Inside My World (Ode to Functor and Monad)

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I like Haskell because it lets me live inside my world.

There are a lot of special worlds out there! And Haskell lets me stay in those worlds, and use all of the tools I normally have when I’m not there. I get to transform normal tools into tools that work in my world.

(This post is meant to be approachable by people unfamiliar with Haskell! That being said, if there is a concept you don’t understand, feel free to leave a comment, tweet me, stop by on irc at freenode’s #haskell, or give Learn You a Haskell a quick read!)

Stuck in Maybe

(Feel free to play along with the code in this section by loading it into ghci, the Haskell interpreter!)

In Haskell, we have a type called Maybe a:

data Maybe a = Nothing | Just a

This says that Maybe a is like an Enumerable type of sorts; The | reads like “or”.

This is like saying

data Bool = False | True

to define a Bool data type. If I have something of type Bool, it can be (literally) False or True. If I have something of type Maybe a, it can be Nothing (nothing is there, it’s empty) or Just x (it contains a value x).

If you are used to an OOP language with templates or generics, this is similar to saying Maybe<a>Maybe<a> is a parameterized type over some a.

This type is useful for functions that might fail:

-- source: http://blog.jle.im/source/code-samples/inside/maybe.hs#L23-41
-- interactive: https://www.fpcomplete.com/user/jle/inside-my-world
-- divideMaybe: Takes two integers and returns -- possibly -- their integer
--      quotient. It succeeds if the denominator is not zero, and fails if
--      it is.
divideMaybe :: Int -> Int -> Maybe Int
divideMaybe _ 0 = Nothing
divideMaybe x y = Just (x `div` y)

-- headMaybe: Takes a list and returns -- possibly -- its first element.
--      Fails if the list is empty, and succeeds with the first element
--      otherwise.
headMaybe :: [a] -> Maybe a
headMaybe []    = Nothing
headMaybe (x:_) = Just x

-- halveMaybe: Takes an integer and returns -- possibly -- its half.  Fails
--      if it is an odd number.
halveMaybe :: Int -> Maybe Int
halveMaybe x | x `mod` 2 == 0 = Just (x `div` 2)
             | otherwise      = Nothing

Aside

Oh hi!

For people new to Haskell:

foo :: Int -> Bool
foo x = ...

declares a function named foo of type Int -> Bool — we use :: to specify type signatures. Int -> Bool means that it takes an Int (named x) and returns a Bool.

I’ll often just say bar :: Bool to say “the value bar (of type Bool)”; you could just read :: as “type of”.

So divideMaybe :: Int -> Int -> Maybe Int means that divideMaybe takes two Ints and returns something of type Maybe Int.

You might have also noticed the pattern matching construct, headMaybe (x:_). This matches the first element in the list to the name x, and the rest of the list to the wildcard, _.

When you want to return a value of type Maybe a, you can either return Just x or Nothing (where x :: a) — they both are members of type Maybe a. That’s what Maybe Int means — an Int that might or might not be there!

If I had something of type Maybe Int, would you know for sure if that Int was there or not (from just the type)? You wouldn’t! You are living in the world of uncertainties.

Welcome to the world of uncertainty.1

The Problem

Okay, well, I have a Maybe Int. Which is nice and all…but…I want to do normal inty-things with it.

That is…I have all these functions that work only on Int!

-- source: http://blog.jle.im/source/code-samples/inside/maybe.hs#L43-50
-- interactive: https://www.fpcomplete.com/user/jle/inside-my-world
addThree :: Int -> Int
addThree = (+ 3)

square :: Int -> Int
square = (^ 2)

showInt :: Int -> String
showInt = show

But…I can’t do these things on Maybe Int!

ghci> addThree (Just 5)
*** SCARY ERROR!
*** addThree takes an Int but you gave it a Maybe Int.
*** What are you trying to do anyway, wise guy.

Aside

In this post, commands at the interactive Haskell interpreter (REPL) ghci are prefaced with the prompt ghci>. If you see ghci>, it means that this is something you’d enter at ghci. If not, it is normal Haskell source code!

