Dear Lucas,
As mentioned by David, I was asking a question regarding lower level
code tree data structure.
But still I enjoy reading your interesting code example. I found this
approach very useful when I was writing a small interpreter where I need
to keep values with different types in an environment table.
Best,
Yue
On 03/15/2011 12:46 AM, Lucas Dixon wrote:
I'm not sure if this is what you meant, but the type of exceptions is
heterogeneous in the sense that all exceptions, no matter what data
they hold, have the same type.
This lets you create lists of different kinds of data, where new types
of data can be added later. It essentially pushes type-checking, via
the exception system, to runtime. You can then package this dynamic
access safely in a functor. The idea is used in Isabelle's generic
contexts, among other places.
(btw - does anyone know who came with this idea? I think the first
time I saw it was in the Isabelle source code 10 years ago or so...)
To quickly get a flavour of the idea here's a little example...
(* exceptions that wrap up some data *)
exception StringExpWrapper of string;
exception IntExpWrapper of int;
(* generic kind of data: an informal name for the data and the
exception that holds the data *)
datatype generic_data =
GenericData of { kindstr : string, dataexn : exn };
(* helper function to wrap up data in exceptions and give them a
kind string *)
fun wrap_int i =
GenericData { kindstr = "int", dataexn = IntExpWrapper i };
fun wrap_string s =
GenericData { kindstr = "string", dataexn = StringExpWrapper s };
fun wrap_generic ex =
GenericData { kindstr = "generic", dataexn = ex };
(* To try and pull integer from generic data; will raise the wrapped
exception if the kind-string is "int" but the exception is not the
IntExpWrapper. *)
fun unwrap_int (GenericData { kindstr = "int", dataexn }) =
((raise dataexn) handle IntExpWrapper x => SOME x)
| unwrap_int _ = NONE;
(* empty list *)
val empty_list = [] : generic_data list;
val list1 = [wrap_string "foo", wrap_int 12, wrap_int 3];
(* we can sum the integers in a generic list... *)
fun sum_ints l =
List.foldr
(fn (data,count) =>
case unwrap_int data of SOME n => count + n | NONE => count)
0 l;
(* a little test to sum of our first generic/heterogeneous list *)
val sum1 = sum_ints list1;
(* we can create new kinds of exception for new type of data we want
to wrap up and add to our list ... *)
exception SomeNewDataExn of int list;
(* now we can extend our old list with elements, even those of the
new type *)
val list2 = [wrap_int 2, wrap_generic (SomeNewDataExn [1,2])] @
list1;
(* and we can apply the sum function again to this new list... *)
val sum2 = sum_ints list2;
best,
lucas
On 10/03/2011 22:31, Yue Li wrote:
To the list.
-------- Original Message --------
Subject: creating heterogeneous array in the Poly/ML codetree
Date: Thu, 10 Mar 2011 16:07:50 -0600
From: Yue Li <y...@cse.tamu.edu>
Organization: CSE Department, Texas A&M
To: David Matthews <david.matth...@prolingua.co.uk>
CC: Gabriel Dos Reis <g...@cs.tamu.edu>
Dear David,
Is there any way to create a heterogeneous array in the Poly/ML codetree
with known size, but whose elements can be values of any type and I
don't
necessarily want to use ML-style datatype. For instance, elements in the
array can be integer values, or function values or arrays?
Best,
Yue
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