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

[Olaf Klinke]

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