In ghci, we also have this command :t that you’ll be seeing often that lets you find the type of something:

ghci> :t True
True :: Bool

In most other languages, to get around this, you would “exit” your uncertain world. That is, you would turn your uncertain 5 into either a certain 5 or an error. Or you would turn your uncertain 5 into either a certain 5 or some “default” value.

That is, you would use functions like these to exit your world:2

-- source: http://blog.jle.im/source/code-samples/inside/maybe.hs#L76-82
-- interactive: https://www.fpcomplete.com/user/jle/inside-my-world
certaintify :: Maybe a -> a
certaintify (Just x) = x
certaintify Nothing  = error "Nothing was there, you fool!"

certaintifyWithDefault :: a -> Maybe a -> a
certaintifyWithDefault _ (Just x) = x
certaintifyWithDefault d Nothing  = d

And then you can just willy-nilly use your normal Int -> Int functions on what you pull out…using various “error handling” mechanisms if it was Nothing.

ghci> addThree (certaintify (headMaybe [1,2,3]))
4
ghci> square (certaintify (halveMaybe 7))
*** Exception: Nothing was there, you fool!
ghci> square (certaintifyWithDefault 0 (halveMaybe 7))
0

But…work with me here. Let’s say I want to live in my uncertain world.

There are a lot of reasons why one might want to do that.

Let’s say you had a function that looked up a person from a database given their ID number. But not all ID numbers have a person attached, so the function might fail and not lookup anyone.

personFromId :: ID -> Maybe Person

And you also had a function that returned the age of a given person:

age :: Person -> Int

What if you wanted to write a function that looked up the age of the person in that database with that ID. The result is going to also be in a Maybe, because the given ID might not correspond to anyone to have an age for.

ageFromId :: ID -> Maybe Int

In this case, it would make no sense to “exit” the world of uncertainty as soon as we get a Maybe Person, and then “re-enter” it somehow when you return the Maybe Int. Our entire answer is shrouded in uncertainty, so we need to stay inside this world the entire time. We want to find a way to deal with values inside a world without leaving it.

So we have a function Person -> Int, and a Maybe Person…darnit. How do we use our age function, without leaving Maybe? We certainly want to re-use the same function somehow, and not write it again from scratch!

Can I have a lift?

So the problem: I have a function a -> b that I want to be able to use on a Maybe a…I want to stay in my Maybe world and use that function on the uncertain value.

If you look at this carefully, we want some sort of “function transformer”. Give our transformer an a -> b, it’ll output a new function Maybe a -> Maybe b. The new function takes an a that may or may not be there, and outputs a b that may or not be there.

We want a function of type (a -> b) -> (Maybe a -> Maybe b)

Let’s make one! It’ll apply the function to the value inside a Just, and leave a Nothing alone.

-- source: http://blog.jle.im/source/code-samples/inside/maybe.hs#L84-88
-- interactive: https://www.fpcomplete.com/user/jle/inside-my-world
inMaybe :: (a -> b) -> (Maybe a -> Maybe b)
inMaybe f = liftedF
  where
    liftedF (Just x) = Just (f x)
    liftedF Nothing  = Nothing

What can we do with it?

ghci> let addThreeInMaybe = inMaybe addThree
ghci> addThreeInMaybe (Just 7)
Just 10
ghci> addThreeInMaybe Nothing
Nothing
ghci> (inMaybe square) (Just 9)
Just 81
ghci> (inMaybe showInt) Nothing
Nothing
ghci> (inMaybe showInt) (Just 8)
Just "8"

Wow! We can now use normal functions and still stay inside my uncertain world. We could even write our ageFromId:

-- source: http://blog.jle.im/source/code-samples/inside/maybe.hs#L68-69
-- interactive: https://www.fpcomplete.com/user/jle/inside-my-world
ageFromId :: ID -> Maybe Int
ageFromId i = (inMaybe age) (personFromId i)

Now we are no longer afraid of dealing with uncertainty. It’s a scary realm, but as long as we have inMaybe…all of our normal tools apply!

