[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