Duplicate class attributes should probably get merged.

[bitemyapp]

In the spirit of "test first" methodology I made a breaking test first,

expected: "<foo class=\"first second third\">content</foo>\n"
        but got: "<foo class=\"first\" class=\"second\" class=\"third\">content</foo>\n"

When I was inspecting results in a browser from a client's application it looked more like <foo class="first">

Any hoodle, since class is a cdata-list it seems to me like the justification for space separated concatenation is there. I'd like to be able to programmatically inject additional classes into my markup using the *{...} syntax.

[bitemyapp]

This behavior would have some justification from past design intent:

https://www.yesodweb.com/blog/2011/03/hamlet-lucius-widgets

If you have a class=someclass and a .otherclass, Hamlet will automatically combine them into class="someclass otherclass".

[snoyberg]

After some review: it looks like this will require too much refactoring to make it work as described. This is a good change, and if someone wants to take a stab at it, I got some initial work on this in bcc1cbf.

[bitemyapp]

For posterity's sake in case anyone else needs quick fix, I injected the additional classes as space-separated text values in Hamlet using the normal #{...} syntax rather than using [(Text, Text)] and *{attrsVarName}.