ghci> let x = headMaybe [2,3,4]        -- x = Just 2
ghci> let y = (inMaybe square) x       -- y = Just 4
ghci> let z = (inMaybe addThree) y     -- z = Just 7
ghci> (inMaybe (> 5)) z
Just True
ghci> let x' = halveMaybe 7            -- x' = Nothing
ghci> let y' = (inMaybe square) x'     -- y' = Nothing
ghci> let z' = (inMaybe addThree) y'   -- z' = Nothing
ghci> (inMaybe (> 5)) z'
Nothing

Functor

This concept of “bringing functions into worlds” is actually a useful and generalizable concept. In fact, in the standard libraries, there’s a typeclass (which is like an interface, sorta, for you Java/OOP people) that provides a common API/interface for “worlds that you can bring functions into.”

We call it Functor, and this “bring into world” function is called fmap.

It should come as no surprise that Maybe is a Functor, so fmap does take any function a -> b and “lifts” it into the Maybe world, turning it into a Maybe a -> Maybe b.

fmap for Maybe is incidentally exactly our inMaybe.

ghci> (fmap square) (headMaybe [4,5,6])
Just 16
ghci> (fmap square) (halveMaybe 7)
Nothing

Aside

Any “legitimate” instance of Functor must satisfy a couple of properties — “laws”, so to speak. These laws basically ensure that whatever instance you define is useful and sensible, and follow what sort of meaning fmap is supposed to convey.

  1. fmap (f . g) should equal fmap f . fmap g; that is, lifting composed functions be the same as composing lifted functions. ((.) is the function composition operator)
  2. fmap id thing should leave thing unchanged.

Some notes before we move on!

First of all, even though we have been writing things like (fmap f) x, the parentheses are actually unnecessary due to the way Haskell associates function calls. So (fmap f) x is the same as fmap f x, and we’ll be writing it that way from now on.

Secondly, an infix operator alias for fmap exists: (<$>). That way, you can write fmap f x as f <$> x, which is meant to look similar to f $ x:

ghci> addThree $ 7
10
ghci> addThree <$> Just 7
Just 10
ghci> addThree <$> Nothing
Nothing

(For those unfamiliar, f $ x = f x)

Sort of a big deal

Let’s pause and reflect to see that this is sort of a big deal, and see what problem Functor just solved.

In another language, you might somehow have a Maybe<Int> (using generics syntax). And you have lots and lots and lots of functions that take Ints. Heck, why would you even ever have a function take a Maybe<Int>? A function would be like:

class Monster {
    void deal_damage(int damage) {};
}

where your deal_damage function would take an integer. So Maybe Int is useless! You either have to re-write deal_damage to take a Maybe Int, and have two versions of it, or you turn your Maybe Int into an Int somehow.

In this light, Maybe is a huge nuisance. It is a big, annoying thing to deal with and it probably results in a lot of boilerplate, making you either duplicate functions or extract Maybe values every time you get one.

But now…now, Maybe is not a nuisance, and there is no boilerplate. All your functions now…just work, as they are!

And this is a big deal.

“Pre-lifting”

Okay, so we now can turn a -> b into Maybe a -> Maybe b.

That might be nice, but if you scroll up just a bit, you might see that there are other functions that might be interesting to apply on a Maybe a.

What about halveMaybe :: Int -> Maybe Int? Can I use halveMaybe on a Maybe Int?

ghci> let x = divideMaybe 12 3     -- x = Just 4 :: Maybe Int
ghci> halveMaybe x
*** SCARY ERROR!
*** halveMaybe takes an Int but you gave it
*** a Maybe Int.  Please think about your life.

Oh no! Maybe we can’t really stay inside our Maybe world after all!

This might be important! Let’s imagine this trip down our world of uncertainty — let’s say we wanted a function halfOfAge

halfOfAge :: ID -> Maybe Int

That returns (possibly), half of the age of the person corresponding to that ID (and Nothing if the person looked up has an odd age. Because odd ages don’t have halves, of course.). Well, we already have ageFromId :: ID -> Maybe Int, but we want to apply halveMaybe to that Maybe Int. But we can’t! Because halveMaybe only works on Int!

