[erlang-questions] maps or records?

Torben Hoffmann <>
Mon Feb 29 21:02:24 CET 2016



Jesper Louis Andersen <> writes:

> [ text/plain ]
> On Mon, Feb 29, 2016 at 4:28 PM, Siraaj Khandkar <>
> wrote:
>
>> In other words, I'm saying it is very possible to have this cake (of
>> public structure) and eat it too (in private, without anyone accessing the
>> rapidly-changing, secret cream) :)
>
>
> You are so close to the idea of view types, and Torben hinted me I should
> say something about them :)
>
> One advantage of a function is that it doesn't have to give you the field
> outright, but can provide ways to dynamically cast that structure into
> another structure you export. Two really good examples is that you can
> return some kind of tuple depending on the contents of the data and thus
> you can build a case match on the data. This ad-hoc
> for-the-purpose-construction of a datatype can often yield far more precise
> code since it splits analysis of what is *in* the datastructure from
> computation which says what to *do* about that data. Say we have a #uri{}
> record. The path component has many possible representations: a binary(), a
> list of binaries, an iterator, and so on. Different view-functions would
> give you different ways to handle that binary.
>
> Another nice thing is that functions can give you are
> zipper/derivative/delimiting-continuation over the data in the structure.
> The path-component can then be unfolded one path-entry at a time:
>
> pth(X) ->
>     case http_uri:unfold_path(X) of
>         none -> ...;
>         {some, C, X2} ->
>            ... C ... pth(X2)
>     end.
>
> or you can imagine a gb_trees/gb_sets like iterator over the data structure.
>
> A plain map() cannot give any of these ideas, and its transparency also
> tightly couples your code to the map() structure. So one has to carefully
> weigh what kind of interface you want in the long run. I much prefer views
> of the data structures if possible since it allows for more freedom in the
> long run, but it requires you to be able to figure out what kind of
> functions you would need a priori on the data. On the other hand, it
> provides for much better hiding of what is going on inside the module,
> which allows you more flexibility regarding backwards compatibility.
>
> In other words: exporting records across application boundaries tend to
> lock them down, so be pretty sure they are set in stone. Also think hard if
> you want every user to implement the analysis code. The view is much like
> in a database: do you export the raw records, or do you expose a
> transactional API through stored procedures which tell you what you can do
> with the data?
>
>
Why attempt to write a half-baked answer when you can ask Jesper to
explain it well?!?! [1]
 
Look at Cowboy and Webmachine for examples of how to provide functions
to work with a request.

Both have a very rich API for extracting relevant things out of the
request record.
No need to start poking into that record on your own.

Cheers,
Torben

[1] One Erlang beer token to you, Jesper.

-- 
Torben Hoffmann
Architect, basho.com
M: +45 25 14 05 38


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