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

On Tue, May 15, 2018 at 7:27 AM, Mig Mit 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 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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