Imperative OOP Ceremony: do-Notation Pt. 1

1 Apr 2015 Stephan Schneider View Markdown source

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Sections

Method Cascades

In the last tutorial we saw how Programmable Fluent Interfaces can be implemented and that they are referred to as Monads in FP lingo. We will now start to use Haskell's built-in Monad type class instead of our own Fluent interface...

import Prelude hiding ((.))
x.f = f x

data Shape a = Circle { radius :: a } deriving (Show)

instance Monad Shape where
  object >>= remake = remake(object.radius)
  return = Circle

...but we will keep on using our own method synonyms bind and make.

bind = flip(>>=)
make = return

Everything should be working as before.

import Control.Monad
import Control.Applicative
import Data.Functor
import Prelude hiding ((.))
x.f = f x

data Shape a = Circle { radius :: a } deriving (Show)
instance Functor Shape where { fmap = liftM }
instance Applicative Shape where { pure = return; (<*>) = ap }
  
instance Monad Shape where
  object >>= remake = remake(object.radius)
  return = Circle

bind = flip(>>=)
make = return

-- show
doubleShapeArea(r) = make(r * sqrt(2))

circ = make(2) :: Shape(Double)

main = do {
  print(circ);
  print(circ.bind(doubleShapeArea));
}
-- /show

Let's add a redundant .bind(make) to the method chain (it trivially returns its argument) and wrap both doubleShapeArea and make in closures in order to reflect the values being passed through the method chain.

import Control.Monad
import Control.Applicative
import Data.Functor
import Prelude hiding ((.))
x.f = f x

data Shape a = Circle { radius :: a } deriving (Show)
instance Functor Shape where { fmap = liftM }
instance Applicative Shape where { pure = return; (<*>) = ap }
instance Monad Shape where
  object >>= remake = remake(object.radius)
  return = Circle

bind = flip(>>=)
make = return

area(Circle r) = r^2 * pi

doubleShapeArea(r) = make(r * sqrt(2))

circ = make(2) :: Shape(Double)

-- show
main = do {
  print(circ.bind(doubleShapeArea).bind(make));
  print(circ.bind(\(a) -> a.doubleShapeArea).bind(\(b) -> make(b)));
}
-- /show

As we can imagine, a is bound to circ.radius, i.e. 2.0, and b is bound to radius of a Circle object with twice as much area as circ, i.e. a radius of 2.8284271247461903. Both a and b exist only inside the closures \(a) -> a.doubleShapeArea and \(b) -> b.make.

But, if we chose to bind \(b) -> b.make to doubleShapeArea inside the closure \(a) -> a.doubleShapeArea, a remains available to make and we can apply make to a instead of b, thereby ignoring doubleShapeArea's return value.

import Control.Monad
import Control.Applicative
import Data.Functor
import Prelude hiding ((.))
x.f = f x

data Shape a = Circle { radius :: a } deriving (Show)
instance Functor Shape where { fmap = liftM }
instance Applicative Shape where { pure = return; (<*>) = ap }
instance Monad Shape where
  object >>= remake = remake(object.radius)
  return = Circle

bind = flip(>>=)
make = return

area(Circle r) = r^2 * pi

doubleShapeArea(r) = make(r * sqrt(2))

circ = make(2) :: Shape(Double)

-- show
main = do {
  print(circ.bind(\(a) -> a.doubleShapeArea.bind(\(b) -> make(b))));
  print(circ.bind(\(a) -> a.doubleShapeArea.bind(\(b) -> make(a))));
}
-- /show

We can also make some computations with a and b like sqrt(a^2 + b^2), which yields the radius of a circle that combines the area of two circles with radius a and b.

import Control.Monad
import Control.Applicative
import Data.Functor
import Prelude hiding ((.))
x.f = f x

data Shape a = Circle { radius :: a } deriving (Show)
instance Functor Shape where { fmap = liftM }
instance Applicative Shape where { pure = return; (<*>) = ap }
instance Monad Shape where
  object >>= remake = remake(object.radius)
  return = Circle

bind = flip(>>=)
make = return

area(Circle r) = r^2 * pi

doubleShapeArea(r) = make(r * sqrt(2))

circ = make(2) :: Shape(Double)

