[Haskell-beginners] Foldable for (,)

Jonathon Delgado voldermort at hotmail.com
Sun Apr 23 10:06:00 UTC 2017

If a tuple only has one value, why do functions for operating over sets make sense at all? I can see from your explanations why the answers could be considered correct (if a particular convention is assumed), but why does the operation make sense at all? It seems like we're asking for the length of a single value, its product, etc.

Francesco Ariis wrote: 

> I've seen many threads, including the one going on now, about why we need 
> to have: 
> length (2,3) = 1 
> product (2,3) = 3 
> sum (2,3) = 3 
> or (True,False) = False 
> but the justifications all go over my head. Is there a beginner-friendly 
> explanation for why such seemingly unintuitive operations should be 
> allowed by default? 
Hello Jonathon, 
    the proponents of `Foldable (a,)` see `(2,3)` not as a pair of 'equal' 
values, but as a value *and* an annotation, much like some other folks 
see Either as having a value (Right a) *or* an annotation (usually an 
error in the form of Left e). 

So to go back to your examples: 

     ^ ^ 
     | +------------- I am the value 
     +--------------- I am an annotation (and since tuples arguments can 
                      be heterogeneous, I could be a String, a Bool, 

If you agree with this paradigm, `length`, `sum` and friend become a 
bit less icky. 

I would prefer tuples to be unbiased, but this intuition helped me 
connect with the people on the other side of the line. 
Does this help? 

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