[Haskell-cafe] Re: Non Empty List?

[GüŸnther Schmidt]

Hi Tillmann,

thank you for the elaborate example. This is exactly what it took for me 
to realize that you and Jake have indeed given me the solution that I need.

I had a very hard time getting my head around it, but to my defense I'm 
usually working on this stuff in the wee hours. :)


Günther



Tillmann Rendel schrieb:
Hi Günther,

GüŸnther Schmidt wrote:
data Container a = Single a | Many a [a]      but the problem above 

I need a data structure as in my example without the [] being possible 
to be empty.

So lets write a variant of list which cannot be empty. A usual list is 
empty, or a head and a tail:

  data List a
    = Empty
    | HeadAndTail a (List a)

Now, our kind of list should be just one element, or a head and a tail:

  data NonEmptyList a
    = JustOne a
    | HeadAndNonEmptyTail a (NonEmptyList a)

So we can replace [] by NonEmptyList in your definition to get what you 
want:

  data Container a
    = Single a
    | Many a (NonEmptyList a)

However, since Container and NonEmptyList are isomorphic, we can use one 
instead of the other:

  data Container a
    = Single a
    | Many a (Container a)

Whether this is a good idea depends on the purpose of the types, but I 
guess it is.

  Tillmann


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