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

[amindfv at gmail.com]

> El 1 abr 2017, a las 05:32, Ivan Lazar Miljenovic <ivan.miljenovic at gmail.com> escribió:
> 
> 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).

My concern is not that people should be able to write trivial-to-check code without actually testing it. My concern (because it has happened to me in real code!) is people may write complex functions with difficult-to-inspect results.

The bug that was trickiest to track down was a numerical calculation involving a list whose length was almost always 1, that I performed 'length' on. I wrapped the list in a tuple elsewhere in the code, ghc happily typechecked it, and only in rare cases did I get very weird results. This was deep within a larger calculation, and someone could have easily shipped something like this to production.

Tom


> 
> 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
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