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

[Jakub Daniel]

> 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.

That is like being surprised about

length [1..]

and

length [[1..]]

though. And I imagine nobody would argue that doesn't make sense. I kind of see why people want better name than length but I see no really good argument why the instance is bad. There are no doubt other generalised notions. It also was an unfair question to ask what mathematicians would think the length of (1,1) was. Because the question is not asked in context where we ask about length for the type (a,-) not (-,-).

Jakub


> 
>> 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/
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