> You are right. The problem is not in IO here, it's that if you have access
> to data-constructor, you can do things like:
>
> six :: TypeValInt 6
> six = TypeValInt 5
>
> Initially, I was making an assumption that you won't be using a
> data-constructor. After thinking about it a bit, I should note that my code
> isn't much different from just using a "smart constructor" approach, e.g.
> hiding a real MyP constructor, and instead providing a function:
>
> mkMyP (a, b) = MyP (a, b) (b, a)
>
> and exporting only this function. This would make sure all your users only
> create a valid set of data.
>
>
> On Tue, Jun 23, 2015 at 5:05 PM, Imants Cekusins <imantc at gmail.com> wrote:
>
>> On 23 June 2015 at 14:54, Kostiantyn Rybnikov <k-bx at k-bx.com> wrote:
>> > Hi Matt. I don't know how bad is this, but here's what I came up with.
>> ...
>>
>> this modified for IO version accepts any input, including that which
>> should have caused error:
>>
>> or did I do something wrong?
>>
>>
>> {-# LANGUAGE DataKinds #-}
>> {-# LANGUAGE KindSignatures #-}
>> {-# LANGUAGE ExistentialQuantification #-}
>> {-# LANGUAGE PolyKinds #-}
>> {-# LANGUAGE StandaloneDeriving #-}
>>
>> module PairsMatchedKR where
>>
>> import GHC.TypeLits
>>
>> data TypeValInt (n::Nat) = TypeValInt Int
>>     deriving (Show)
>>
>> one :: TypeValInt 1
>> one = TypeValInt 1
>>
>> two :: TypeValInt 2
>> two = TypeValInt 2
>>
>> data MyP a b = MyP (TypeValInt a, TypeValInt b) (TypeValInt b, TypeValInt
>> a)
>>     deriving (Show)
>>
>> main :: IO ()
>> main = do
>>     putStrLn "Hello!"
>>     x1 <- getLine
>>     x2 <- getLine
>>     x3 <- getLine
>>     x4 <- getLine
>>
>>     print (MyP (tvi x1, tvi x2) (tvi x3, tvi x4))
>>
>> class TypeVal (g :: a -> *)
>> instance TypeVal TypeValInt
>>
>> data MyPGen a b = forall g. (TypeVal g, Show (g a), Show (g b))
>>                => MyPGen (g a, g b) (g b, g a)
>> deriving instance Show (MyPGen a b)
>>
>>
>> tvi:: String -> TypeValInt (n::Nat)
>> tvi = TypeValInt . read
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