We can’t even use fmap, because:

ghci> :t fmap halveMaybe
fmap halveMaybe :: Maybe Int -> Maybe (Maybe Int)

Wrong wrong wrong! We don’t want a Maybe Int -> Maybe (Maybe Int), we want a Maybe Int -> Maybe Int! fmap lifts “both sides” of the function, but we only want, in this case, to “lift” the input.

This is a disaster!

But wait, calm down. We have overcome similar things before. With our recent journey to Functor enlightenment in mind, let’s try to look for a similar path.

We had an a -> b that we wanted to apply to a Maybe a, we used fmap to turn it into a Maybe a -> Maybe b. So we have a a -> Maybe b here that we want to apply to a Maybe a. The plan is simple! We turn an a -> Maybe b into a Maybe a -> Maybe b. Let’s pretend we had such a function.

liftInput :: (a -> Maybe b) -> (Maybe a -> Maybe b)

How should we expect this to behave?

Well, let’s think this through case-by-case.

If we want to apply halveMaybe to a number that isn’t there…well…it should also return a number that isn’t there. It should propagate the not-thereness.

If we want to apply halveMaybe to a number that is there…well, just apply it to that number, and take that result! If the result is there, then you have a result there. If the result is not there, then you don’t.

We have enough to write this out ourselves:

-- source: http://blog.jle.im/source/code-samples/inside/maybe.hs#L90-94
-- interactive: https://www.fpcomplete.com/user/jle/inside-my-world
liftInput :: (a -> Maybe b) -> (Maybe a -> Maybe b)
liftInput f = liftedF
  where
    liftedF Nothing  = Nothing
    liftedF (Just x) = f x
ghci> :t liftInput halveMaybe
Maybe Int -> Maybe Int
ghci> let x = divideMaybe 12 3     -- x = Just 4 :: Maybe Int
ghci> (liftInput halveMaybe) x
Just 2
ghci> let y = divideMaybe 12 0     -- y = Nothing :: Maybe Int
ghci> (liftInput halveMaybe) y
Nothing

Neat! Now we don’t have to fear a -> Maybe b’s…we can use them and still stay in our world, without leaving our world of uncertainty!

-- source: http://blog.jle.im/source/code-samples/inside/maybe.hs#L71-72
-- interactive: https://www.fpcomplete.com/user/jle/inside-my-world
halfOfAge :: ID -> Maybe Int
halfOfAge i = (liftInput halveMaybe) (ageFromId i)

Monad

Like with Functor and fmap, this general pattern of turning an a -> f b into an f a -> f b is also useful to generalize.

In general, you can think functions a -> world b as functions that “bring you into your world”. We would like to turn it, in general, into world a -> world b. Lifting the input only, so to speak.

We say that if a world has such a way of lifting the input of such a function (plus some other requirements), it implements the Monad typeclass3.

Monad is a typeclass (which is kinda like an interface), so that means that if Maybe is a Monad, it “implements” that way to turn a a -> Maybe b into a Maybe a -> Maybe b.

We call this (a -> Maybe b) -> (Maybe a -> Maybe b) function “bind”.

Now, embarrassingly enough, “bind” actually isn’t called bind in the standard library…it actually only exists as an operator, (=<<).

(Remember how there was an operator form of fmap? We have both fmap and (<$>)? Well, in this case, we only have the operator form of bind, (=<<). Yeah, I know. But we live with it just fine!).

(=<<) is exactly our liftInput for Maybe. Let’s try it out:

ghci> :t (=<<) halveMaybe
Maybe Int -> Maybe Int
ghci> let x = divideMaybe 12 3     -- x = Just 4 :: Maybe Int

-- use it as a prefix function
ghci> (=<<) halveMaybe x
Just 2
ghci> let y = divideMaybe 12 0     -- y = Nothing :: Maybe Int

-- use it as an infix operator
ghci> halveMaybe =<< y
Nothing

And now maybe we can finally rest easy knowing that we can “stay inside Maybe” and never have to leave it.

Aside

The “other thing” that Monad has to have (the other thing that the “interface” demands, besides (=<<)) is a way to “bring a value into your world”.

This function is called return.

