[Haskell-beginners] Multiple declarations of value constructor

[Peter Hall]

Something like this?


class TwoValues a where
   val1 :: a
   val2 :: a

data MyTypeA = ValueA1 | ValueA2
data MyTypeB = ValueB1 | ValueB2

instance TwoValues MyTypeA where
    val1 = ValueA1
    val2 = ValueA2

instance TwoValues MyTypeB where
    val1 = ValueB1
    val2 = ValueB2


---
val1 :: MyTypeA -- ValueA1
val2 :: MyTypeB -- ValueB2


Peter



On 19 December 2012 15:52, Emanuel Koczwara <poczta at emanuelkoczwara.pl>wrote:

> Hi,
>
> -- tests.hs
> data MyType1 = Value1 | Value2
> data MyType2 = Value1 | Value2
>
>
> Prelude> :l tests
> [1 of 1] Compiling Main             ( tests.hs, interpreted )
>
> tests.hs:3:16:
>     Multiple declarations of `Main.Value1'
>     Declared at: tests.hs:1:16
>                  tests.hs:3:16
>
> tests.hs:3:25:
>     Multiple declarations of `Main.Value2'
>     Declared at: tests.hs:1:25
>                  tests.hs:3:25
> Failed, modules loaded: none.
>
> If so, how are numbers defined? I can use 1 :: Int or 1 :: Integer. Why
> not Value1 :: MyType1 and Value1 :: MyType2?
>
> Emanuel
>
>
>
>
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