mostly-adequate-guide
Search…
Chapter 09: Monadic Onions
Pointy Functor Factory
Before we go any further, I have a confession to make: I haven't been fully honest about that
of method we've placed on each of our types. Turns out, it is not there to avoid the new keyword, but rather to place values in what's called a default minimal context. Yes, of does not actually take the place of a constructor - it is part of an important interface we call Pointed.A pointed functor is a functor with anofmethod
What's important here is the ability to drop any value in our type and start mapping away.
1
IO.of('tetris').map(concat(' master'));
2
// IO('tetris master')
3
​
4
Maybe.of(1336).map(add(1));
5
// Maybe(1337)
6
​
7
Task.of([{ id: 2 }, { id: 3 }]).map(map(prop('id')));
8
// Task([2,3])
9
​
10
Either.of('The past, present and future walk into a bar...').map(concat('it was tense.'));
11
// Right('The past, present and future walk into a bar...it was tense.')
Copied!
If you recall,
IO and Task's constructors expect a function as their argument, but Maybe and Either do not. The motivation for this interface is a common, consistent way to place a value into our functor without the complexities and specific demands of constructors. The term "default minimal context" lacks precision, yet captures the idea well: we'd like to lift any value in our type and map away per usual with the expected behaviour of whichever functor.One important correction I must make at this point, pun intended, is that
Left.of doesn't make any sense. Each functor must have one way to place a value inside it and with Either, that's new Right(x). We define of using Right because if our type can map, it should map. Looking at the examples above, we should have an intuition about how of will usually work and Left breaks that mold.You may have heard of functions such as
pure, point, unit, and return. These are various monikers for our of method, international function of mystery. of will become important when we start using monads because, as we will see, it's our responsibility to place values back into the type manually.To avoid the
new keyword, there are several standard JavaScript tricks or libraries so let's use them and use of like a responsible adult from here on out. I recommend using functor instances from folktale, ramda or fantasy-land as they provide the correct of method as well as nice constructors that don't rely on new.Mixing Metaphors
onion
You see, in addition to space burritos (if you've heard the rumors), monads are like onions. Allow me to demonstrate with a common situation:
1
const fs = require('fs');
2
​
3
// readFile :: String -> IO String
4
const readFile = filename => new IO(() => fs.readFileSync(filename, 'utf-8'));
5
​
6
// print :: String -> IO String
7
const print = x => new IO(() => {
8
console.log(x);
9
return x;
10
});
11
​
12
// cat :: String -> IO (IO String)
13
const cat = compose(map(print), readFile);
14
​
15
cat('.git/config');
16
// IO(IO('[core]\nrepositoryformatversion = 0\n'))
Copied!
What we've got here is an
IO trapped inside another IO because print introduced a second IO during our map. To continue working with our string, we must map(map(f)) and to observe the effect, we must unsafePerformIO().unsafePerformIO().1
// cat :: String -> IO (IO String)
2
const cat = compose(map(print), readFile);
3
​
4
// catFirstChar :: String -> IO (IO String)
5
const catFirstChar = compose(map(map(head)), cat);
6
​
7
catFirstChar('.git/config');
8
// IO(IO('['))
Copied!
While it is nice to see that we have two effects packaged up and ready to go in our application, it feels a bit like working in two hazmat suits and we end up with an uncomfortably awkward API. Let's look at another situation:
1
// safeProp :: Key -> {Key: a} -> Maybe a
2
const safeProp = curry((x, obj) => Maybe.of(obj[x]));
3
​
4
// safeHead :: [a] -> Maybe a
5
const safeHead = safeProp(0);
6
​
7
// firstAddressStreet :: User -> Maybe (Maybe (Maybe Street))
8
const firstAddressStreet = compose(
9
map(map(safeProp('street'))),
10
map(safeHead),
11
safeProp('addresses'),
12
);
13
​
14
firstAddressStreet({
15
addresses: [{ street: { name: 'Mulburry', number: 8402 }, postcode: 'WC2N' }],
16
});
17
// Maybe(Maybe(Maybe({name: 'Mulburry', number: 8402})))
Copied!
Again, we see this nested functor situation where it's neat to see there are three possible failures in our function, but it's a little presumptuous to expect a caller to
map three times to get at the value - we'd only just met. This pattern will arise time and time again and it is the primary situation where we'll need to shine the mighty monad symbol into the night sky.I said monads are like onions because tears well up as we peel back each layer of the nested functor with
map to get at the inner value. We can dry our eyes, take a deep breath, and use a method called join.1
const mmo = Maybe.of(Maybe.of('nunchucks'));
2
// Maybe(Maybe('nunchucks'))
3
​
4
mmo.join();
5
// Maybe('nunchucks')
6
​
7
const ioio = IO.of(IO.of('pizza'));
8
// IO(IO('pizza'))
9
​
10
ioio.join();
11
// IO('pizza')
12
​
13
const ttt = Task.of(Task.of(Task.of('sewers')));
14
// Task(Task(Task('sewers')));
15
​
16
ttt.join();
17
// Task(Task('sewers'))
Copied!
