Higher Order Functions

Introduction

In OCaml, working with functions quickly becomes second nature. We like to think that functions describe the world, and like that, we usually write functions that describe how something behaves.

Take for example a function that says hello to a person by name:

# let say_hi name = print_string ("Hello, " ^ name ^ "!\n") ;;
val say_hi : string -> unit = <fun>

We can call this function several times, to say "hello" to several people:

#say_hi "Xavier";;
Hello, Xavier!
- : unit = ()

# say_hi "Sabine";;
Hello, Sabine!
- : unit = ()

# say_hi "Joe";;
Hello, Joe!
- : unit = ()

If we wanted to say "hello" to the same person multiple times, we'd just repeat the same line of code.

# say_hi "Camel";;
Hello, Camel!
- : unit = ()

# say_hi "Camel";;
Hello, Camel!
- : unit = ()

# say_hi "Camel";;
Hello, Camel!
- : unit = ()

One way we can avoid having to repeat these lines every time is by writing a function to say "hi" 3 times:

# let say_hi_3_times name =
  say_hi name;
  say_hi name;
  say_hi name
;;
val say_hi_3_times : string -> unit = <fun>

In this function we can see a few behaviors:

• It says "hi" to the same name.
• It repeats it exactly 3 times.

But what would happen if we wanted to say "hi" 2 times? Or 4 or 12 times?

When this happens, it usually means that the function is making certain decision that it shouldn't. In other words, the function knows something (like the number of times).

So instead, we will create a function that let's the caller decide how many times to say "hi." We do this by requiring a new argument, in this case, times:

# let rec say_many_hi times name =
  if times < 1 then ()
  else begin
    say_hi name;
    say_many_hi (times - 1) name
  end
;;
val say_many_hi : int -> string -> unit = <fun>

Much better. Now we can call:

# say_many_hi 3 "Xavier";;
Hello, Xavier!
Hello, Xavier!
Hello, Xavier!
- : unit = ()

# say_many_hi 12 "Camel";;
Hello, Camel!
Hello, Camel!
Hello, Camel!
Hello, Camel!
Hello, Camel!
Hello, Camel!
Hello, Camel!
Hello, Camel!
Hello, Camel!
Hello, Camel!
Hello, Camel!
Hello, Camel!
- : unit = ()

Unfortunately, reusing this repetition behaviour isn't so easy because we have hard-coded our call to say_hi.

To make this reusable, we can let the caller decide what our function should do:

# let rec repeat times thing_to_do =
  if times < 1 then ()
  else begin
    thing_to_do;
    repeat (times - 1) thing_to_do
  end
;;
val repeat : int -> 'a -> unit = <fun>

But what should thing_to_do be? Our intuition may be that we can call:

# repeat 3 (say_hi "Camel");;
Hello, Camel!
- : unit = ()

But our program only outputs one salutation:

Hello, Camel!

That is not what we want! We want it to say "hello" to Camel 3 times.

In OCaml, the arguments are evaluated before the function itself, so in this case, we ended up saying "hi" before we even got to the repeat function.

We can let repeat call say_hi many times by delaying the function's execution by wrapping it with another function, like this:

# repeat 3 (
fun () ->
    say_hi "Camel");;

This means we must refactor our repeat function, by replacing thing_to_do with thing_to_do (), to call our new function:

let rec repeat times thing_to_do =
  if times < 1 then ()
  else (
    thing_to_do ();
    repeat (times - 1) thing_to_do)
;;

After renaming thing_to_do to fn we get a nice little repeat function:

# let rec repeat times fn =
  if times < 1 then ()
  else begin
    fn ();
    repeat (times - 1) fn
  end
;;
val repeat : int -> (unit -> 'a) -> unit = <fun>

And we can use repeat to recreate our original say_many_hi, or to repeat any work any number of times:

# let say_many_hi times name = repeat times (fun () -> say_hi name);;
val say_many_hi : int -> string -> unit = <fun>
  
# let print_big_space () = repeat 10 print_newline;;
val print_big_space : unit -> unit = <fun>

This is the power of Higher-Order Functions. They empower you to create complex behaviors from simpler functions.

