On 2/11/25 15:44, Martin Baker wrote:
might work like the concept of Dictionaries in Python which are used
to store data values in key:value pairs.
Is already available in FriCAS in several domains.
http://fricas.github.io/api/AssociationList.html
http://fricas.github.io/api/HashTable.html
http://fricas.github.io/api/XHashTable.html
There was some difference that always used to cause me problems
because categories are more statically defined than interfaces but I
have forgotten the details now.
I suggest you read the Aldor User Guide.
http://www.aldor.org/docs/aldorug.pdf
For example, in Java you might have an interface called 'shape' and
this interface has functions such as 'draw' so classes that
implement shape such as 'square' and 'circle' all have there own
implementations of 'draw'.
But that is exactly the same as in SPAD.
Categories can provide implementations of some functions.
One should, however, take care that such implementations do not rely on
any representation details of an element. Such implementations can be
overridden by an respective domain, but the general idea is that
categories do not know about the representation. It's just an abstract
collection of function signatures without the notion of the concrete
carrier set is one speaks in terms of universal algebras (or rather
multisorted algebras).
Some other part of the code may have a List of 'shape' and if you
call 'draw' on each of the elements it would call the appropriate
version of draw depending on whether it is a 'square' or a 'circle'.
That you can also do in FriCAS, but it is not exactly the way you do it
in Java. A list must have elements of the same type, i.e. in FriCAS you
can only have List(T) for a concrete type T. If you have Circle and
Square as domains realizing the category Shape, then you can either have
List(Circle) or List(Square), but not a mixture of them. What you need
is a domain, that "represents" both of them. You find such a general
domain describe in Section 21.10 in aldorug.pdf, the domain "Object".
That domain is also in FriCAS and is called "Any". And yes, it bundles
an element together with its type information. (That is what all
object-oriented programming languages do per default.)
In FriCAS/Aldor an element (i.e. its representation in memory) usually
does not know its type, (i.e. there is no reference to the respective
domain). The "Any", "Object" construction makes this explicit. In that
sense, FriCAS/Aldor is different from other OO programming languages.
I mention this just as a reminder of the sort of problems new users
may have when coming to SPAD from other languages. I think it would
be good if there was something on fricas.github.io listing potential
issues like this for new users (and me).
Maybe I should add such an explanation into some help section if the
above at least helps you. (Feedback appreciated.)
Ralf
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