[Haskell-cafe] Re: Non Empty List?

GüŸnther Schmidt gue.schmidt at web.de
Thu Jun 4 19:39:12 EDT 2009

Hi Jake,

Jake McArthur schrieb:
> GüŸnther Schmidt wrote:
>> data Container a = Single a | Many a [a]      
>> but the problem above is that the data structure would allow to 
>> construct a Many 5 [] :: Container Int.
> I think you meant to do either
>     data Container a = Single a | Many a (Container a)

nope, I pretty much meant what I wrote above, the solution here would 
mean deeply nested containers, since it's a recursive data structure.

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

It's quite possible that in order to achieve this I would need to split 
this in 2 separate data declarations.

The idea behind this is that an "a" can "pocket" siblings, but only one 
level deep and that an "a's" list of "pocketed/swallowed" siblings must 
not be empty, because otherwise it would automatically be an "Single a".

Sorry, I really don't know how to put this better.


> or
>     data Container a = Container a [a]
> - Jake

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