Re: Cached queries

[Eric Bezault]

On 07/01/2024 23:41, Paolo Redaelli wrote:
> I found myself using this pattern over and over again:
>
> |class FOO|
>
> |feature {ANY}|
>
> |    costly_query: SOMETHING is|
>
> |        do|
>
> |            if cached_costly_query=Void then compute_costly_query end|
>
> |            Result := cached_costly_query|
>
> |        end|
>
> |feature {} -- Implementation|
>
> |    cached_costly_query: like costly_query|
>
> |    compute_costly_query is|
>
> |        do|
>
> |            -- ......|
>
> |        ensure cached_costly_query/=Void|
>
> |        end|
>
> |end|
>
>
> Do you know of a more "precanned" / more elegant / better way of doing it?

In ECMA Eiffel (attached mode) you can write:

  costly_query: SOMETHING
        attribute
                Result := -- costly computation
        ensure
                costly_query_not_void: Result /= Void
        end

Otherwise, if `costly_query' is never modified, you can use
a once-per-object:

  costly_query: SOMETHING
        once ("OBJECT")
                Result := -- costly computation
        ensure
                costly_query_not_void: Result /= Void
        end

I don't know whether these are supported in Liberty.
They are in ISE Eiffel and Gobo Eiffel.

--
Eric Bezault
mailto:ericb@gobosoft.com
http://www.gobosoft.com