class T::Types::FixedHash

in the list.
Takes a hash of types. Validates each item in a hash using the type in the same position

def describe_obj(obj)

@override Base

"Expected {a: String}, got Hash".
instead of
"Expected {a: String}, got {a: TrueClass}"
This gives us better errors, e.g.: 
def describe_obj(obj)
  if obj.is_a?(Hash)
    "type #{serialize_hash(obj.transform_values(&:class))}"
  else
    super
  end
end

def initialize(types) 

def initialize(types)
  @types = types.transform_values {|v| T::Utils.coerce(v)}
end

def name

@override Base 
def name
  serialize_hash(@types)
end

def recursively_valid?(obj)

@override Base 
def recursively_valid?(obj)
  return false unless obj.is_a?(Hash)
  return false if @types.any? {|key, type| !type.recursively_valid?(obj[key])}
  return false if obj.any? {|key, _| !@types[key]}
  true
end

def serialize_hash(hash) 

def serialize_hash(hash)
  entries = hash.map do |(k, v)|
    if Symbol === k && ":#{k}" == k.inspect
      "#{k}: #{v}"
    else
      "#{k.inspect} => #{v}"
    end
  end
  "{#{entries.join(', ')}}"
end

def subtype_of_single?(other)

@override Base 
def subtype_of_single?(other)
ther
ixedHash
ing `subtype_of?` here instead of == would be unsound
es == other.types
e

def valid?(obj)

@override Base 
def valid?(obj)
  return false unless obj.is_a?(Hash)
  return false if @types.any? {|key, type| !type.valid?(obj[key])}
  return false if obj.any? {|key, _| !@types[key]}
  true
end