abstract_requirements.cr

module Moo
  # Moo expects `foo` to return an Int32
  abstract def foo : Int32

  # `bar` adds 1 to it
  def bar : Int32
    foo + 1
  end
  
  # And maybe it provides other methods...
  def bar2
    42
  end
end

# MooImpl decides inovking "foo" is an error. A real example
# is an IO which is read-only and raises in the
# `write(slice, count)` method.
class MooImpl
  def foo
    raise "Not implemented"
  end
end

# The above will make `bar` have the type `NoReturn`, but it should still compile,
# even though NoReturn != Int32.