If we have two layers of the same type, we can smash them together with
join. This ability to join together, this functor matrimony, is what makes a monad a monad. Let's inch toward the full definition with something a little more accurate:Monads are pointed functors that can flatten
Any functor which defines a
join method, has an of method, and obeys a few laws is a monad. Defining join is not too difficult so let's do so for Maybe:1
Maybe.prototype.join = function join() {
2
return this.isNothing() ? Maybe.of(null) : this.$value;
3
};
Copied!
There, simple as consuming one's twin in the womb. If we have a
Maybe(Maybe(x)) then .$value will just remove the unnecessary extra layer and we can safely map from there. Otherwise, we'll just have the one Maybe as nothing would have been mapped in the first place.Now that we have a
join method, let's sprinkle some magic monad dust over the firstAddressStreet example and see it in action:1
// join :: Monad m => m (m a) -> m a
2
const join = mma => mma.join();
3
​
4
// firstAddressStreet :: User -> Maybe Street
5
const firstAddressStreet = compose(
6
join,
7
map(safeProp('street')),
8
join,
9
map(safeHead), safeProp('addresses'),
10
);
11
​
12
firstAddressStreet({
13
addresses: [{ street: { name: 'Mulburry', number: 8402 }, postcode: 'WC2N' }],
14
});
15
// Maybe({name: 'Mulburry', number: 8402})
Copied!
We added
join wherever we encountered the nested Maybe's to keep them from getting out of hand. Let's do the same with IO to give us a feel for that.1
IO.prototype.join = () => this.unsafePerformIO();
Copied!
Again, we simply remove one layer. Mind you, we have not thrown out purity, but merely removed one layer of excess shrink wrap.
1
// log :: a -> IO a
2
const log = x => new IO(() => {
3
console.log(x);
4
return x;
5
});
6
​
7
// setStyle :: Selector -> CSSProps -> IO DOM
8
const setStyle =
9
curry((sel, props) => new IO(() => jQuery(sel).css(props)));
10
​
11
// getItem :: String -> IO String
12
const getItem = key => new IO(() => localStorage.getItem(key));
13
​
14
// applyPreferences :: String -> IO DOM
15
const applyPreferences = compose(
16
join,
17
map(setStyle('#main')),
18
join,
19
map(log),
20
map(JSON.parse),
21
getItem,
22
);
23
​
24
applyPreferences('preferences').unsafePerformIO();
25
// Object {backgroundColor: "green"}
26
// <div style="background-color: 'green'"/>
Copied!
getItem returns an IO String so we map to parse it. Both log and setStyle return IO's themselves so we must join to keep our nesting under control.My Chain Hits My Chest
chain
You might have noticed a pattern. We often end up calling
join right after a map. Let's abstract this into a function called chain.1
// chain :: Monad m => (a -> m b) -> m a -> m b
2
const chain = curry((f, m) => m.map(f).join());
3
​
4
// or
5
​
6
// chain :: Monad m => (a -> m b) -> m a -> m b
7
const chain = f => compose(join, map(f));
Copied!
We'll just bundle up this map/join combo into a single function. If you've read about monads previously, you might have seen
chain called >>= (pronounced bind) or flatMap which are all aliases for the same concept. I personally think flatMap is the most accurate name, but we'll stick with chain as it's the widely accepted name in JS. Let's refactor the two examples above with chain:1
// map/join
2
const firstAddressStreet = compose(
3
join,
4
map(safeProp('street')),
5
join,
6
map(safeHead),
7
safeProp('addresses'),
8
);
9
​
10
// chain
11
const firstAddressStreet = compose(
12
chain(safeProp('street')),
13
chain(safeHead),
14
safeProp('addresses'),
15
);
16
​
17
// map/join
18
const applyPreferences = compose(
19
join,
20
map(setStyle('#main')),
21
join,
22
map(log),
23
map(JSON.parse),
24
getItem,
25
);
26
​
27
// chain
28
const applyPreferences = compose(
29
chain(setStyle('#main')),
30
chain(log),
31
map(JSON.parse),
32
getItem,
33
);
Copied!
