def extend(head, tail):
'''Return the concatenation of two list terms.'''
if types.isNil(head):
return tail
if types.isNil(tail):
return head
factory = tail.factory
for elm in reversed(head):
tail = factory.makeCons(elm, tail)
return tail
def length(term):
'''Length of a list term.'''
length = 0
while not types.isNil(term):
assert types.isCons(term)
length += 1
term = term.tail
return length
def fromTerm(cls, trm):
res = []
tail = trm
while not types.isNil(tail):
if not types.isCons(tail):
raise ValueError('bad path', trm)
idx = tail.head
if not types.isInt(idx):
raise ValueError('bad index', idx)
res.append(idx.value)
tail = tail.tail
res.reverse()
return cls(res)
def item(term, index):
'''Get item at given index of a list term.'''
if index < 0:
raise IndexError('index out of bounds')
while True:
if types.isNil(term):
raise IndexError('index out of bounds')
if not types.isCons(term):
raise TypeError('not a list term', term)
if index == 0:
return term.head
index -= 1
term = term.tail
def empty(term):
'''Whether a list term is empty or not.'''
return types.isNil(term)