[mnarayan01]

Then you'd need an extra set of delimiters, e.g.:

  sig {{name: String, returns: Integer}}

[BenMorganIO]

But that would make the hash braces redundant so you could just use parenthesis.

[mnarayan01]

That's a block returning a Hash; see bhuga's sibling comment where he notes that they're using blocks to lazy load the constants in the type definition, which may seem silly for e.g. Integer, but consider e.g. some high-dependency Rails model which requires auto-loading 10,000 other classes.