[erlang-questions] spec declaration for single list parameter with fixed size

[Roberto Ostinelli]

2011/7/4 Kostis Sagonas <kostis@REDACTED>

> Jesper Louis Andersen wrote:
>
>> On Sun, Jul 3, 2011 at 11:58, Michael Richter <ttmrichter@REDACTED>
>> wrote:
>>
>>> On 3 July 2011 16:10, Roberto Ostinelli <roberto@REDACTED> wrote:
>>>
>>>> a very stupid spec declaration question. how can you declare the specs
>>>> of
>>>> a function which has a single list with multiple arguments?
>>>>
>>>> For instance, consider this function:
>>>>
>>>
>>>  myfun([One, Two]) ->
>>>>
>>>
>> This is better since you then have a product type rather than a
>> recursive type on lists. It is much simpler to work with from a typing
>> perspective. Also note that you would not easily be able to type
>> ["hello", 5] in a statically typed language without resorting to
>> either some kind of type tagging, polymorphic variants or existential
>> types.
>>
>> My advice would be to alter the structure of the function such that it
>> is easier to type. It tends to be safer from a programming perspective
>> as well.
>>
>
> Both Jesper and Michael have given very good advice on this topic, but for
> the record, I guess, let me add my two cents here.
>
> First of all, as others have mentioned, when grouping together a fixed
> number of items, tuples rather than lists should better be used. Not only
> does this make sense from a typing perspective, but for more than one item
> tuples are also more memory efficient and thus slightly faster. For example
> [One, Two] needs 4 words while {One, Two} needs 3.
>
> Coming back to the original question, it's pretty clear that what the type
> language does it to make a abstraction over all terms (which belong to some
> type) and thus cannot possibly express all programmer intentions accurately.
> For example, it is currently not possible to declare lists with a certain
> number of elements, partly due to the fact that the type language has
> decided to make [T] an alias for list(T). Even if this constraint was lifted
> (e.g. [T] stood for a one element list for items of type T, [T1,T2] for a
> two element list of items of type T1 and T2, etc.) the type language would
> not be able to express all programmer intentions (e.g. lists of an even
> number of elements, lists whose length is a prime number, lists where the
> middle element is 42, etc.). This is fundamental limitation of all type
> languages: no matter how expressive they become, there are things that
> cannot (conveniently) be expressed in them.
>
> Kostis
>

Jesper and Michael,

thank you for your feedback on this. I am aware of the necessity of using
tuples in this condition instead of lists. However, I was using a library
whose callback function is getting arguments as list.

Kostis, thank you for clarifying this.

Cheers,

r.
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