Created
April 13, 2015 10:50
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module Peano | |
O = [].freeze | |
def succ(n) | |
n | [n] | |
end | |
def pred_rec(n, p, q) | |
if n == p | |
q | |
else | |
pred_rec(n, succ(p), succ(q)) | |
end | |
end | |
def pred(n) | |
if n == O | |
O | |
else | |
pred_rec(n, succ(O), O) | |
end | |
end | |
def add(x, y) | |
if y == O | |
x | |
else | |
succ(add(x, pred(y))) | |
end | |
end | |
end | |
class Array | |
def to_int | |
if self == Peano::O | |
0 | |
else | |
1 + Peano.pred(self).to_int | |
end | |
end | |
end | |
class Integer | |
def to_peano | |
if self == 0 | |
Peano::O | |
else | |
Peano.succ((self - 1).to_peano) | |
end | |
end | |
end | |
include Peano | |
p O.to_int | |
# ==> 0 | |
p succ(O).to_int | |
# ==> 1 | |
p succ(succ(O)).to_int | |
# ==> 2 | |
p pred(succ(succ(O))).to_int | |
# ==> 1 | |
p pred(add(10.to_peano, 3.to_peano)).to_int | |
# ==> 12 |
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