Another example are non-empty containers, e.g. for `NonEmpty` one cannot have
a total `fromList :: [a] -> NonEmpty a`.
Regards,
Marcin
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‐‐‐ Original Message ‐‐‐
On Sunday, September 19th, 2021 at 08:49, David Feuer
wrote:
> No, fromList is too much. Consider
>
> data Foo a = Foo (IORef String) [a] deriving Foldable
>
> What IORef should fromList use?
>
> On Sun, Sep 19, 2021, 2:44 AM Anthony Clayden
> wrote:
>
> > (Moving this discussion to glasgow-users. It's just not appropriate on the
> > cafe.)
> >
> > > I am no longer a novice, and yet would still have a hard time making any
> > >use of the laws as written in constructing instances. Instead, I'd ignore
> > >the laws and write a natural intuitive instance, and it would invariably
> > >work.
> >
> > Seems my approach is very similar to Viktor's. My (very informal)
> > understanding of the Laws looks nothing like the docos. I regard Foldable
> > structures as merely more efficient ways to hold a List. Then I expect
> > 'moral equivalences':
> >
> > > toList . fromList ~=~ id -- going via the Foldable structure> fromList .
> > > toList ~=~ id> toList ~=~ foldr (:) []
> >
> > But those aren't equalities. 'moral equivalence' means the Lists have the
> > same elements, not necessarily in the same order; the structures have the
> > same elements but possibly in a different arrangement -- that is, in the
> > `Tree` example, there might be `Empty` scattered about, and elements held
> > variously in `Leaf`s vs `Node`s. So more accurately:
> >
> > > fromList . toList . fromList === fromList -- i.e. there's a 'canonical'
> > > arrangement> toList . fromList . toList === toList -- i.e. there's a
> > > 'canonical' List ordering
> >
> > (That triple-journey business is a similar style to defining Lattice
> > pseudocomplements https://en.wikipedia.org/wiki/Pseudocomplement#Properties
> > -- if I can chuck in some math theory.)
> >
> > I'd expect all other methods to be one of: `reduceStuff === reduceStuff .
> > toList` or `mapStuff === fromList . mapStuff . toList`.
> >
> > But! there's no method `fromList` in Foldable. Why not?/please explain.
> > (Are there Foldable structures which we can't load from a List? At least
> > assuming the List is finite.) `fromList` is the first thing I write after
> > declaring the datatype, so I can easily load up some test data. There is
> > one example `fromList` in the doco. Is that not generalisable? `foldMap
> > Leaf` would be brutal, but should work? `foldMap singleton` ? (But there's
> > no method `singleton`.)
> >
> > ___
> >
> > Glasgow-haskell-users mailing list
> >
> > Glasgow-haskell-users@haskell.org
> >
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users
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