[erlang-questions] Exporting a record type

[lloyd@REDACTED]

Hi Jesper,

Well, I finally found time to try out the code you proposed.

It is so cool!

I dawned on me that I could do:

repr_rel(#book { isbn = I } = Book, isbn)           -> {I, Book};
repr_rel(#book { title = T } = Book, title)         -> {T, Book};
repr_rel(#book { author = A } = Book, author)       -> {A, Book};
repr_rel(#book { publisher = P } = Book, publisher) -> {P, Book};
repr_rel(#book { pub_date = P } = Book, pub_date)   -> {P, Book};
repr_rel(#book { genre = G } = Book, genre)         -> {G, Book}.

And thanks for introducing the sofs library. I've often wondered how I might use it.Your code opens the door.


Three quick questions:

1) I'm guessing repr is your abbreviation for replica and rel for relation. Is this correct?

2) I've been trying to figure out a similarly compact way to set values, but can't without explicitly referencing the book record structure. Any suggestions?

3) Pardon my thick skull, but the book/0 declaration in the -opaque and -export_type attributes still puzzles me. Can I use book() in external modules? But without a parameter for a book instance, just exactly how would it be used?

Much much appreciate your help.

Lloyd



-----Original Message-----
From: "Jesper Louis Andersen" <jesper.louis.andersen@REDACTED>
Sent: Monday, July 13, 2015 6:03am
To: "Lloyd R. Prentice" <lloyd@REDACTED>
Cc: "Gordon Guthrie" <gguthrie@REDACTED>, "erlang-questions" <erlang-questions@REDACTED>
Subject: Re: [erlang-questions] Exporting a record type

On Sun, Jul 12, 2015 at 5:06 PM, Lloyd R. Prentice <lloyd@REDACTED>
wrote:

> Also, in your repr/2 code (which is very suggestive of neat things one can
> do) what is the significance of the View variable? As I see it now, it's
> simply a tag like thumbnail_overview, but I'm not comfortable that my
> understanding is correct.


The idea, at least, is this: a "view" of some data, book, record, ..., is a
way to cast that data into another structure by transforming it. I.e.,
"view" the data in another form. In statically typed languages, some
languages support built-in view-types, so you can define these
transformations formally, but in Erlang you have to make a more dynamic
resolution.

In its simple form, the view is just an atom(), requesting the view. But in
a more advanced form, the view is a language which can be executed to
construct a view of the data:

...
repr(Book, {relation, R}) ->
    repr_rel(Book, R).

repr_rel(#book { genre = G } = Book, genre) ->
    {G, Book}.

Now, suppose you have a list of books, Bs:

Rel = [repr(B, {relation, genre}) || B <- Bs],
R = sofs:relation(Rel),
F = sofs:relation_to_family(R),
sofs:to_external(F).

If for instance you have books [#book { genre = fantasy } = A, #book {
genre = fantasy } = B, #book { genre = scifi } = C],

Then the above would return

[{fantasy, [A, B]}, {scifi, [C]}].

There are numerous tricks that can be played here, depending on what
transformations you need.



-- 
J.