### Intro

I had never wanted to be a programmer exactly, but as I was introduced to Haskell, certain aspects of it appealed to me and I grew to love it. I present them here to help me organize my own thoughts and remember why Haskell is fun. One of the things I most enjoy in life is when a bunch of facts that you usually learn independently and seem separate can be fitted together into a tidy, coherent package or progression. Haskell offers a lot of opportunities for this.

Today I’d like to share with you the good news about currying. Most of what follows is in the book and there are approximately 10,000 blog posts about currying already, so why write one more? Because I’d like to take it a step farther than most people do.

The joy of currying for me was in finding that there was consistency in how *kinds* work – kinds being the types of types – such that I could know how some functors would work based on what I already knew.

Maybe I take joy in strange things.

## The Function Datatype

It took me some time to come to grips with the idea that a lot of types aren’t just *types*; they’re *type constructors* and function-like in that they must be applied to some argument to construct a value of that type. It took me a bit longer to understand that the function arrow you see in type signatures is also a type constructor. It doesn’t look or act precisely like other type constructors, but don’t let that fool you.

The `(->)`

type constructor is a product type with two parameters and no data constructors:

We have two polymorphic type parameters here, `a`

and `b`

. A function takes an `a`

argument and returns `b`

. It has no data constructors, but it does construct a value – constructing a function constructs value of type `a -> b`

.

But these two parameters only allow for one input and one output. So it must be true that Haskell functions take one argument and return one result.

We don’t know anything about our parameters, though, so they can be any type – including more functions. They may be different types, or they may be the same type. In other words, one function could take a function as the input argument and return a function as the output. If it does one or the other or both, we have a *higher-order function*.

*Currying* takes advantage of this ability of functions to return more functions. We know that all functions take one argument and return one result, but we also need the ability to write multiparameter functions. Currying, which happens by default in Haskell, allows us to write multiparameter functions that are *really* nested single-parameter functions.

The `(->)`

associates to the right so that this:

that appears to have three arguments is really:

Applying the function to an `a`

results in another function and so on.

So, any “multiparameter” function is a series of nested one-argument functions, and any of those arguments could theoretically be functions. It’s this default currying that enables the cool thing called *partial application*. We’ll look at that in a moment.

## Higher-Order Functions

Right, so the `(->)`

type constructor has two parameters, and either argument can itself be a function. When we pass a function in as input to a function, we write the type like this:

with parentheses around that first input that is itself a function. If we didn’t, it would associate to the right, and we’d end up with a mess. Parenthesizing it means that function is *one argument* to our larger function. The right associativity and currying still apply, so in some way, what we have is:

but this is done by default.

One of the important points about functional programming is supposed to be that *functions are first class*. Since Haskell is my first language, I suppose I take it for granted and never understood the significance (I get why higher-order functions are useful and good, but it just seems so bloody natural to me). But what it means is what we’ve seen: functions can be passed as arguments or returned as values. They may also be stored in variables or data structures, like other values.

## Currying and Partial Application

Partial application and why it’s useful is covered quite well by a number of other blog posts so I’ll try not to go on too long about it, and anyway I want to get to the exciting stuff soon.

While the function type associates to the right, function application associates to the left. So,

where `f`

is the function and `x`

and `y`

are the input arguments.

When we partially apply a function, we see a reduction in the parameters of the function’s type signature:

```
(+) :: Num a => a -> a -> a
-- partially applied
Prelude> let add5 y = 5 + y
Prelude> :t add5
add5 :: Num a => a -> a
```

Our new function, `add5`

, will accept one input argument and return a result. Not terribly exciting, but, in fact, this gives us a way to define any number of useful functions, such as:

In both of those, we’ve partially applied folding functions to create functions that take one list input, without having to rewrite the other arguments over and over. Pretty sweet.

We can even compose partially applied functions:

`enumFrom`

takes one input and generates a list, while both `filter`

and `take`

require lists as their *second* inputs, not their first ones:

By applying them to one argument each (`5`

and `even`

), they become functions prepared to accept the list generated by `enumFrom`

as input.

Currying allows you to build more specialized functions via partial application, so the order of arguments is a design concern with functions you’ll want to reuse. For example, you’re more likely to want to build a filtering function that will filter all the zeroes out of any list it is given, which can be easily done by partially applying `filter`

to a test for nonequality with `0`

, than to build one where `filter`

is partially applied to some list first and awaits its filtering criterion. A similar idea gives us the partially applied folds that work as `sum`

and `reverse`

functions.

## Kinds

All of that is very nice, but for me the really cool part came when I discovered that the types of types, called *kinds*, work the same way, and knowing this about kinds gives you quite a lot of information about how other things work, things that seem complicated and mind-bending when you approach them cold. *So many things* just made sense when I realized they’re a logical progression from how the function type works.

So, in case you’re not familiar with kinds already, here’s a very short rundown. Kinds are represented by stars, `(*)`

or `Type`

. Some datatypes, the ones that don’t take any arguments, are essentially type *constants* rather than type constructors, just as some data constructors are constant values (think of `True`

and `False`

– they don’t *construct* a value, really, they just *are*). Those datatypes, such as `Bool`

, are concrete types so they have one star:

That tells us that the *kind* of `Bool`

is just one star, or `Type`

; you can also read this as. It’s not a constructor; that is, it’s not a function. It’s a concrete type – like a value at the type level.

On the other hand, `List`

and `Maybe`

, datatypes which must be applied to a type argument before they can construct a value of that type, are represented at the kind level with a function arrow:

See, they’re functions! You apply them to an argument and construct a type. You may also see this as `Type -> Type`

: when applied to a `Type`

, they return a `Type`

where `Type`

is the kind of concrete types.

