def slice(self, start: int, length: int, span: Span) -> 'Array':
if start + length > len(self.elements):
raise errors.FailureError(
span=span,
message=
'Start {} with length {} out of bounds of array length {}.'.
format(start, length, len(self.elements)))
return Array(tuple(self.elements[start:start + length]))
def update(self, i: int, v: Value, span: Span) -> 'Array':
if i >= len(self.elements):
raise errors.FailureError(
span=span,
message='Index {} out of bounds of array length {}.'.format(
i, len(self.elements)))
return Array(
tuple(v if i == j else e for j, e in enumerate(self.elements)))