I actually meant
data Container a = Many a(Container a)
but here is what I don't understand (fyi, I am a beginner) how can you
construct this container? I can do
let a = Many "somestring" - and I will get back a function but I can not do
let a = Many 'a' "somestring" - because the second param is not
(Container a) type.
let a = Many 'a' (Container ['a','a']) - doesn't work either because
Container is a type not a constructor (right or I am missing
something?).
So I have two questions:
1. When I do
let b = Many "somestring" , I get
:t b
b :: Container[Char] -> Container[Char]
what is it and why I am allowed to pass just one parameter to Many
(and how can I use it)?
2. How can you construct that container?
data Container a = Many a(Container a)
let a = ????
On Fri, Jun 5, 2009 at 4:01 PM, Tillmann Rendel
Hi,
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.
Tillmann