[Haskell-cafe] dynamic web form generation for algebraic data types

[Olaf Klinke]

On Thu, 2023-03-02 at 07:34 -0800, Ivan Perez wrote:
> All of this reminds me of Conal Elliott's Tangible Values.
> 
> http://conal.net/papers/Eros/
> 
> (Not for web but the idea still applies.)
> 
> Ivan

Yes, the concept has been re-invented multiple times, as is the case
with most good ideas. TVs are general enough to bolt a web interface on
top, I guess. 

Seems that Conal, too, has not solved the sum type problem. 
The tangible value GADT 
https://hackage.haskell.org/package/TV-0.5.0/docs/Interface-TV-Input.html
has a Pair constructor but not for Sum. I wonder whether there is a
deeper categorical reason for this. 

So I should re-phrase my question: 
Has anyone extended tangible values to Generic types and given them a
modern html interface? 

Olaf

> 
> On Thu, 2 Mar 2023 at 07:02, Olaf Klinke <olf at aatal-apotheke.de> wrote:
> 
> > On Thu, 2023-03-02 at 15:51 +0200, Georgi Lyubenov wrote:
> > > Hey Olaf,
> > > 
> > > This is not an answer to your question, but I was reminded of Grace[0],
> > > which is a language
> > > with a "browser"[1] that allows you to type in terms and get back
> > > webpages based on those terms
> > > "automagically", which sounds like exactly what you need. I don't know
> > > how it's implemented, so I don't know if
> > > it is actually relevant to you, but it is worth noting that Grace itself
> > > is implemented in Haskell.
> > > 
> > > Cheers,
> > > Georgi
> > > 
> > > [0] https://github.com/Gabriella439/grace
> > > [1] https://trygrace.dev/
> > > 
> > > On 3/2/23 12:54, Olaf Klinke wrote:
> > > > Dear Cafe,
> > > > 
> > > > has anyone ever attempted (and maybe succeeded) in building dynamic
> > > > forms using one of the Haskell web frameworks (Yesod,Servant,...)?
> > > > By "dynamic" form I mean a form that changes the number of fields based
> > > > on selections the user makes in other fields of the form.
> > > > 
> > > > For example, say one has an algebraic data type
> > > > 
> > > >      data T = Number Int | Check Bool T
> > > > 
> > > > A form for such a type would initially consist of radio buttons or a
> > > > drop-down list with options "Number" and "Check" that lets the user
> > > > select the constructor. When "Number" is selected, an <input
> > > > type="number"> field is shown. When "Check" is selected, an <input
> > > > type="checkbox"> is displayed next to another form of this kind.
> > > > 
> > > > In the end, one would use the GHC.Generics machinery to generate forms
> > > > for a wide range of algebraic data types. I've seen this in the Clean
> > > > language's iTask library [1] and it's very convenient.
> > > > Of course this would involve a lot of JavaScript like
> > > > document.createElement() as well as book-keeping how to re-asseble the
> > > > fields into a T value upon submission. At least the latter is already
> > > > handled by libraries such as yesod-form.
> > > > 
> > > > Olaf
> > > > 
> > > > [1] https://cloogle.org/src/#itasks/iTasks/UI/Editor/Generic
> > > > [2] https://github.com/haskell-servant/servant-swagger/issues/80
> > > > 
> > > > 
> > 
> > Thanks for the pointer!
> > 
> > The Grace README says under Notable Omissions:
> > 
> > > Recursion or recursive data types
> > > 
> > > Grace only supports two built-in recursive types, which are List and
> > > JSON, but does not support user-defined recursion or anonymous
> > > recursion.
> > > 
> > > User-defined datatypes
> > > 
> > > All data types in Grace are anonymous (e.g. anonymous records and
> > > anonymous unions), and there is no concept of a data declaration
> > 
> > The tutorial shows how Grace function inputs are mapped to forms, where
> > functions with List input indeed have a form that is "dynamic" in the
> > sense I defined. Otherwise there is only one input field per function
> > argument. That means complex types are to be input in JSON syntax and
> > parsed.
> > Instead of a DSL, I'd prefer a shallow embedding into Haskell, so that
> > one can leverage all the available machinery.
> > Yet Grace already goes a long way towards what I am after.
> > 
> > Olaf
> > 
> > 
> > 
> > 
> > 
> > 
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