Trying to explain Haskell-style pure IO to a Python programmer, in the hope that I will understand it better myself (!).

pureio.py

#
# A crude demo of how IO can be pure yet achieve something.
# All functions are pure, including main(). Only __main__ has effects.
#
class IO:
    def __init__(self, f):
        self.f = f
    
    def run(self, x):
        return self.f(x)

    def bind(self, io):
        def f(x): io.run(self.run(x))
        return IO(f)

def printio(s):
    def f(_): print s
    return IO(f)

def hello():
    def f(x): print "Hello " + x
    return IO(f)

def readlineio(prompt):
    def f(_): return raw_input(prompt)
    return IO(f)

def main():
    return printio("Hello").bind(readlineio("What's your name?").bind(hello()))

if __name__ == "__main__":
    #main() # doesn't print anything
    main().run(None)