Types that have more than two arguments have even more stars. A datatype called `Either`

takes two parameters:

and the `a`

is in the `Left`

side of the sum type, while the `b`

goes to the `Right`

. Two parameters means it has three stars:

To construct data of this type, then, we need to apply it to two arguments. Now, just like functions, this associates to the right, and you can partially apply it:

```
Prelude> :k (Either Int)
(Either Int) :: * -> *
Prelude> :k (Either Int String)
(Either Int String) :: *
```

Woot! It does just what we expect from watching what happened to the types of functions as we partially applied them.

And just the way there’s an interaction between argument order and likelihood of partial application for functions, there’s a similar interaction here. To understand why, let’s look at how `Functor`

works.

## The Functor will see you now

In case you’re not familiar with `Functor`

already, a hand-wavy explanation is that it’s a generalization of a `map`

function. Just as you can `map`

a function over a list:

You can `fmap`

a function over a lot of different types of structured data:

But Functor needs a type that is kind `* -> *`

– if it’s `*`

then there’s nothing to apply a function to; if it’s `* -> * -> *`

then there’s too much to apply a function to, so `* -> *`

is the kind that’s just right for `Functor`

:

The `f`

structure that we lift the function over needs to be kind `* -> *`

. Incidentally, the infix operator for `fmap`

is `<$>`

so the following are equivalent:

So, how do we get a `Functor`

for `Either`

? We saw above what partially applying `Either`

does to its kind signature, and we see that fmapping over `Either`

values looks like this:

Our function can’t be applied to whatever is inside the `Left`

because what is inside `Left`

is our `a`

and that’s what we had to partially apply to make `Either`

have the kind `* -> *`

. When we write a `Functor`

instance for `Either`

(or tuples, or any datatype with two parameters^{1}) to tell the compiler how `fmap`

should work for that type, we have to partially apply the type constructor which makes the first type argument (or arguments, if our datatype has more than two parameters) part of the structure that `Functor`

lifts over.^{2}

Anyway, so we’re finally getting to the part that was really exciting for me, although it seems to follow so naturally from what came before, it’s almost a disappointment. *Almost.*

## The Functor of Functions

*Friends*, the function type itself has a `Functor`

instance. It also has `Applicative`

and `Monad`

instances, but first thing’s first.

Since the function type has two parameters, it is kind `* -> * -> *`

so right away you know we have to apply that first argument before we can get a `Functor`

for it.

It feels a little different from `Either`

, though, because now it isn’t whatever type is safely contained within the `Left a`

of our `Either`

type that becomes part of the structure – now, it’s the first input to our function that will be part of the “structure.”

For the `Functor`

, this only amounts to function composition. We can see this by comparing the types:

```
-- function composition
(.) :: (b -> c) -> (a -> b) -> (a -> c)
-- fmap
<$> :: (a -> b) -> f a -> f b
-- we can change letters without changing the meaning
:: (b -> c) -> (a -> b) -> (a -> c)
:: (b -> c) -> f b -> f c
-- f here is ((->) a)
-- that is, a partially applied function
:: (b -> c) -> (a -> b) -> (a -> c)
:: (b -> c) -> ((->) a) b -> ((->) a) c
-- change the prefix notation into infix
:: (b -> c) -> (a -> b) -> (a -> c)
:: (b -> c) -> (a -> b) -> (a -> c)
```

They’re the same! Which means the `Functor`

of functions isn’t terribly exciting! What is exciting is that something that at first seemed weird and alienating – *what do you mean you fmap a partially applied function over another partially applied function?* – isn’t so weird. It follows from what we already knew.

The `Applicative`

of functions is pretty exciting, though, because it allows us to create a context in which two functions are awaiting the same input and a third function can apply to the result of both of those functions, once that input is supplied.

Waaaaat? No, for real:

Our two partially applied functions, `(^2)`

and `(+10)`

, are waiting for an input; the tie-fighter-looking operator between them is our applicative operator. We’re also fmapping `(+)`

over that whole deal so that the results of both those functions will be the two arguments to the addition function. Essentially, something like this:

Meh, what a lot of trouble. Who wants to write all that out all the time?

Of course we don’t want to do that, so we just give it a name:

```
function :: Integer -> Integer
function = ((+) <$> (^2) <*> (+10))
Prelude> function 4
30
Prelude> function 5
40
```

These trivial arithmetic examples in blog posts admittedly always seem…well, trivial. But it turns out to be a handy pattern:

We’ll return `True`

whenever the input is between 14.5 and 20.1 (but not equal to either).

So partial application and currying have allowed us to create a context in which we have functions sort of hanging around waiting for some input to come from *somewhere* in the environment, and we don’t need to keep stringing that argument explicitly through every function. The `Monad`

and transformer versions, called `Reader`

and `ReaderT`

, are probably more common than this bare applicative.

Sometimes people ask us why we didn’t split Haskell Programming from First Principles into two volumes. What it comes down to is that all the really important stuff is in the first half, but people often think that to understand Haskell, they need the second half. But much of what comes in the second half is just more complex ways to nest lambdas and apply and compose functions, which is what the first half is all about.

*Many thanks to John DeGoes and Adam McCullough for their thorough, though not at all brutal, feedback.*

As an aside, in a lot of cases, such as with Either and tuples, the parameters

*may*be different types, remember, they’re`a`

and`b`

(although the names themselves are meaningless), although they are not required to be. But you don’t know that before they’ve been applied to anything. If`a`

and`b`

*were*different types, your function may not be able to apply to both of those types anyway.↩︎Since ordinary

`fmap`

can’t apply a function to the`a`

in`Left a`

, it’s common for people to use it for failure cases. You may notice a relationship between this and the interaction between partial application and argument order of functions we saw above.↩︎