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

Olaf Klinke olf at aatal-apotheke.de
Tue Mar 14 21:03:47 UTC 2023

> > From briefly reading the wiki page
> https://wiki.haskell.org/Tangible_Value, it looks like it relies on
> Applicative functors for combining values.
> So maybe the missing piece for sum-types was Selective functors
> https://hackage.haskell.org/package/selective?
> Gustavo Bicalho

Well spotted! I did not know selective applicative functors (SAFs),
thanks for mentioning! Indeed `branch` seems to be all that is
necessary to get forms for sum types going. In the talk [1] Andrey
mentions that SAFs have a bind, but only for enumerable inputs. 
The papers state that "all effects are declared statically". If we
equate "effect" with "form input" then for recursive types like lists
this would mean to pre-define an infinite number of fields and have the
user select a finite number of them. I wonder whether that is feasible
with html forms. 
Maybe forms (in the html sense) are a red herring for what I am after.
Both the Grace and tangible values demos immediately process changes to
inputs, there is no distinction between inputting the data and having
the data processed: they are event-driven systems. As is JavaScript
with its `oninput` and `onchange` functions. 
So there are two sub-problems here: The first problem is to
programmatically generate html elements by JavaScript functions that
can build all finite values of a given Generic Haskell type, and
nothing else. The second problem is to scrape the thusly generated
elements and re-assemble them into a Haskell value on the server side.
The second problem is usually solved by html forms, but there may be
other means. 


[1] https://dl.acm.org/doi/abs/10.1145/3341694#

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