Another example are non-empty containers, e.g. for `NonEmpty` one cannot have a total `fromList :: [a] -> NonEmpty a`.
Regards, Marcin Sent with ProtonMail Secure Email. ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐ On Sunday, September 19th, 2021 at 08:49, David Feuer <david.fe...@gmail.com> 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 <anthony.d.clay...@gmail.com> > 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 > > > > Glasgowfirstname.lastname@example.org > > > > http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users
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