from math import sqrt,sin;
import functools as ft;

#------------------------------------------------------------------------------#

def unit(x):
    return x;

def bind(x,f):
    if x==None:
        return None;
    else:
        return f(x);
# end def

#------------------------------------------------------------------------------#

def fSqrt(x):
    if x<0: return None;
    else:   return sqrt(x);

mSqare=ft.partial(bind,f=fSqrt);
mSin=ft.partial(bind,f=sin);

x=4;
y=mSqare(x);
assert y==2.0
    
x=-1;
y=mSqare(x);
assert y==None;

x=-1;
y=mSqare(x);
y=mSin(y);
assert y==None;