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

[Andreas Abel]

In the end, I should be grateful for the FTP.  It will provide amusement 
for years.

Q: What is the length of pi?
A: Dunno, infinity?
Q: And what is the length of just pi?

GHCi, version 7.10.3: http://www.haskell.org/ghc/  :? for help
Prelude> length (Just pi)
1

ROFL.

On 01.04.2017 12:32, Ivan Lazar Miljenovic wrote:
> 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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-- 
Andreas Abel  <><      Du bist der geliebte Mensch.

Department of Computer Science and Engineering
Chalmers and Gothenburg University, Sweden

andreas.abel at gu.se
http://www.cse.chalmers.se/~abela/