-- show
main = do {
  print(circ.bind(\(a) -> a.doubleShapeArea.bind(\(b) -> make(sqrt(a^2 + b^2)))));
}
-- /show

Let's indent our code to reflect the Method Cascade.

import Control.Monad
import Control.Applicative
import Data.Functor
import Prelude hiding ((.))
x.f = f x

data Shape a = Circle { radius :: a } deriving (Show)
instance Functor Shape where { fmap = liftM }
instance Applicative Shape where { pure = return; (<*>) = ap }
instance Monad Shape where
  object >>= remake = remake(object.radius)
  return = Circle

bind = flip(>>=)
make = return

area(Circle r) = r^2 * pi

doubleShapeArea(r) = make(r * sqrt(2))

circ = make(2) :: Shape(Double)

-- show
main = do {
  print(
    circ.bind(\(a) ->
      a.doubleShapeArea.bind(\(b) ->
        make(sqrt(a^2 + b^2))
      )
    )
  );
}
-- /show

As we can see, with each bind we add a new variable to the scope of the Method Cascade.

Side note: I say variable instead of value because values declared in Method Cascades can be redeclared in an inner closure, thereby overwriting a previously declared value of an outer closure. This happens in circ.bind(\(a) -> a.doubleShapeArea.bind(\(a) -> make(a))), where a is been bound to the radius of circ and then to the radius of the return value of doubleShapeArea.

do-Notation

Let's change our indentation step by step in order to reflect this pattern. First, we append the closing parenthesis of the Method Cascade to the innermost closure.

import Control.Monad
import Control.Applicative
import Data.Functor
import Prelude hiding ((.))
x.f = f x

data Shape a = Circle { radius :: a } deriving (Show)
instance Functor Shape where { fmap = liftM }
instance Applicative Shape where { pure = return; (<*>) = ap }
instance Monad Shape where
  object >>= remake = remake(object.radius)
  return = Circle

bind = flip(>>=)
make = return

area(Circle r) = r^2 * pi

doubleShapeArea(r) = make(r * sqrt(2))

circ = make(2) :: Shape(Double)

-- show
circ' =
  circ.bind(\(a) ->
    a.doubleShapeArea.bind(\(b) ->
      make(sqrt(a^2 + b^2))))

main = do {
  print(circ');
}
-- /show

Now we seperate the .bind(\(...) -> snippets and align the Method Cascade.

import Control.Monad
import Control.Applicative
import Data.Functor
import Prelude hiding ((.))
x.f = f x

data Shape a = Circle { radius :: a } deriving (Show)
instance Functor Shape where { fmap = liftM }
instance Applicative Shape where { pure = return; (<*>) = ap }
instance Monad Shape where
  object >>= remake = remake(object.radius)
  return = Circle

bind = flip(>>=)
make = return

area(Circle r) = r^2 * pi

doubleShapeArea(r) = make(r * sqrt(2))

circ = make(2) :: Shape(Double)

-- show
circ' =
  circ              .bind(\(a) ->
  a.doubleShapeArea .bind(\(b) ->
  make(sqrt(a^2 + b^2))))

main = do {
  print(circ');
}
-- /show

As we can see, Method Cascades that use a Programmable Fluent Interfaces introduce a variable to the scope of each chained method, in our case a and b, that are extracted from objects that implement the Fluent Interface. It's like writing a <- circ and b <- a.doubleShapeArea. Haskell uses this pattern extensively and therefore provides syntactic sugar called do-notation that allows for using this patterns exactly like that.

import Control.Monad
import Control.Applicative
import Data.Functor
import Prelude hiding ((.))
x.f = f x

data Shape a = Circle { radius :: a } deriving (Show)
instance Functor Shape where { fmap = liftM }
instance Applicative Shape where { pure = return; (<*>) = ap }
instance Monad Shape where
  object >>= remake = remake(object.radius)
  return = Circle

area(Circle r) = r^2 * pi

doubleShapeArea(r) = return(r * sqrt(2))

circ = return(2) :: Shape(Double)

-- show
circ' = do {
  a <- circ;
  b <- a.doubleShapeArea;
  return(sqrt(a^2 + b^2));
}

main = do {
  print(circ');
}
-- /show

The naming of Haskell's built-in return (which we called make) reflects that Method Cascades that use a Programmable Fluent Interface are powerful enough to model computations.