[Haskell-cafe] Convert Either to Tree - Occurs check

Neil Brown nccb2 at kent.ac.uk
Fri Oct 22 06:30:42 EDT 2010

On 22/10/10 09:23, André Batista Martins wrote:
> Tks for the answer,
> the data structure of Either is:
> data   Either  a  b   =   Left  a  |  Right  b    deriving  (Eq,  Ord,  Read,  Show)
> one example of what i want convert is:
>   Left(Right(Left(Left())))

The problem here is that the type of Left () is:

 > Either () a

The type of Left (Left ()) is:

 > Either (Either () a) b

The type of Right (Left (Left ())) is:

 > Either c (Either (Either () a) b)

and finally, the type of Left (Right (Left (Left ()))) is:

 > Either (Either c (Either (Either () a) b)) d

That is, each level in the tree must have a different type.  For this 
reason, you can't sensibly use Either for tree types of varying depth (a 
type-class would help, but I doubt it's what you want).  A sensible type 
for a tree is the one you gave in your original post, TreeE.  So why do 
you want to encode the tree with Either (not really possible) and then 
convert to your TreeE type?  Why not just start out with the values in 
your tree type?


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