Extending mkYesodSubData

[jprider63]

For background, please see this post on the google group. To solve the type variable not being in scope problem, I've deviated from my original plan of using a &{} syntax. Instead I've:

• Updated mkYesodGeneral so that it can accept type variables as well. For example, you can now do: mkYesodSubData "MySubsite master". This also fixed a non-exhaustive pattern matching bug.
• Updated ttToType so that type variables can be parsed in routes. This now can look something like: /#{MyDataId master} MyHandlerR GET
• Added contexts as an argument to mkYesodGeneral'. These contexts are added to the instances generated. When there are contexts, I switched to use standalone deriving instead of regular deriving since I was running into this issue.

So far, it seems like everything is working as I'd like. You can see what I currently have on my vars branch. I don't think there are any breaking changes (I'm not sure if every template-haskell version works properly though).

I'd like to make a few more changes.

• Implement a context parser so that we can just write mkYesodSubData "(MyClass master) => MySubsite master" instead of passing in a manually constructed Cxt.
• Change the [Either String [String]] to [String] in mkYesodGeneral since the context parser would subsume this functionality.

Do you have any thoughts before I continue?

[jprider63]

I just created PR #1366. This implements the context parser and everything mentioned above it. There shouldn't be any breaking changes.

I didn't change mkYesodGeneral since this would be a breaking change with mkYesodWith exported. I can create a separate pull request if you'd like to save this change for the next major version bump.