Playing (informally) with the Either monad in Python

chain.py

# Rationale: How to chain functions with type (a -> transformation_of_a) #
#            as they were (a -> a)?
#
# If the functions were "normal" (a -> a), we'd write simply: fun3(fun2(fun1(1)))

def fun1(x):
  return ("ok", 1*x) if x > 1 else ("fail", "fun1: value of x is ugly: %s" % x) 

def fun2(x):
  return ("ok", 2*x) if x > 2 else ("fail", "fun2: value of x is ugly: %s" % x) 
    
def fun3(x):
  return ("ok", 3*x) if x > 7 else ("fail", "fun3: value of x is ugly: %s" % x)

# Option1: Wrapper

def w(func, value):
  if value[0] == "ok":
    return func(value[1])
  elif value[0] == "fail":
    return value
  else:
    raise ValueError(value)
   
print(w(fun3, w(fun2, fun1(1))))    
print(w(fun3, w(fun2, fun1(2))))
print(w(fun3, w(fun2, fun1(3))))
print(w(fun3, w(fun2, fun1(4))))

# Option1b: Create wrapper chain.

def reduce_w(funcs, initial_value):
    head, *tail = funcs
    value = head(initial_value)
    for func in tail:
      value = w(func, value)
    return value

print("using reduce_w:", reduce_w([fun1, fun2, fun3], 2)) 

# Option2: Lift functions

def lift(func):
    def _wrapper(value):
        return w(func, value)
    return _wrapper
    
print("using lifting:", lift(fun3)(lift(fun2)(fun1(2))))

# Option2b: Use lifted functions and start by "lifting" the initial value (like Haskell's "return")

lfun1, lfun2, lfun3 = map(lift, [fun1, fun2, fun3])

def prepare(value):
    return ("ok", value)

print("using lifting 2:", lfun3(lfun2(lfun1(prepare(2)))))

# ('fail', 'fun1: value of x is ugly: 1')
# ('fail', 'fun2: value of x is ugly: 2')
# ('fail', 'fun3: value of x is ugly: 6')
# ('ok', 24)
# using reduce_w: ('fail', 'fun2: value of x is ugly: 2')
# using lifting: ('fail', 'fun2: value of x is ugly: 2')
# using lifting 2: ('fail', 'fun2: value of x is ugly: 2')