Constraints on definition of `length` should be strengthened

[Ben Franksen]

Am 07.04.2017 um 00:48 schrieb Nathan Bouscal:
> It *is* actually a useful instance and it is used in practice. It's not
> that better Haskell wouldn't have an biased pair type with these instances,
> it's that it would *also* have an unbiased one with the instances that such
> a type could support.

Nobody (I think) claimed that the biased type isn't useful. We merely
discuss whether it would not be more useful to define new types for that
with names that convey the intent, and leave (,) and Either unbiased as
their name (and special notation) suggests.

> The issue seems to be that people don't like the
> biased type having special syntax that wrongly gives an unknowing reader
> the impression that the type is unbiased. 

This is not the only reason, see above.

> This is a reasonable position,
> but getting rid of the tuple instances isn't a reasonable way to act on
> that position: 1) they're going to be defined anyway, 

Would you say the same for non-law-abiding instances for, say, class Monad?

> but also 2) it's not
> helpful to just pretend the type is unbiased when it isn't. 

We are used to pretend a lot in Haskell. We cannot capture all
properties in types, but we expect them to hold nevertheless. Are you
saying that this is bad because, well someone is going to come and
define a Monad instance that doesn't obey the laws anyway, so let's not
pretend the Monad laws hold?

> It would be
> coherent to argue for the removal of the special tuple syntax (though
> coherent doesn't mean reasonable; this would break everything), but it's
> not coherent to argue for crippling tuples so we can pretend they're
> something they aren't.

Pretending that a thing is actually something other than it really is is
the whole idea of high level languages. All data and code are just bits
in the end. You can enforce certain re-interpretations of these bits
using a type system. But what matters is that we intend a Char to be a
character and consciously avoid asking "what it really is" i.e. how it
is represented.

Cheers
Ben