[Haskell-cafe] Re: Non Empty List?

Tillmann Rendel rendel at cs.au.dk
Fri Jun 5 16:01:04 EDT 2009


please write to the whole list, not just me. There are a lot of people 
around who can help you.

MH wrote:
> Rendel do you mind to explain to me how Container a = Many a
> (Container [a]) prevents user from creating an empty list?
> I did try the following:
> let a = Many "string"
> a :: Container [Char] -> Container [Char]
> let b = Many []
> b :: forall a. Container [a] -> Container [a]
> but I can not understand what is the difference.

You cannot prevent users from creating an empty list, because lists are 
already defined in the Haskell prelude, and you cannot change that 
definition. But you can define your own datatype Container which 
prevents users from creating empty containers.

Your datatype

   data Container a = Many a (Container [a])

is quite complicated, because you have (Container a) on the left hand 
side of the equation, but (Container [a]) on the right hand side. Is 
that on purpose?

Anyway, your Container has the property that a (Container a) is never 
empty, because it always contains at least one value of type a, namely 
the first parameter of Many. To see that, we can write a function which 
extracts that element:

   getContainerElement :: Container a -> a
   getContainerElement (Many element something) = element

It is not possible to write such a function for ordinary lists without 
calling error or entering a nonterminating loop:

   getListElement :: [a] -> a
   getListElement (element : something) = element
   getListElement [] = error "no element around :("

So ordinary lists can be empty, but your Containers can't.


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