Here's some other examples from the real world:

# let say_hi_to_many names = List.iter say_hi names;;
val say_hi_to_many : string list -> unit = <fun>


# module StringSet = Set.Make(String);;
module StringSet :
  sig
    type elt = string
    type t = StringSet.t
    val empty : t [...]
  end

# let only_once fn names =
  names
  |> StringSet.of_list
  |> StringSet.iter fn;;
val only_once : (string -> unit) -> string list -> unit = <fun>
 
# let yell_hi name =
  name
  |> String.uppercase_ascii
  |> say_hi;;
val yell_hi : string -> unit = <fun>
  
# let call_for_dinner names = only_once yell_hi names;;
val call_for_dinner : string list -> unit = <fun>

Common Higher-Order Functions

In the wild, there's certain patterns that repeat over and over again. It's useful to be familiar with them because they are part of the common vocabulary of a functional programmer. Some of them are:

• Currying and uncurrying
• Pipelining, composition, and chaining
• Iterating
• Filtering
• Mapping
• Folding (or reducing)
• Sorting
• Binding (or flat mapping)

Currying and Uncurrying

Since in OCaml all functions really just take one parameter, when you call add x y, you're actually calling two functions! ((add x) y)

Sometimes it helps to apply parts of a function in different orders, and sometimes it helps to make a function really take all its parameters at once.

This is what we call currying and uncurrying:

• A curried add function will be called like add x y.
• An uncurried add functoin will be called liked add (x, y). Note how this is really just one argument!

Before we get to some examples, let's define some helper functions that will help us curry and uncurry functions.

Uncurrying

Our uncurry helper is a function that takes one function as input and returns another function. It is essentially a wrapper.

The input function must have type 'a -> 'b -> 'c. This is the type of any function that takes 2 parameters.

The output function will have type ('a * 'b) -> 'c. Notice how the arguments 'a and 'b are now bundled together in a tuple!

Here's our helper:

(* [uncurry] takes a function that is normally curried,
   and returns a function that takes all arguments at once. *)
# let uncurry f (x, y) = f x y;;
val uncurry : ('a -> 'b -> 'c) -> 'a * 'b -> 'c = <fun>

If we wanted to write uncurry for more arguments, we'd just make a new uncurry3 or uncurry4 or even uncurry5 function that would work exactly the same:

# let uncurry4 f (w, x, y, z) = f w x y z;;
val uncurry4 : ('a -> 'b -> 'c -> 'd -> 'e) -> 'a * 'b * 'c * 'd -> 'e = <fun>

Uncurrying can be very useful when you're dealing with lists (which we do a lot in OCaml) and when the list happens to have tuples.

Take for example this list of tuples of names and favorite emojis:

# let people = [
  "🐫", "Sabine";
  "🚀", "Xavier";
  "✨", "Louis";
]
;;

If we wanted to do something with any of these elements, we'd need to split the tuple, and call a function:

# let greet emoji name =
  Printf.printf "Glad to see you like %s, %s!\n" emoji name
;;

let emoji, name = List.hd people in
# greet emoji name
;;

But we can also uncurry our greet function to operate over the entire tuple!

# uncurry greet (List.hd people)
;;

Currying

On the other hand, sometimes we have functions that already work with tuples, and we'd like to use them like functions that take multiple arguments.

For that we can define a little curry helper that will take a function as input, and return another function as output. It is essentially a wrapper.

The input function must have type: ('a * 'b) -> 'c – this is the type of any function that one tuple with 2 parameters.

The output function will have type 'a -> 'b -> 'c – notice how the arguments 'a and 'b are now unbundled!