For example, for Maybe, we need a way to take a normal value like an Int and “bring it into” our world of uncertainty — an Int -> Maybe Int. For Maybe, semantically, to bring something like 7 into the world of uncertainty…well, we already know the 7 is there. So to bring a 7 into Maybe, it’s just Just 7

For an instance of Monad to be considered legitimate, there are a few rules/laws that return and (=<<) must obey when used together in order to be useful (just like for Functor). If you define nonsensical return and (=<<), of course, your instance won’t be very useful anyway, and you wouldn’t be able to reason with how they work together. The laws sort of are some way of ensuring that your instance is useful and sensible, and that (=<<) and return make sense at all.

Aside

Now, for some strange reason, it is actually much more popular to use (>>=) over (=<<); (>>=) is just (=<<) backwards:

ghci> halveMaybe =<< Just 8
Just 4
ghci> Just 8 >>= halveMaybe
Just 4

This is really weird! I mean…really really weird! Why would you ever put the function you are applying after the value you are applying it to? That’s like having x :: a and f :: a -> b, and doing x f or something!

Why is this style the norm? Who knows!4 People are just weird!

For the rest of this article, we will be using (=<<); just be aware that you might see (>>=) out in the wild more often!

Recap

Thanks to Functor and Monad, we now have a way to confidently stay in our world of uncertainty and still use normal functions on our uncertain values — we only have to use the right “lifters”.

If you have an x :: Maybe a and you have a:

  • f :: a -> b, then use fmap or (<$>)fmap f x or f <$> x

  • f :: a -> Maybe b, then use (=<<)f =<< x

Armed with these two, you can comfortably stay in Maybe without ever having to “get out of it”!

A big picture

Again, the big picture is this: sometimes we get values inside contexts, or worlds. But we have functions like a -> world b that produce values inside your world.

Which normally would leave you “high and dry”, because you can’t, say, apply that same function twice. You either have to write a new world a -> world b version or some other boilerplate.

With Functor, we can make normal functions treat our world values like normal values; with Monad, we can do the same with functions that “bring us into” worlds. With these two together…maybe contexted values aren’t so bad after all!

Other Worlds

on Worlds

You might have noticed that up until now I have used the word “world” pretty vaguely.

When I say that a value is “inside” a “world”, I mean that it “lives” inside the context of what that world represents. A Maybe a is an a living in the Maybe “world” — it is an a that can exist or not exist. Maybe represents a context of existing-or-not-existing.5

But there are other worlds, and other contexts too. And though I have shown you what Functor and Monad look like for Maybe…you probably need to see a few more examples to be really convinced that these are general design patterns that you can apply to multiple “values in contexts”.

It’s important to remember that fmap and (=<<) don’t really have any inherent semantic meaning…and their usefulness and “meaning” come from just the specific instance. We saw what they “did” for Maybe, but their meaning came from Maybe itself. For other worlds, as we will see, we can make them mean completely different things.

Aside

There are some important nuances that might trip you up! Though useful worlds are instances of Monad, it is improper to say that “Monads are worlds/values in contexts”. That’s not what Monads are. Monads are just Monads (the two functions and their laws), no more and no less.

In our usage here, Functor and Monad mean only “these things implement some sort of fmap and (=<<), etc., and those two are useful.” That is, the interface offered by Functor and Monad are useful for our specific world. But there are plenty of Functors and Monads that are not “worlds”.

Anyways, here is a whirlwind tour of different worlds, to help you realize how often you’ll actually want to live in these worlds in Haskell, and why having fmap and (=<<) are so useful!

The world of future values

(Play along with this section too by loading the source!)

In Haskell, we have a Reader r world. You can think of (Reader r) a as a little machine that “waits” for something of type r, then uses it to (purely) make an a. The a doesn’t exist yet; it’s a future a that will exist as soon as you give it an r.

-- source: http://blog.jle.im/source/code-samples/inside/reader.hs#L17-27
-- interactive: https://www.fpcomplete.com/user/jle/inside-my-world
-- futureLength: A future `Int` that will be the length of whatever the
--      list it is waiting for will be.
futureLength :: (Reader [a]) Int

-- futureHead: An future `a` that will be the first element of whatever the
--      list it is waiting for will be.
futureHead   :: (Reader [a]) a

-- futureOdd: A future `Bool` that will be whether the `Int` it is waiting
--      for is odd or not.
futureOdd    :: (Reader Int) Bool

futureLength is a “future Int”; an Int waiting (for an [a]) to be realized. futureHead is a “future a”, waiting for an [a].