I swapped out any
map/join with our new chain function to tidy things up a bit. Cleanliness is nice and all, but there's more to chain than meets the eye - it's more of a tornado than a vacuum. Because chain effortlessly nests effects, we can capture both sequence and variable assignment in a purely functional way.1
// getJSON :: Url -> Params -> Task JSON
2
getJSON('/authenticate', { username: 'stale', password: 'crackers' })
3
.chain(user => getJSON('/friends', { user_id: user.id }));
4
// Task([{name: 'Seimith', id: 14}, {name: 'Ric', id: 39}]);
5
​
6
// querySelector :: Selector -> IO DOM
7
querySelector('input.username')
8
.chain(({ value: uname }) =>
9
querySelector('input.email')
10
.chain(({ value: email }) => IO.of(`Welcome ${uname} prepare for spam at ${email}`))
11
);
12
// IO('Welcome Olivia prepare for spam at [email protected]');
13
​
14
Maybe.of(3)
15
.chain(three => Maybe.of(2).map(add(three)));
16
// Maybe(5);
17
​
18
Maybe.of(null)
19
.chain(safeProp('address'))
20
.chain(safeProp('street'));
21
// Maybe(null);
Copied!
We could have written these examples with
compose, but we'd need a few helper functions and this style lends itself to explicit variable assignment via closure anyhow. Instead we're using the infix version of chain which, incidentally, can be derived from map and join for any type automatically: t.prototype.chain = function(f) { return this.map(f).join(); }. We can also define chain manually if we'd like a false sense of performance, though we must take care to maintain the correct functionality - that is, it must equal map followed by join. An interesting fact is that we can derive map for free if we've created chain simply by bottling the value back up when we're finished with of. With chain, we can also define join as chain(id). It may feel like playing Texas Hold em' with a rhinestone magician in that I'm just pulling things out of my behind, but, as with most mathematics, all of these principled constructs are interrelated. Lots of these derivations are mentioned in the fantasyland repo, which is the official specification for algebraic data types in JavaScript.Anyways, let's get to the examples above. In the first example, we see two
Task's chained in a sequence of asynchronous actions - first it retrieves the user, then it finds the friends with that user's id. We use chain to avoid a Task(Task([Friend])) situation.Next, we use
querySelector to find a few different inputs and create a welcoming message. Notice how we have access to both uname and email at the innermost function - this is functional variable assignment at its finest. Since IO is graciously lending us its value, we are in charge of putting it back how we found it - we wouldn't want to break its trust (and our program). IO.of is the perfect tool for the job and it's why Pointed is an important prerequisite to the Monad interface. However, we could choose to map as that would also return the correct type:1
querySelector('input.username').chain(({ value: uname }) =>
2
querySelector('input.email').map(({ value: email }) =>
3
`Welcome ${uname} prepare for spam at ${email}`));
4
// IO('Welcome Olivia prepare for spam at [email protected]');
Copied!
Finally, we have two examples using
Maybe. Since chain is mapping under the hood, if any value is null, we stop the computation dead in its tracks.Don't worry if these examples are hard to grasp at first. Play with them. Poke them with a stick. Smash them to bits and reassemble. Remember to
map when returning a "normal" value and chain when we're returning another functor. In the next chapter, we'll approach Applicatives and see nice tricks to make this kind of expressions nicer and highly readable.As a reminder, this does not work with two different nested types. Functor composition and later, monad transformers, can help us in that situation.
Power Trip
Container style programming can be confusing at times. We sometimes find ourselves struggling to understand how many containers deep a value is or if we need
map or chain (soon we'll see more container methods). We can greatly improve debugging with tricks like implementing inspect and we'll learn how to create a "stack" that can handle whatever effects we throw at it, but there are times when we question if it's worth the hassle.I'd like to swing the fiery monadic sword for a moment to exhibit the power of programming this way.
Let's read a file, then upload it directly afterward:
1
// readFile :: Filename -> Either String (Task Error String)
2
// httpPost :: String -> String -> Task Error JSON
3
// upload :: String -> Either String (Task Error JSON)
4
const upload = compose(map(chain(httpPost('/uploads'))), readFile);
Copied!
Here, we are branching our code several times. Looking at the type signatures I can see that we protect against 3 errors -
readFile uses Either to validate the input (perhaps ensuring the filename is present), readFile may error when accessing the file as expressed in the first type parameter of Task, and the upload may fail for whatever reason which is expressed by the Error in httpPost. We casually pull off two nested, sequential asynchronous actions with chain.All of this is achieved in one linear left to right flow. This is all pure and declarative. It holds equational reasoning and reliable properties. We aren't forced to add needless and confusing variable names. Our
upload function is written against generic interfaces and not specific one-off apis. It's one bloody line for goodness sake.For contrast, let's look at the standard imperative way to pull this off:
1
// upload :: String -> (String -> a) -> Void
2
const upload = (filename, callback) => {
3
if (!filename) {
4
throw new Error('You need a filename!');
5
} else {
6
readFile(filename, (errF, contents) => {
7
if (errF) throw errF;
8
httpPost('/uploads', contents, (errH, json) => {
9
if (errH) throw errH;
10
callback(json);
11
});
12
});
13
}
14
};
Copied!