Here's our helper:

(* [curry] takes a function that is normally curried,
   and returns a function that takes all arguments at once. *)
let curry f x y = f (x, y)
;;

If we wanted to write curry for more arguments, we'd just make a new curry3 or curry4 or even curry5 function that would work exactly the same:

let curry4 f w x y z = f (w, x, y, z)
;;

Currying can be very useful when you're dealing with lists (which we do a lot in OCaml) and when the list has a single value, but your function takes more than one.

Take for example this list of names and an uncurried revealing function:

let names = [
  "Sabine";
  "Xavier";
  "Louis";
]
;;

let reveal (title, name) =
  Printf.printf "But it was %s, %s!\n" title name
;;

If we wanted to use reveal on a name, we have to put it into a tuple, and then do the call. Like this:

List.iter (fun name ->
  let title = "The OCamler" in
  reveal (title, name)) names
;;

But we can also curry our reveal function to take 2 arguments!

List.iter (curry reveal "The OCamler") names
;;

Readability Notes

Currying and uncurrying is all fun and games until your code gets very hard to read.

Sometimes it makes sense to curry a function, and sometimes the clearer thing is to manually wrap it in a function.

For example, this is a pipeline with a lot of currying/uncurrying that would most likely be easier to read and maintain if we manually wrote out the wrapper functions:

let do_and_return f x = f x; x
;;

let flip (x, y) = (y, x)
;;

names 
|> List.map (do_and_return (greet "👋🏼"))
|> List.map (Fun.flip List.assoc (List.map flip people))
|> List.iter (curry reveal "The OCamler")
;;

Versus a much more readable and easier to maintain version:

let find_by_name name1 =
  List.find (fun (_emoji, name2) -> name1 = name2) people
;;

(* first iterate over the names, greeting them *)
names |> List.iter (greet "👋🏼")
;;

(* then find the right emoji by name *)
names
|> List.map find_by_name
|> List.iter (fun (emoji, _name) -> reveal ("The OCamler", emoji))
;;

Pipelines, Composition, and Chaining

In OCaml we use functions a lot, so values go from one function to the other forming what we like to call pipelines.

let a = foo () in
let b = bar a in
let c = baz b in
(* ... *)

Of course, we can always call functions in a nested fashion, to avoid the extra variables and all that typing:

let c = baz (bar (foo ())) in
(* ... *)

But this is not so easy read sometimes, especially as the number of functions grows, as it goes from the inside out.

To avoid this we have use the |> operator:

let c = foo () |> bar |> baz in 
(* ... *)

This operator translates to the exact same nested calls we would've done by hand, and is really no magic. It is defined as a function:

(* the pipeline operator *)
let (|>) x fn = fn x

It receives a value x and a function fn and as soon as it has both, it calls fn with x. This lets us invert the order and build pipelines that read left-to-right or top-to-bottom instead of inside-out.

But what happens when our functions have more than one argument?

Let's look at an example of string manipulation. We want to get the domain name from an email.

let email = "ocaml.mycamel@ocaml.org"
;;

email
|> String.split_on_char '@'
|> Fun.flip List.nth 0
|> Option.map (fun str -> String.sub str 0 5)
|> Option.get
;;

Thanks to OCaml currying functions by default, it is practical to partially apply a function with only some of its arguments, and leave the last one to be passed along in the pipeline.

This is true for functions that have the most important argument in the last position (which we call t-last) and for functions that use labeled arguments and allow the most important argument to be passed last by passing all the names arguments first (which we usually call t-first).

These two cases sound very similar, but have a big practical difference when it comes to usability. Let's revisit our example above using labeled argument versions of those functions:

open StdLabels

module List = struct
  include List
  let nth_opt t ~at = nth_opt at t
end

email
|> String.split_on_char ~sep:'@'
|> List.nth_opt ~at:0
|> Option.map (String.sub ~off:0 ~len:5)
|> Option.value ~default:"new-user"
;;

(** NOTE(@leostera): this example kinda sucks, i'd like one where the use of labels greatly improves the readability but since ListLabels.nth_opt doesn't take an argument then we still need that nasty fun flip :( will get back t othis)

Iterating

We usually think of iteration when we think of looping, and going through collections of things:

• loop through elements in a list
• go over the keys in a map

But in OCaml the pattern for iteration can be extended to other kinds of data types, like optional values or results, or trees and lazy sequences.