We use the function runReader to “force” the a out of the (Reader r) a:

-- given a `(Reader r) a` and an `r`, uses that `r` to finally get the `a`:
runReader :: (Reader r) a -> r -> a
ghci> runReader futureLength [1,2,3]
3
ghci> runReader futureHead [1,2,3]
1
ghci> runReader futureOdd 6
False
ghci> runReader futureOdd 5
True

Welcome to the world of future values.

Aside

It is important to note here that (Reader Int) Bool and (Reader [Int]) Bool do not exist in the same world. One lives in a Reader Int world — a world of future values awaiting an Int. The other lives in a Reader [Int] world — a world of future values awaiting an [Int].

Let’s say I have a future Int. Say, futureLength, waiting on an [a]. And I have a function (< 5) :: Int -> Bool. Can I apply (< 5) to my future Int, in order to get a future Bool?

At first, no! This future Int is useless! I can’t even use it in any of my normal functions! Time to reach for the exit button?

Oh — but, because Reader [a] is a Functor, I can use fmap to turn (< 5) :: Int -> Bool into fmap (< 5) :: (Reader [a]) Int -> (Reader [a]) Bool!

-- source: http://blog.jle.im/source/code-samples/inside/reader.hs#L34-38
-- interactive: https://www.fpcomplete.com/user/jle/inside-my-world
futureShorterThan :: Int -> (Reader [a]) Bool
futureShorterThan n = fmap (< n) futureLength

futureShorterThan5 :: (Reader [a]) Bool
futureShorterThan5 = futureShorterThan 5
ghci> runReader futureShorterThan5 [1,2,3]
True
ghci> runReader (futureShorterThan 3) [1,2,3,4]
False

And voilà, we have a future Bool. We turned an Int -> Bool into a function that takes a future Int and returns a future Bool. We applied (< 5) to our future length, to get a future Bool telling us if that length is less than 5.

So futureShorterThan is a function that takes an Int and turns it into a future Bool. Let’s go…deeper. What if I wanted to apply futureShorterThan to a future Int? To still get a future Bool?

I can’t apply futureShorterThan to a future Int straight-up, because it only takes Int. Boo! But, wait — Reader [Int] is a Monad, so that means I can take the Int -> (Reader [a]) Bool and turn it into a (Reader [a]) Int -> (Reader [a]) Bool using (=<<)!

Using (=<<), we turned a function from Int to a future Bool to a function from a future Int to a future Bool.

futureShorterThan       :: Int              -> (Reader [a]) Bool
(=<<) futureShorterThan :: (Reader [a]) Int -> (Reader [a]) Bool

Hm. Let’s try this out on a future Int we have…we can use futureHead :: (Reader [Int]) Int.

-- source: http://blog.jle.im/source/code-samples/inside/reader.hs#L40-41
-- interactive: https://www.fpcomplete.com/user/jle/inside-my-world
futureShorterThanHead :: (Reader [Int]) Bool
futureShorterThanHead = futureShorterThan =<< futureHead

So, we are applying futureShorterThan to the Int we got from futureHead. And so we get a future Bool that tells us if that future Int we got from the input list is shorter than the input list.

ghci> runReader futureShorterThanHead [1,2,3]
False
ghci> runReader futureShorterThanHead [5,2,3]
True

Neat!

Now, we can live in a world of “future values”, and now use all of our “normal” functions on future values!

In another language, we might have had to do this complicated dance of “forcing” futures to get the value (to “exit” the world of future values), applying functions to that value, and going “back into” the future.

But now, we don’t have to be scared of future values. We can work with them just as well as if they were normal values, and “leave” them as futures the entire time, without ever forcing them until when we really need to, and only force them once.

Who said futures were complicated, anyway?

The world of “IO”

(The source code for this section is also available online for you to play with!)

And now we go into the most infamous of Haskell worlds, IO.

