Recently I took the plunge into writing F# on OS-X and Linux (Ubuntu specifically). This is the first of a new series I’m starting (along with my other ongoing series on JavaScript workflow and practices). In this article particularly I’m just going to provide an overview and notes of key paradigms in functional programming. Most of these notes are derived from a great series called “Thinking Functionally“.

## #1 Thinking Functionally: Introduction

This is the article that kicks off the series. The emphasis is focused around realizing that a different way of thinking needs applied when using a functional language. The difference between functional thinking and imperative. The key topics of the series includes:

- Mathematical Functions
- Functions and Values
- Types
- Functions with Multiple Parameters
- Defining Functions
- Function Signatures
- Organizing Functions

## #2 Mathematical Functions

Functional programming, its paradigms and its origins, are all rooted in mathematics. A mathematical function looks something like this:

Add1(x) = x + 1

- The set of values that can be used as input to the function is called the
*domain*. - The set of possible output values from the function is called the
*range*. - The function is said to
*map*the*domain*to the*range*.

If F# the definition would look like this.

let add1 x = x + 1

The signature of the function would look like this.

val add1 : int -> int

**Key Properties of Mathematical Functions**

- A function always gives the same output value for a given input value.
- A function has no side effects.

**Pure functions**

- They are trivially parallelizable.
- I can use a function lazily.
- A function only needs to be evaluated once. (i.e. memoization)
- The function can be evaluated in any order.

**Prospectively “Unhelpful” properties of mathematical functions**

- The input and output values are immutable.
- A function always has exactly one input and one output

## #3 Function Values and Simple Values

Looking at this function again:

let add1 x = x + 1

The x means:

- Accept some value from the input domain.
- Use the name “x” to represent that value so that we can refer to it later.

It is also referred to as x is bound to the input value. In this binding it will only ever be that input value. This is not an assignment of a variable. Emphasis on the fact that there are no “variables”, only values. This is a critical part of functional thinking.

**Function Values:** this is a value that provides binding for a function to a name.

**Simple Values:** This is what one might think of as constants.

let c = 5

**Simple Values vs. Function Values**

Both are bound to names using the keyword *let*. The key difference is a function needs to be application to an argument to get a result, and a simple value doesn’t, as it is the value it is.

**“Values” vs. “Objects”**

In F# most things are referred to as “values”, contrary to “objects” in C# (or other languages like JavaScript for that matter). A value is just a member of a domain, such as a domain of ints, string, or functions that map ints to strings. In theory a value is immutable and has no behavior attached to it. However in F#, even some primitive values have some object-like behavior such as calling a member or property with dot notation syntax like “theValue”.Length or something similar.

**Naming Values**

Most of the naming is the general ruleset that applies to practically every language. However there is one odd feature that comes in handy in some scenarios. Let’s say you want to have a name such as “this is my really long and flippantly ridiculous name” for a domain. Well, what you can use is the “this is my really long and flippantly ridiculous name“ to have that be the name. It’s a strange feature, but I’ll cover more of it in subsequent articles.

In F# it is also practice to name functions and values with camel case instead of pascal case (camelCase vs. PascalCase).

## #4 How Types Work with Functions

Function signatures look like this, with the arrow notation.

val functionName : domain -> range

Example functions:

let intToString x = sprintf "x is %i" x // format int to string let stringToInt x = System.Int32.Parse(x)

Example function signatures:

val intToString : int -> string val stringToInt : string -> int

This means:

- intToString has a domain of int which it maps onto the range string.
- stringToInt has a domain of string which it maps onto the range int.

**Primitive Types**

Standard known types you’d expect: string, int, float, bool, char, byte, etc.

**Type Annotations**

Sometimes type might not be inferred, if that is the case, the compiler can take annotations to resolve type.

let stringLength (x:string) = x.Length let stringLengthAsInt (x:string) :int = x.Length

**Function Types as Parameters**

A function that takes other functions as parameters, or returns a function, is called a higher-order function (sometimes abbreviated as HOF).

Example:

let evalWith5ThenAdd2 fn = fn 5 + 2

The signature looks like:

val evalWith5ThenAdd2 : (int -> int) -> int

Looking at an example executing. The example:

let times3 x = x * 3 evalWith5ThenAdd2 times3

The signature looks like:

val times3 : int -> int val it : int = 17

Also these are very sensitive to types, so beward.

let times3float x = x * 3.0 evalWith5ThenAdd2 times3float

Gives an error:

error FS0001: Type mismatch. Expecting a int -> int but given a float -> float

**Function as Output**

A function value can also be the output of a function. For example, the following function will generate an “adder” function that adds using the input value.

let adderGenerator numberToAdd = (+) numberToAdd

**Using Type Annotations to Constrain Function Types**

This example:

let evalWith5ThenAdd2 fn = fn 5 +2

Becomes this:

let evalWith5AsInt (fn:int->int) = fn 5

…or maybe this:

let evalWith5AsFloat (fn:int->float) = fn 5

**The “unit” Type**

When a function returns no output, it still requires a range, and since there isn’t a void function in mathematical functions this supplants the void as return output. This special range is called a “unit” and has one specific value only, a “()”. It is similar to a void type or a null in C# but also is not, in that it is the value “()”.

**Parameterless Functions**

For a WAT moment here, parameterless functions come up. In this example we might expect “unit” and “unit”.

let printHello = printf "hello world"

But what we actually get is:

hello world val printHello : unit = ()

Like I said, a very WAT kind of moment. To get what we expect, we need to force the definition to have a “unit” argument, as shown.

let printHelloFn () = printf "hello world"

Then we get what we expect.

val printHelloFn : unit -> unit

So why is this? Well, you’ll have to dive in a little deeper and check out the F# for fun and profit “Thinking Functional” entry on “How Types Work with Functions” for the full low down on that. Suffice it I’ve documented how you get what you would expect, the writer on the site goes into the nitty gritty of what is actually happening here. Possibly, you might have guessed what is happen already. There is also some strange behavior that takes place with forcing unit types with the ignore function that you may want to read on that article too, but if you’re itching to move on, and get going faster than digging through all of this errata, keep going. I’m going to jump ahead to the last section I want to cover for this blog entry of notes, currying.

## #5 Currying

Breaking multi-parameter functions into smaller one-parameter functions. The way to write a function with more than one parameter is to use a process called currying. For more history on this check out Haskell Curry the mathematician to dive deep into his work (*which also covers a LOT of other things beside merely currying, like combinatory logic and Curry’s paradox for instance*).

Basic example:

let printTwoParameters x y = printfn "x=%i y=%i" x y

This of course looks neat enough right? Well, if we look at the compiler rewrite, it gets rather interesting.

let printTwoParameters x = let subFunction y = printfn "x=%i y=%i" x y subFunction

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