Add Parameter Type Validation

[dougblack]

Think of a way to do automatic type validation on parameters and 400 when invalid.

[marksteve]

I particularly like how RequestParser in Flask-Restful works. Maybe a similar api would work here too?

[joslinm]

I don't see this enough in APIs. Two projects that might be of interest to you:

• Params::Validate
• Sinatra Param

This sounds like a fun feature. I'll work a little on this issue and submit a PR when/if I think I got something.