state.py

# State is represented as (value, state)

returnST = lambda a: lambda s: (a, s)
# The 'ss' thing is some trick to get around the inability to assign in a
# lambda expression, and get around calculating x(s) twice
bindST = lambda x: lambda f: lambda s: \
        (lambda ss: runST(f(ss[0]))(ss[1]))(x(s))

runST = lambda f: lambda s: f(s)

getST = lambda s: (s, s)
putST = lambda a: lambda _: ((), a)


fun1 = bindST(getST)(lambda i: returnST(i * 2))
fun2 = bindST(fun1)(lambda i: putST(i))

print runST(fun2)(10)
# Returns ((), 20)