I implemented the Maybe monad using template specialization. Feedback welcome!

https://bitbucket.org/qznc/d-monad/src/ba39f2551af0e2e90a40653af92c048fed519a18/generic.d?at=master

However, a few questions or possibly shortcomings of D came up.

1. I could not specify the inheritance between the templates. A Monad is an Applicative, which is a Functor. Is there some template magic to check "There is a template like this 'Functor!Maybe'"?

2. A template cannot restrict template specializations. Specifically, the Functor(T) template should restrict Functor(T:Maybe), such that it must declare functions or templates of a specific signature.

3. Shouldn't a function be a delegate? I assume this question came up before, but there seems to be interest to make it easier. My code specifies function parameters three times. It should be a delegate and the caller should use std.functional.toDelegate to convert it. Why not an implicit cast? Performance seems to be the only problem.