IO is “kind of like” our Reader r world — an IO Int is an Int that doesn’t yet exist…but that will be computed by a CPU/computer when a CPU executes it.

In this sense, an IO Int is kind of like a little packet of Assembly or C code — it contains instructions (assembly commands, machine language) for a computer to do this and that and eventually produce an Int. An IO String could, for example, be a little packet of C code that reads a file and outputs the contents of that file. The String doesn’t exist yet — but it will, once the computer executes those commands.

If you’ve ever used a Unix operating system, there is a shell command ls that lists the contents of a directory. The actual ls program is kind of like an IO [FilePath]. The [FilePath] does not “exist” inside" ls — rather, ls is a program that promises a list of FilePaths when it is executed by the computer or interpreter.

So an IO String doesn’t “contain” a String — it is a program that promises a String in the future, when a computer eventually executes it,6.

One common IO object we are given is getLine :: IO String. getLine is kind of like the Unix program cat — it promises a String, and it gets that String by taking in from standard input. That is, it is a program that, when executed by a computer, pulls a line from stdin, and returns that as the String it promises. getLine contains instructions for a computer to get a String from stdin. A future/promised String.

We want to apply length :: String -> Int to that future/promised String, to get us a future/promised Int. Again, we can’t apply length to getLine directly — but because IO is a Functor, we can use fmap length :: IO String -> IO Int.

getLine             :: IO String
length              :: String -> Int
fmap length         :: IO String -> IO Int
fmap length getLine :: IO Int

Neat!

We had a function that only worked on String, but we made it work the “future/promised” String of IO String

Let’s look at a function returning an IO action wc, which takes a filename and returns a program that, when executed, promises an Int — the number of lines in that file.

-- source: http://blog.jle.im/source/code-samples/inside/io.hs#L19-19
-- interactive: https://www.fpcomplete.com/user/jle/inside-my-world
wc :: String -> IO Int

So wc "file.txt" would evaluate to a computation that, when executed by a computer, produces an Int (by loading the file from disk using system calls, reading it, and counting the lines).

wc is a function that takes a (non-future, normal) String.

But what if we wanted to apply wc to getLine, the IO String we had? We want to apply wc to that “future String”. We can’t apply it directly. We want to turn our String -> IO Int into an IO String -> IO Int.

Luckily, IO is a Monad, so we have (=<<) at our disposal.

getLine        :: IO String
wc             :: String -> IO Int
(=<<) wc       :: IO String -> IO Int
wc =<< getLine :: IO Int

Neat!

What does wc =<< getLine do, as a program? How does it compute that Int?

Conceptually, it all sort of “makes sense” if you look at it from a high level view. getLine is an IO String — a future String. wc takes a String and returns a future Int. If we “applied wc to getLine”, we would be applying wc to that future String, to get a future Int.

And so there we have it — we don’t ever have to actually work “directly” with computed values a that are received from IO. All we ever have to do is work with IO a, and we can use all of our normal functions on that IO a, as if they were normal as.

In that way, we don’t have to be scared of working with “future computable values” — we can use all of our normal tools on them!

Even more

There are lots of other worlds besides just Maybe, Reader r, and IO. Each one comes with their own unique semantics/meanings/contexts, and their own answer for what fmap and (=<<) are supposed to “mean”.

Here are some others — with brief descriptions.

  1. The world of Either e, which is like Maybe in which things may or may not be there. But in Either e, when things aren’t there, they come with a reason why they are not (of type e).

  2. The world of [], where things are the results of computations that have ambiguous answers. I wrote a series of posts on this :)

  3. The world of State s, which is a world of future things awaiting an s, which modify the s in the process.

  4. The world of Parser, which is a world of things that an input string will be parsed into.

There are many more! The great thing about Haskell is that with fmap and (=<<), it is easy to work with values inside these worlds with all of your normal functions, without any real extra effort. Normal functions, normal values, values inside worlds…we have them all at our disposal.

Haskell lets me stay inside my world!

Final Notes

For some further reading, Gabriel Gonzalez’s “Functor Design Pattern” post covers a similar concept for people more familiar with Haskell and explains it more elegantly than I ever could have.