Iterating in OCaml means that if there is a value (or more), we'd like to apply a function to it.

Iterating over Lists

A list in OCaml is a linked-list that is composed by a head (the first element) and a tail (the rest of the list).

We can iterate over lists by pattern matching on then. When doing so, we either get an empty list ([]), or we get a pattern with a head and a tail (n :: rest). On the branch with a head and a tail, we can directly use the head value and apply a function to it, and then recurse with the tail.

let rec print_nums nums =
  match nums with
  (* if the list is empty, we do nothing *)
  | [] -> ()
  (* if the list is not empty... *)
  | n :: rest ->
    (* we print the first element *)
    Printf.printf "%d\n" n;
    (* and repeat over the rest of the list *)
    print_nums rest

Now if we wanted to do something else with each element of the list, we could just ask for a function that will run over them:

let rec print_all fn nums =
  match nums with
  | [] -> ()
  | n :: rest ->
    fn n;
    print_all fn rest

And this way we can call print_all with any function fn, so it really is just iterating over the list and running a function over every element.

That's exactly how List.iter is defined in the standard library.

Iterating over Optionals and Results

Another kind of data it is common to iterate over in OCaml is optional and result values. Normally, we want to run a function only if the option has Some value in it or if there is an Ok value. If there is no value in it, or if we have an Error, we don't want to do anything.

We can do this by pattern-matching on our value and calling our function over the inner value in the Some or Ok branch.

let run_if_some opt fn =
  match opt with
  | Some value -> fn value
  | None -> ()
;;
 
let run_if_ok res fn =
  match res with
  | Ok value -> fn value
  | Error _  -> ()
;;

This is how Option.iter and Result.iter are defined in the standard library.

Iterating over Maps and Sets

Larger collections of data like maps and sets, are also common in OCaml. We have dedicated modules for them but they have a functor interface. This means you can't really use Set or Map directly, but you have to call the module-level function Set.Make to create your own custom version of the Set module for the specific types you want to store in it.

Once you create your Set or Map module, you'll find they provide functions to convert their values into lists.

With either of those functions, we can put together an iterator over maps or sets:

let iter values collection fn  =
  let values : 'a list = values collection in 
  List.iter fn values
;;
  
module StringSet = Set.Make(String);;
module IntMap = Map.Make(Int);;

let iter_map map fn = iter IntMap.bindings map fn ;;
let iter_set set fn = iter StringSet.elements set fn ;;

You'll notice that we did not use pattern-matching this time around to iterate over the values of the Map or the Set directly. This is because the representation of Sets and Maps is private.

The actual implementation of iteration functions for Maps and Sets does use pattern-matching under the hood.

Iterating over Lazy Sequences

Usually, a module that implements some data type will provide functions that iterate over it.

But some data is lazy, and it only lets us access one element at a time. So if the data looked like [1,2,3] you'd only be able to compute the 3rd value after accessing the first and second ones.

Lazy sequences in OCaml are represented with the Seq module, which has a function called uncons to get the next element. This function also returns the new sequence that we can use to get the 2nd element, and so on.

let rec iter seq fn = 
  match Seq.uncons seq with
  | None -> ()
  | Some (value, seq2) -> 
    fn value;
    iter seq2 fn
;;

This function will try to get the first element of the sequence, and if there is one, run our function fn over it. Then it will repeat over the new sequence (seq2) which starts at the second element.

This is almost exactly how Seq.iter is defined in the standard library.

Iterating over custom data types

So far we've seen how to iterate over data types from the standard library. Now we'll see how to iterate over our own data type for trees.

