[web-devel] Updates to Julius interpolation

[Michael Snoyman]

Hi all,

Following some discussions on the issue both recently and many months ago,
I've just released some updates to how Julius interpolation works. The
basic idea is that we want to make interpolation of properly escaped JSON
data the default, and the interpolation of unchecked Javascript code the
exception to the rule. Concretely, take the following Julius template:

    let greeting = "Say \"Hello\"" :: String
     in [julius|alert("#{greeting}")|]

In Julius 1.0, greeting is interpolated directly into your Javascript
template, resulting in the output:

    alert("Say "Hello"")

Importantly, the double quotes are *not* escaped. Instead, we would like
`greeting` to be treated as JSON, the double quotes within the text to be
escaped, and the surrounding double quotes to be automatically added. I've
just released versions 1.0.2 and 1.1 of shakespeare-js (the package
providing Julius), and it is now valid to say:

    let greeting = "Say \"Hello\"" :: String
     in [julius|alert(#{toJSON greeting})|]

Resulting in:

    alert("Say \"Hello\"")

What if you really want to spit out raw Javascript code? You can do so via
the `rawJS` function:

    let somecode = "alert(\"Hello\")" :: String
     in [julius|#{rawJS somecode}|]

I've set up this release system to try and maintain backwards
compatibility, while simultaneously making it a compile-time error when you
misuse the library. For the former, I've released shakespeare-js 1.0.2,
which keeps the same interpolation semantics as 1.0.1, but adds in the
rawJS function. You can safely add `rawJS` to virtually any existing #{}
interpolation in Julius without any change in behavior.

The difference with version 1.1 is that the old interpolations will no
longer work. Under the surface, this is because I've removed the
ToJavascript instances for String and Text. You can now only interpolate
JSON values (by using toJSON) or raw Javascript (by using rawJS).

Down the road, it will likely make sense to add in ToJavascript instances
for common types which first convert to JSON. However, doing so in this
first release would mean that old code would silently change its semantics.
Hopefully you will now be able to upgrade to 1.0.2 without any breakage,
and in the near future upgrade to 1.1 and get error messages where you need
to insert rawJS calls.

I am not yet updating the yesod-platform to use the new versions of these
packages; I'd like to ask people to test things out and report any issues
they run into so that we can make a properly stable yesod-platform release
soon.

Thanks,
Michael
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