compacted variables V can be Attribute or Element

[Skyfold]

I wanted to use compact HTML for a form, but could not find a way to set the checked attribute dynamically. Attribute that take an argument already work nicely with V, but CheckedA and other boolean Attribute don't have an argument to fill dynamically.

In other words, you are left with trying to write:

form = Input :@ V @"checked"

This will not type check, nor should it type check. That V @"checked" could be anything with a Convert instance which may not result in valid html.

Still, I wanted to use compact HTML and I wanted to ensure V @"checked" was a valid Attribute for Input. So, I changed:

data Put (n :: Symbol) = forall a. Convert a => Put a
data V (name :: Symbol) = V

Into:

data Put (n :: Symbol) (a :: Type) = Convert a => Put a
data V (name :: Symbol) (t :: Type) = V

and added a new rule for the Lawful type family

  Lawful relation x (V _ t) = Lawful relation x t

Now I could write

form = Input :@ (V @"checked" @(Maybe CheckedA))

and be sure that CheckedA was a valid attribute in this context.

At the same time, I am now sure that when rendering only a Maybe CheckedA gets put in that variable:

renderCompactString (compactHTML form) (Put @"checked" (Just CheckedA))

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One nice addition to this implementation is that you do not have to specify the type:

renderCompactString (compactHTML $ Input :@ (V @"test")) (Put @"test" (Just CheckedA))

will happily type check. In other words, you may not need to fix existing code.

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Also, I added type synonyms for all Attribute and Element to make specifying the type cleaner.

[Skyfold]

TODO:

1. Needs update to README to explain how compact HTML has changed
2. Needs examples added to Haddock (would adding doctest be ok?)
3. Needs a new performance check to see how inserting blocks of html affects speed
4. Could use a few tests