We'll define our tree type to include 2 constructors. One for a leaf node, which is a node at the end of the tree. The other one for nodes that have children.

type 'value tree =
 | Leaf of 'value 
 | Node of 'value tree * 'value
;;

So our data type allows us to represent tree-like data:

graph TD
  Node1([Node]) --> ML
  Node1 --> Node2
  Node2([Node]) --> Caml((Caml))
  Node2 --> Node3
  Node3([Node]) --> CamlLight([CamlLight])
  Node3 --> Leaf
  Leaf  --> OCaml

Now before we define our iteration function, its important to define what iteration means for our data type. Do we want to iterate from the bottom of the tree? Do we want to iterate from the top? Should we do middle-out?

For our example, we'll iterate from the top down as we go along:

let rec iter tree fn = 
  match tree with
  | Leaf value -> fn value
  | Node (tree2, value) -> 
      fn value;
      iter tree2 fn
;;

Again, we iterate by pattern matching on values, applying a function to the deconstructed value and recursing over the remaining data.

Mapping

In contrast to iterating, sometimes we want to apply a function over some data and we want to keep the results without changing the shape of the data.

For example, if we have a list of users, maybe we want to get a list of usernames. Or if we have an optional password, we may want to encrypt it only if it is set.

This is called mapping.

Mapping Lists

Mapping lists is very similar to iterating over them. We pattern match on a list, get the head of it, run a function over it, and recurse over the body.

The main difference is that instead of throwing away the resulting value from running our function over the elemnts, we will reconstruct a list from it.

let rec map list fn =
  match list with
  | [] -> []
  | head :: tail -> (fn head) :: (map tail fn)
;;

Note how we use the :: constructor to both deconstruct the list and reconstruct it.

Mapping Options

Mapping an optional value is only meaningful when we want to change the contents in case there is a value inside our option. That is, if we have a None there is nothing to map, so we can only map Some x values.

let map opt fn =
  match opt with
  | Some value -> Some (fn value)
  | None -> None
;;

Note that both sides of the match return the same thing: if we had a None we return None, if we have a Some we return a Some. This way, the structure is preserved.

Mapping Results

When we have a result, mapping becomes a little trickier. We now have 2 possible ways in which we can change the internal value, and they're both fully valid maps!

We can map the value in the Ok value constructor, or we can map the error value in the Error reason constructor.

(* maps a result over the ok value *)
let map_ok res fn =
  match res with
  | Ok value -> Ok (fn value)
  | Error reason -> Error reason
;;

(* maps a result over the error value *)
let map_err res fn =
  match res with
  | Ok value -> Ok value
  | Error reason -> Error (fn reason)
;;

Both of these are useful in different situations, such as wanting to change the type of errors, or only perform operations once we have an Ok value.

Mapping Custom Data Types

When working with our custom data types, such as the tree we used in the Iterating section, we should try to always preserve the structure of the data. That is, if we map over it, we'd expect the same nodes and connections between nodes, but with different values in them.

let rec map tree fn =
  match tree with
  | Leaf value -> Leaf (fn value)
  | Node (tree2, value) -> Node (map tree2 fn, fn value)
;;

Note that the structure of the tree is preserved, but every time we encounter a value, we update it with (fn value).

Folding (or reducing)

Sometimes we want to iterate over data and collect the results as we go from element to element. This behavior is called "folding" or "reducing", and it is very useful for summarizing data.

For example, if we have a list of numbers from 1 to 10, we can add them together via a fold:

let sum = List.fold_left (+) 0 [1;2;3;4;5;6;7;8;9;10];;

If we wanted to implement a sum over the custom tree type we saw in the Iterating chapter, we could do it like this:

type 'value tree =
 | Leaf of 'value 
 | Node of 'value tree * 'value
;;

let rec sum_tree tree =
  match tree with
  | Leaf value -> value
  | Node (tree2, value) -> value + (sum_tree tree2)
;;

And if we generalize this to apply any transformation over our tree and reduce it to a single value we will need to replace our + function with an argument fn:

let rec fold_tree tree fn =
  match tree with
  | Leaf value -> fn value
  | Node (tree2, value) -> fn value (fold_tree tree2 fn)
;;

But we quickly run into a problem: our fn function is meant to combine two items, so in the Leaf branch, what is the second item?

