[Haskell-cafe] template haskell library for OO-like selector syntax?

[Dennis Raddle]

On Tue, May 15, 2018 at 7:27 AM, Mig Mit <migmit at gmail.com> wrote:

> Dennis probably means that when in imperative language you'd write
>
> employee.department.manager.salary += 1000
>
> in vanilla Haskell it would be like
>
> newSalary = employee.department.manager.salary + 1000
> newManager = employee.department.manager {salary = newSalary}
> newDepartment = employee.department {manager = newManager}
> newEmployee = employee {department = newDepartment}
>
> Lazyness have nothing to do with it. This is actually where lenses usually
> come in, although I'd argue that the data should rather be restructured.
>
>
That's what I mean.

In what sense can I restructure the data? My current application has lots
of "composite" data... conceptually, "things" each of which hold a lot of
"other things" each of which hold a lot of "still other things". And
sometimes I want to update a single one of those "still other things".

Lenses seem like a good solution, but if you have a better idea, let me
know.

D





>
> On 2018. May 15., at 16:01, Vanessa McHale <vanessa.mchale at iohk.io> wrote:
>
> >In Haskell, if using immutable data, this would requiring doing a data
> update on every level
>
> I'm not sure what you mean but in either case it doesn't have to do with
> immutability. Haskell is lazy, so your updates will only happen if the
> field is later accessed. The other fields in the record will only be
> updated if they A) change and B) are later accessed. If you just updated
> one field of a record the other fields will still have pointers to the old
> data.
>
> Otherwise, I agree with Matt's answer. Lenses handle this problem nicely
> :)
> On 05/14/2018 07:41 PM, Dennis Raddle wrote:
>
> In my current application, I'm really missing the ability from OO to do
> things like
>
> someData.field1.field2 = "foo"
>
> What's do I mean?
>
> - field selectors are scoped so that names don't clash with each other or
> global names
>
> - this way of selecting data can be used both for reading it, and for
> updating specific fields of some data deep in a data hierarchy (In Haskell,
> if using immutable data, this would requiring doing a data update on every
> level).
>
> Any attempt to do this in Haskell, for me anyway, results in an explosion
> of names and a lot of boilerplate code and a resulting heavy syntax.
>
> Is there a way of obtaining OO-like concise syntax, perhaps with Template
> Haskell?
>
> Dennis
>
>
>
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> --
>
>
>
> *Vanessa McHale*
> Functional Compiler Engineer | Chicago, IL
>
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