[Haskell-cafe] Is it possible to represent such polymorphism?

[Yves Parès]

2-tuple and 3-tuple *are not the same type*.
So to do this you must use typeclasses.
Plus you have to deal with the type parameters

class To3Tuple a where
   expand :: a -> (Int, Int, Int)

instance To3Tuple (Int, Int, Int) where
   expand = id

instance To3Tuple (Int, Int) where
   expand (x,y) = (x,y,1)


Here I had to force my tuples to be tuples of integers.
It's more complicated if you want polymorphism.


2011/10/2 Du Xi <sdiyazg at sjtu.edu.cn>

> --I tried to write such polymorphic function:
>
> expand (x,y,z) = (x,y,z)
> expand (x,y) = (x,y,1)
>
> --And it didn't compile. Then I added a type signature:
>
> expand::a->b
> expand (x,y,z) = (x,y,z)
> expand (x,y) = (x,y,1)
>
> --It still didn't compile. I think the reason is that the following is
> disallowed:
>
> f::a->b
> f x = x
>
> --Is it possible to get around this and write the "expand" function? Of
> course, x and y may be of different types
>
>
>
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