Folding requires us to define a zero value, a starting point for an accumulator, that will be used when the collection or data type is "empty".

Some data types don't have a good "empty" value. Our tree for example does not. Lists do have an empty list. Options have a None constructor. Results' don't have a good "empty" value either.

So to fix our fold_tree we just need to pass in a zero or accumulator value:

let rec fold_tree tree fn acc =
  match tree with
  | Leaf value -> fn value acc
  | Node (tree2, value) -> fn value (fold_tree tree2 fn acc)
;;

And voila! Our function now types correctly and we can use it to reduce our trees down to any value.

Sorting

Another common behavior usually implemented with higher-order functions is sorting collections.

Both Array.sort and List.sort implement an interface where if you pass in a function that knows how to compare 2 elements, they'll adapt the sorting to use this comparison.

For arrays, this operation mutates the array in-place:

let array = [| 4;9;1;10 |];;

(* sorts the array in ascending order *)
Array.sort (fun a b -> a - b) array;;

(* sorts the array in descending order *)
Array.sort (fun a b -> b - a) array;;

For lists, this operation returns a new sorted list:

let list = [4;9;1;10] ;;
let asc = List.sort (fun a b -> a - b) list ;;
let desc = List.sort (fun a b -> b - a) list ;;

Most OCaml modules include a compare function that can be pass in to sort:

let int_array = [|3;0;100|];;
Array.sort Int.compare int_array;;

List.sort String.compare ["z";"b";"a"];;

List.sort Bool.compare [true;false;false];;

Binding

One last common higher-order pattern in functional programming is the ability to join data from within. For historical reasons, this is normally called a bind.

For example, if we have a list, and map over it with a function that returns a list, then we'll have a list of lists. Sometimes we want this, but some times we would rather the new list was flattened instead of nested.

To do this with lists we can use the concat_map function, which looks like this:

let twice x = [x;x];;
let double ls = List.concat_map twice ls;;

double [1;2;3];; (* [1;1;2;2;3;3] *)

Binding as early returns

This same pattern is useful to build chains of functions that short circuit on specific values.

For example, if you had to retrieve a user from a database, and only if there is a user try access the user's email, you could use Option.bind to short-circuit on the first operation:

type user = {
    email: string option
}

let get_user () = None ;;
let get_email user = user.email ;;

let email = Option.bind (get_user ()) get_email ;;

In this example, because our get_user () call returns None, get_email will never be called. Only if get_user returns Some user does get_email be called.

This also applies to result values:

type user = {
    email: string
}

let get_user () = Error `no_database ;;
let get_email user = Ok user.email ;;

let email = Result.bind (get_user ()) get_email ;;

The main difference is that in this case Result.bind is biased towards Ok value, so that if a result value is an Error reason, it will short-circuit through the binds and just return the first error that appears.

Let-ops

Unfortunately, calling Result.bind can be a bit awkward. We can't pipe our value through a series of binds like we can do with calls to map. For example, this isn't valid:

let email = get_user ()
           |> Result.bind get_email
           |> Result.bind extract_domain
           |> Result.bind validate_domain
           ;;

Thankfully, OCaml lets us redefine a subset of operators called let-operators that can be used to streamline the use of bind calls by making them look very close to normal let-bindings:

(* first we will declare out let* operator to be equal to Result.bind *)
let (let*) = Result.bind

let* user = get_user () in
let* email = get_email user in
let* domain = extract_domain email in
validate_domain domain

This has the advantage of making code a lot more readable, without changing the behavior we've come to expect from bind calls.

Async code

Async libraries for OCaml that implement Promises/Futures usually also have a bind function that allows you to chain computations.