[Haskell-beginners] Structural restrictions in type constructor

[Martin Vlk]

But this is checking the values in the implementation, not a type level
build time guarantee, isn't it?

M

Imants Cekusins:
> we could use pattern matched constructor:
> 
> module PairsMatched where
> 
> data MyP t = MyP (t,t)(t,t) deriving Show
> 
> 
> myP_ctor:: Eq t => (t,t)->(t,t)->MyP t
> myP_ctor (a1,a2) (a3,a4)
>    | a1 == a4 && a2 == a3 = MyP (a1,a2) (a3,a4)
>    | otherwise = error "mismatched pairs"
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