Functor, Applicative, Monad, Foldable, Traversable instances for (, , ) a b

[Ivan Lazar Miljenovic]

On 1 April 2017 at 19:42, Henrik Nilsson
<Henrik.Nilsson at nottingham.ac.uk> wrote:
> Hi all,
>
>> It is perfectly to limit the concept of "length" to
>>
>> * linear structures (lists, arrays, etc.)
>>   [branching structures do not have a length, they may have a size or
>>   maximum depth]
>>
>> * which have variable length
>>   [why would you otherwise ask for the length?]
>
> Just for the record, I could not agree more.
>
> Looking up "length", e.g. Merriam-Webster:
>
>     1a :  the longer or longest dimension of an object
>
> What we really are talking about here is counting the number of
> elements in a data structure. That is clearly applicable to lists
> as well as trees and tuples interpreted in particular ways.
>
> But calling it "length" is not particularly helpful. Nor the effort to
> reappropriate the very concept of "length" through a categorical
> interpretation.
>
> It would have been a lot better if a name suggesting a sense of
> counting, or at least a name suggesting that this is something different
> than the usual length, would have been picked. Maybe elementcount
> or ever flength.

I agree, but on the other hand this should have been done _before_
FTP; at this stage, I lean towards marking this down as one of the
known warts of Haskell - possibly with a 3 major version GHC cycle to
migrate to a better name once the bikeshedding has been completed -
and moving on rather than being revisionist and using this as an
excuse to go backwards.

So far, the arguments against these instances that I've seen are:

* The name of the length function is misleading for such instances
(with all respect to Tony, agreed; but see above); I don't think I've
seen anyone else complain about any of the other function names
though.

* Since these classes/instances are now exported from the Prelude,
previous code that wouldn't have have type-checked are now valid and
thus produce unexpected results (I was against the Prelude export when
FTP was being debated, but now - with the possible exception of people
learning to program - I think this is a matter of needing to be more
aware of the types and using our wonderful REPL environments to test
functions rather than blindly just writing code without validating the
code logic).

TL;DR: I didn't 100% agree with the way FTP was done precisely due to
the problems people have with these instances, but now let's not go
backwards (and the tuple instances I've found very useful).

>
> Names matter.
>
> /Henrik
>
>
>
>
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-- 
Ivan Lazar Miljenovic
Ivan.Miljenovic at gmail.com
http://IvanMiljenovic.wordpress.com