Regarding heist, I've got a template such as:
<script>
var json = ???;
</script>
<h1>Example</h1>
Is there a way to substitute the ??? string with another string?
I think the following function might be the solution https://hackage.haskell.org/package/heist-1.0.1.0/docs/Heist-Splices-Json.html#v:bindJson but I have difficulty understanding that function and or what markup to use in the template.
No. You cannot substitute anything inside a <script> tag because the text inside a script tag is not treated as HTML. It's treated as plain text. If this was not done, you wouldn't be able to write JS code like if ( x < 42 ) because the less than would get treated as the beginning of a tag.