1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
|
-- Half of a natural number:
import Test.Prop
data Peano = O | S Peano
deriving (Eq,Show)
toPeano :: Int -> Peano
toPeano n = if n==0 then O else S (toPeano (n-1))
fromPeano :: Peano -> Int
fromPeano O = 0
fromPeano (S p) = 1 + fromPeano p
add :: Peano -> Peano -> Peano
add O p = p
add (S p) q = S (add p q)
half :: Peano -> Peano
half y | add x x === y
= x
where x free
main :: Int
main = fromPeano (half (toPeano 100))
test_half = fromPeano (half (toPeano 100)) -=- 50
|
