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

[Tony Morris]

Right, length is particularly unfortunate in that it is a bad example
for the purposes of parametricity. We can express additional properties,
as well as types, to make up for this shortcoming. Still better than
relying on glyphs, which are notoriously unreliable, misleading,
inefficient and impratical to use to convey useful information about the
behaviour of a function.

I will leave out the expression of these properties, so as not to rob
anyone of a fun and useful exercise. Ultimately, I genuinely do not
care. I routinely work on teams with people who do not care. We use
other tools for reasoning about programs; I argue, they are more
efficient and reliable. If anyone wants to disagree, that's fine, call
it pineapples or whatever may be chosen, and we can all move on. I
simply need that function, whatever it might be called.

PS: in a dream land, where hardware efficiencies are of no concern,
length = fmap (const ()).


On 06/04/17 18:12, Henrik Nilsson wrote:
> On 04/06/2017 04:15 AM, Tony Morris wrote:
>> By "length" here I mean, the function whose name is lima echo november
>> golf tango hotel. I truly do not care what it is called. Follow the
>> types, and the types of the types.
>
> I think the nub of the matter is that the types (and the types of the
> types) (in a language like Haskell) actually are not enough. There are
> (literally) infinitely many functions with the same type as "length".
> One might argue that few of those make "sense", but that is always
> going to be a precarious argument unless the setting is made clear.
>
> More generally, if we were to extrapolate and truly refer to things
> by their full specification, then that would be very cumbersome indeed.
> By analogy, mathematical theorems are full specifications, and they
> usually have names.
>
> Names matter, at least to most people (I dare guess), and in particular
> when we rely on the names to convey missing information, as is the
> case with "length" given its (Haskell) type.
>
> Best,
>
> /Henrik
>
>
>
>
>
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