[Chart] data-accessor, lens, and Chart

[Ben Gamari]

    tl;dr: I like lens and ported Chart to use it for the following reasons,

     * data-accessor has an annoying namespace conflict with stateref
     * lens provides more functionality and is therefore more worth my time learning
     * lens provides (IMHO) better error messages
     * lens is more likely to enjoy better long-term support
     * lens doesn't require an odd composition operator
     * I already use lens in my code

    do others think this is a worthwhile change in Chart (after perhaps a
    major version bump) or should this constitute a new package?



I'll admit, I've never really felt comfortable with data-accessor.

Certainly, it's a fine package and has played an important role in the
evolution of lens-like constructs in Haskell. That being said, it has
never really gained enough mindshare to be worth learning and using
myself. This may be in part due to the fact that it is a fairly quirky
piece of code. It shares a namespace with a module from stateref (a
fairly consistent point of friction), requires unfamiliar composition
operators, and features just enough functionality to make it useful yet
too few to be compelling for one-off use. This is compounded by its less
than stellar error messages and further by the unhelpful type signatures
produced by its TH-generated accessors (e.g. accessors are identified by
the type T).

Recently, I've began playing around with lens. In contrast to
data-accessor, to me lens feels worth using. Far more than a data
accessor library, it can encode concepts I previously struggled
to express. For this reason, lens use has slowly crept in to my everyday
code (although I still only use a fraction of its namespace). The fact
that it provides nice data accessors is almost a secondary concern.

Recently I've wondered how much effort would be required to port Chart
to lens. Needing an mechanical project to wake up to, I gave it a try[1]
this morning. Perhaps unsurprisingly, the changes were all quite
straightforward (almost entirely accomplished with regular expressions
shuffling around underscores). User code was even easier to port,
requiring in most cases s/^:/./ and s/^=/.~/

I would be happy to see these changes make their way upstream. That
being said, lens is heavier dependency than data-accessor which may
scare some away (although it seems pretty likely that lens will become
ubiquitous before long). Given the nature of the change, it would be
best if it were accompanied by a major version bump.

What do others think?

Cheers,

- Ben


 [1] https://github.com/bgamari/haskell-chart/tree/lens