Feature idea: Hashie::Bash - the Builder Hash

[maxlinc]

I'm working on a Swagger library for ruby. I wanted a Swagger::Builder so projects like grape-swagger could walk the grape DSL and build a swagger object.

I'd already described the Swagger structure as a Dash so I could load complete Swagger files. I wanted to be able to use the same definition in order to build a Swagger object, but the required constraints would get in my way, forcing me to work with a normal Hash or Mash until the object was complete.

I implemented an experimental Bash mixin so I could get around that limitation:

A Bash is a 'builder' Hash that can be used to build a

# Dash ('defined' or 'discrete' Hash). It provides a
# `build` method that turns the Bash into a Dash. It enforces
# the same constraints and coercions as a Dash except for 'required' properties,
# which are not enforced until converting to a Dash via `build`.

See the Swagger::Builder spec for an example of how a Bash-based builder would work.

This let's me keep the "discrete" behavior of a Dash - so I get immediate errors if I try to use set an undefined property. Type coercions also happen immediately, and I'll get a Hashie:: CoercionError if I use the wrong type. These errors will happen on the line where I try to make the assignment, rather than much later when I'm converting a Hash to a Dash. I don't get the property 'x' is required errors until I call swagger_builder.build, but since that's an error of omission it wouldn't have a line number anyways.

The implementation has the advantage of being self-contained - it doesn't require changes to any Hashie classes (and doesn't monkeypatch them) but it achieves this through fragile and complex metaprogramming that dynamically subclasses Dash objects. I'm creating an issue rather than a PR in the hopes that someone has an idea for a simpler implementation that updates Dash to be more Bash friendly.

[dblock]

I'll take a look, in the meantime I have to say that I love the "it achieves this through fragile and complex metaprogramming" :)