[Haskell-cafe] ANNOUNCE: partial-lens 0.0.1

[roconnor at theorem.ca]

On Wed, 21 Dec 2011, Erik Hesselink wrote:

> How does this relate to the Maybe lenses in fclabels [1]?
>
> Erik
>
> [1] http://hackage.haskell.org/packages/archive/fclabels/1.0.4/doc/html/Data-Label-Maybe.html

It appears to be somewhere between similar and the same.

*** Comparison of API

Data.Label.Maybe.get corresponds to Data.Lens.Partial.getPL

Data.Label.Maybe.set roughly corresponds to Data.Lens.Partial.trySetPL 
except that trySetPL will bail out early if the reference is null.  We can 
match the signature of set more precisely by:

Data.Label.Maybe.set l v r ~ Data.Lens.Partial.trySetPL l r <*> pure v

Data.Label.Maybe.modify would correspond to Data.Lens.Partial.tryModPL if 
I had implemented it ... which maybe I ought to.

Data.Label.Maybe.embed corresponds to a composition of totalLens and 
maybeLens.  More specifically

Data.Label.Maybe.embed l ~ Data.Lens.Partial.maybeLens . Data.Lens.Partial.totalLens l

Data.Label.MaybeM.gets roughly corresponds to 
Data.Lens.Partial.Lazy.accessPlus except that accessPlus is particular to 
StateT because partial-lens is a Haskell 98 compliant package.  I need to 
write partial-lens-fd which will contain a function precisely 
corresponding to Data.Label.MaybeM.gets

I don't have Data.Label.MaybeM.asks, because there was no corresponding 
functionality in data-lens.  We should probably add a version of this.

*** Comparison of representation

The usual differences between data-lens and fclabels applies to 
partial-lens as well.  The representation for data-lens and 
partial-lens allows modify to be done with one case analysis on a record 
since the getter and setters are combined into one coalgebra whereas in 
fclabels two case analysis must be done: one for the getter and one for 
the setter.  When chains of lenses are composed, I'm told the differences 
become more apparent.

In partial-lens, the combination of getter and setter into a single 
coalgebraic operations means that the getter and setter are statically 
forced to return Nothing on the same record; but this is not enforced with 
the fclabels representation.

That said, perhaps the MaybeLens from fclabels is trying to do something 
different.  I don't know what laws you expect to hold for the getter and 
setters of a maybe lens since it isn't documented (actually I appear to 
have also forgotten to document the coalgebra laws for a comonad in my 
package) so perhaps MaybeLens are intended to be more general than partial 
lenses.

For example maybe a user wants to make it illegal to set the birth date to 
be greater than the death date in a record.  In this case getting the 
birth date will succeed, but setting will fail if the provided birth date 
out of bounds.  This is possible to write using MaybeLens, but is 
impossible with partial lenses since with partial-lenses either the 
reference is null, meaning getting and setting both fail, or it is not 
null which means that getting and setting both succeed.

-- 
Russell O'Connor                                      <http://r6.ca/>
``All talk about `theft,''' the general counsel of the American Graphophone
Company wrote, ``is the merest claptrap, for there exists no property in
ideas musical, literary or artistic, except as defined by statute.''