Don’t forget as you’re reading and moving on that it’s not correct to say “Functors are worlds”, or “Monads are worlds”. As I mentioned before in an aside, Monads aren’t “anything” other than the functions and the laws. Rather, if we look at Maybe, etc. as a “world”, then having a Monad interface/instance allows us to do cool things with that world.

Feel free to again play around with the code used here and load it in ghci yourself!

Experienced readers might have noted an unconventional omission of “Applicative Functors”, which (since 2008-ish) traditionally goes somewhere in between the section on Functor and the section on Monad. Applicative Functors, in this context, are handy in that they let you combine two values in worlds together; that is, if you have a Maybe a and a Maybe b, it allows you to use an a -> b -> c to “squash” them into a Maybe c. For the nuances of this, check out a great tutorial by adit that explains the Applicative typeclass well; you should easily be able to connect Applicative Functors to the concepts discussed in this post and see how they are useful :)

As always, if you have any questions, leave them in the comments, or come find me on freenode’s #haskell — I go by jle` :)

(Special thanks to c_wraith and rhaps0dy for their time reviewing this post)


  1. Dun dun dun!

  2. In the standard libraries, certaintify and certaintifyWithDefault exist in the Data.Maybe module as fromJust and fromMaybe, respectively.

  3. It also needs return, which I will mention in due time.

  4. I know! And I’m not telling! Just kidding.

    (>>=) is actually a lot of times more useful than (=<<), despite its awkward reversed nature.

    One major reason is that things end up looking more “imperative” (which may or may not be desired). Imagine divideMaybe 12 3 >>= halveMaybe >>= halveMaybe versus halveMaybe =<< halveMaybe =<< divideMaybe 12 3.

    Believe it or not, usage of Monads was originally motivated by structuring IO actions. So, in that setting, it seemed natural to have an “imperative-y” feel.

    Also, in the popular “do notation” syntactical sugar, (>>=) is used in the desugaring and not (=<<), so (>>=) pops out naturally when reasoning about Monads coming through do notation.

    Also, whenever you use lambda syntax (like (\x -> f x)), (>>=) might be nice, because lambda syntax carries some sort of inherent left-to-rightness in it with the arrow. It’s also tricky to write “chained binds” using lambda syntax using (=<<) — try writing f >>= (\x -> g >>= (\y -> h x y)) using (=<<) and you’ll see that it’s slightly less natural.

    Still, it is worth being aware that (>>=) is a is a bit “different” from the rest of the pack of normal Haskell function application and composition operators; it is unique in that it is the only one where the backwards form is more common than the normal one.

    Even the term “bind” is often used to refer to both.

    A general guideline is that you ever mix a bind with a (<$>) and/or a ($) and (.), you should prefer (=<<), to prevent your eyes from jumping directions.

  5. In Haskell, “worlds” are represented at the type level as type constructors. Maybe is a type constructor that takes a type like Int and returns a new type, Maybe Int. Not all type constructors represent Worlds, of course.

  6. An important distinction between IO and the other worlds we have looked at is that there is no way to “exit” the world of IO within Haskell. That is, there is no meaningful IO a -> a.

    If you think about it for a while, it kind of makes sense. If IO a is assembly code for a computer…the only thing that can “get” that a is the computer itself — by shifting those registers, ticking that program clock, reading from IO…

    Remember, a Haskell program can only “evaluate” expressions, not “execute” them. The execution is the computer’s job. When you compile a Haskell program, the compiler takes whatever IO () is named main in your program, evaluates it, and compiles it into a binary. Then you, the computer user, can execute that binary like any other binary (compiled from C or whatever). Because you can never “exit” IO in your Haskell code, this makes IO an extreme version of the worlds we mentioned before; in the others, we could “exit” the world if we really wanted to. We only used fmap and (=<<) because it provided for beautiful abstractions. This topic is discussed in depth at an old blog post of mine.

    Because of this, if it weren’t for Functor and Monad, it would be extremely hard to do anything useful with IO! We literally can’t pass an IO a into any normal function. We need Functor and Monad for us to ever work at all with our “future values” with normal functions!

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