Well isn't that the devil's arithmetic. We're pinballed through a volatile maze of madness. Imagine if it were a typical app that also mutated variables along the way! We'd be in the tar pit indeed.
Theory
The first law we'll look at is associativity, but perhaps not in the way you're used to it.
1
// associativity
2
compose(join, map(join)) === compose(join, join);
Copied!
These laws get at the nested nature of monads so associativity focuses on joining the inner or outer types first to achieve the same result. A picture might be more instructive:
monad associativity law
Starting with the top left moving downward, we can
join the outer two M's of M(M(M a)) first then cruise over to our desired M a with another join. Alternatively, we can pop the hood and flatten the inner two M's with map(join). We end up with the same M a regardless of if we join the inner or outer M's first and that's what associativity is all about. It's worth noting that map(join) != join. The intermediate steps can vary in value, but the end result of the last join will be the same.The second law is similar:
1
// identity for all (M a)
2
compose(join, of) === compose(join, map(of)) === id;
Copied!
It states that, for any monad
M, of and join amounts to id. We can also map(of) and attack it from the inside out. We call this "triangle identity" because it makes such a shape when visualized:
monad identity law
If we start at the top left heading right, we can see that
of does indeed drop our M a in another M container. Then if we move downward and join it, we get the same as if we just called id in the first place. Moving right to left, we see that if we sneak under the covers with map and call of of the plain a, we'll still end up with M (M a) and joining will bring us back to square one.I should mention that I've just written
of, however, it must be the specific M.of for whatever monad we're using.Now, I've seen these laws, identity and associativity, somewhere before... Hold on, I'm thinking...Yes of course! They are the laws for a category. But that would mean we need a composition function to complete the definition. Behold:
1
const mcompose = (f, g) => compose(chain(f), g);
2
​
3
// left identity
4
mcompose(M, f) === f;
5
​
6
// right identity
7
mcompose(f, M) === f;
8
​
9
// associativity
10
mcompose(mcompose(f, g), h) === mcompose(f, mcompose(g, h));
Copied!
They are the category laws after all. Monads form a category called the "Kleisli category" where all objects are monads and morphisms are chained functions. I don't mean to taunt you with bits and bobs of category theory without much explanation of how the jigsaw fits together. The intention is to scratch the surface enough to show the relevance and spark some interest while focusing on the practical properties we can use each day.
In Summary
Monads let us drill downward into nested computations. We can assign variables, run sequential effects, perform asynchronous tasks, all without laying one brick in a pyramid of doom. They come to the rescue when a value finds itself jailed in multiple layers of the same type. With the help of the trusty sidekick "pointed", monads are able to lend us an unboxed value and know we'll be able to place it back in when we're done.
Yes, monads are very powerful, yet we still find ourselves needing some extra container functions. For instance, what if we wanted to run a list of api calls at once, then gather the results? We can accomplish this task with monads, but we'd have to wait for each one to finish before calling the next. What about combining several validations? We'd like to continue validating to gather the list of errors, but monads would stop the show after the first
Left entered the picture.In the next chapter, we'll see how applicative functors fit into the container world and why we prefer them to monads in many cases.
Exercises
Considering a User object as follow:
1
const user = {
2
id: 1,
3
name: 'Albert',
4
address: {
5
street: {
6
number: 22,
7
name: 'Walnut St',
8
},
9
},
10
};
Copied!
Use
safeProp and map/join or chain to safely get the street name when given a user1
// getStreetName :: User -> Maybe String
2
const getStreetName = undefined;
Copied!
We now consider the following items:
1
// getFile :: IO String
2
const getFile = IO.of('/home/mostly-adequate/ch09.md');
3
​
4
// pureLog :: String -> IO ()
5
const pureLog = str => new IO(() => console.log(str));
Copied!
Use getFile to get the filepath, remove the directory and keep only the basename,
then purely log it. Hint: you may want to use
split and last to obtain the
basename from a filepath.1
// logFilename :: IO ()
2
const logFilename = undefined;
Copied!
For this exercise, we consider helpers with the following signatures:
1
// validateEmail :: Email -> Either String Email
2
// addToMailingList :: Email -> IO([Email])
3
// emailBlast :: [Email] -> IO ()
Copied!
Last modified 1yr ago