On Sun, Mar 04, 2012 at 10:22:18AM +0100, Ralf Hemmecke wrote:
>> The question can be reformulated: what Spad has for the tagged union?.
>
> Tagged union in SPAD is something like
>
> Union(i: Integer, f: Fraction Integer, ...)
>
Please, where it is described how to use it?
> but I bet, that is not what you want, since it is not equivalent to Haskell.
>
>> The initial goal is explained in the first my letter on this subject.
>
> Yes, I understood that part. But you haven't expressed the WHY.
> Why would you want to put everything into one big type?
I am trying to compose a Spad function parseCall which parses from a
String a function call, for a certain restricted set of functions,
with parsing domain descriptions and the corresponding argument
descriptions, and a package call description -- given all in one String.
I am going to provide a working program for parseCall and then, to ask
whether it is reasonably done.
But I met an obstacle: I do not know how to define a
_recursive tagged type_.
Example
-------
a Tree over a type T is either a Leaf or
a Node containing v: T, left branch Tree, and right branch Tree.
In Haskell, this can be defined by intruducing a user type constructor
Tree and its type:
data Tree a = Leaf a | Node a (Tree a) (Tree a)
For example,
(Node 1 (Node 11 (Leaf 111) (Node 112 (Leaf 1121) (Leaf 1122)))
(Leaf 12)
)
-- :: Tree Integer
represents the tree 1
/ \
11 12
/ \
111 112
/ \
1121 1122
Example: the function f sums the numbers in all the tree:
f :: Tree -> Integer
f tree = case tree of Leaf n -> n
Node n left right -> n + (f left) + (f right)
What is the most appropiate approach in Spad to definig Tree(T) ?
I try Tree INT :
---------------------------------------------
INT ==> Integer
Tree ==> Union(Integer, Node)
)abbrev package FOO Foo
Foo() : with tree1 : INT -> Tree
== add
Node : Type :=
Record(nodeVal : INT, nodeLeft : Tree, nodeRight : Tree)
tree1(n : INT) : Tree == n :: Tree
---------------------------------------------
-- and tree1(2) does not work.
Thank you for advices,
------
Sergei
[email protected]
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