mcheck: check monad for Python

In Haskell terms: Either String a

Allows you to build pipelines of checks on values. For example

import mcheck as c

pipeline = c.compose_many(
  c.lift(lambda x: x > 0, 'Value has to be positive.'),
  c.lift(lambda x: x < 100, 'Value has to be smaller than 100.'),
  c.lift(lambda x: not x & 1, 'Value has to be even.'),
  c.lift(lambda x: not x & (x - 1), 'Value has to be a power of two.')
)

result = pipeline(34)
print(result.message)

will result in the output Value has to be a power of two..

Definition

In terms of (pseudo-)Haskell, we could define Check as follows:

data Check a = Pass a | Fail String 

instance Monad Check where
    return x = Pass x  -- is mcheck.return_

    Fail msg >>= f = Fail msg  -- is mcheck.bind
    Pass x >>= f = f x
    
    fail x = Fail x  -- is not implemented

we also have Kleisli composition >=> which is defined as

(>=>) :: Monad m => (a -> m b) -> (b -> m c) -> a -> m c

and implemented through mcheck.compose and the convenience function mcheck.compose_many which takes an arbitrary number of functions (a -> m b) and folds that list using Kleisli composition.

And then there is mcheck.lift which is technically not liftM (or fmap) which is defined as

liftM :: Monad m => (a -> b) -> m a -> m b

while mcheck.lift is defined as

lift :: (a -> bool) -> String -> (a -> Check a)

so maybe lift is not such a good name after all…