# Haskell Course - Lesson 7 - Functions, Variables, and Signatures

functional programming language
, which means that we'll code programs and applications by creating and
composing functions
. (Remember to click on the purple words for extra context.) On top of that, Haskell is a
purely functional
language. That's why functions are so critical to understand. They're the backbone of the entire language! Luckily, they're not hard to learn π! Let's dive in!

## Functions

A function is an expression that performs a specific task packaged as a unit. (It'll make more sense as you keep reading.)

### Declaring functions

This is an expression to declare a function that checks if a number is greater than 18:

``````greaterThan18 x = x > 18
``````
• `greaterThan18` is the name of the function. When declaring a function, choose a name that makes it easy to know what it does.
• `x` is a symbol serving as a placeholder that'll be replaced by a value when we call (use) the function. It's called a parameter.
• The `=` operator assigns the `x > 18` expression to the `greaterThan18` name.

To the left of the `=` sign, we write the function's name and parameters. And to the right, the expression that'll be contained by this function.

We can write complex expressions and "package" them inside a simple and reusable name. Reusable is the keyword here. Instead of writing the same complex code, again and again, we just write the name of the function that contains it. (Convenient for lazy people like me! π)

Cool, we have a function. Now, let's use it!

### Using functions

Super simple! To use this function, we just have to write the name, a space, and write a number:

``````greaterThan18 30  -- True
``````

When that expression is executed, Haskell replaces all the `x` with `30`, and `greaterThan18 30` becomes `30 > 18`. Then, it evaluates the expression, returning `True`.

Let's explore a little bit more this concept of parameter and result.

### Function parameters and result

Both when defining the function and when using it, parameters are separated by spaces:

``````add2numbers a b = a + b
``````
``````add2numbers 1 2   -- 3
``````

The order of the parameters matters:

``````greeting name lastname = "We meet again, " ++ name ++ " " ++ lastname ++ "!"
``````

When using the function, `name` will always be the first parameter and `lastname` the second.

``````greeting "James" "Bond" -- "We meet again, James Bond!"
``````
``````greeting "Bond" "James" -- "We meet again, Bond James!"
``````

A function has to ALWAYS return a result:

A function will return the final result of evaluating the expression on the right side of `=`. And it always has to return something.

Imagine that we create a function to check the age of people that comes to our restaurant, and we do something like:

``````wrongFunciton x = if x < 250 then "You can come in" -- ERROR
``````

There's no way anyone would be older than 250 years! π§πΌ But Haskell doesn't care! If the possibility of not returning something exists, the compiler will yell at you to fix it! It doesn't matter if it'll ever happen. We have to be ready for anything!

So we fix it:

``````correctFunciton x = if x < 250 then "You can come in" else "Zombieeeeeee!!! π§"
``````

Done! Also, a function can have as many parameters as needed:

``````add6numbers u v w x y z = u + v + w + x + y + z
``````
``````add6numbers  1 2 3 4 5 6  -- 21
``````

OK, we saw that functions can take as many parameters as needed, but how low can you go? π³

## Names/Definitions

Take a look at this function:

``````name = "Daniel"
``````

If we don't have parameters, we have a function that always returns the same exact value, no matter what! π

The kind of function that doesn't take parameters is called a definition or a name.

Because we can't change the value of a definition (the expression on the right side of the `=` always evaluates to the same result), `name` and `"Daniel"` are essentially the same thing. And we can use them interchangeably.

This is what in most programming languages is called a variable. But, "variable" doesn't always mean the same.

When talking about programming in general, a variable is like a box that contains a value. And the variable's name is written on the side of the box. For example:

``````x = 3
``````

The symbol before the `=` is the variable's name. And the value after the `=` is the variable's value. `x = 3` means: I have a "box" named `x` that has a `3` inside it.

Now, after declaring this variable, we can use it the same way we would use the `3`:

``````x = 3
7 + x   --- This will give us 10
``````

Andβin most programming languagesβyou can later change your mind and replace the value inside the box:

``````x = 3
7 + x   --- This will give us 10
x = 5
7 + x   --- This will give us 12
``````

OOOOOOOOhhhhhh but not with Haskell, no, no, no! Once you tell Haskell that `x` means `3`, it will mean `3` forever! Haskell doesn't mess around with undecided people, so you better be sure when you declare a variable!

Haskell's concept of variable is different from most programming languages. Haskell has variables but in the mathematical sense. And you don't see variables changing value halfway through a problem!

Professor: A train leaves Venice at 7:00 pm, averaging 80 mph. Another train headed in the same direction leaves Venice at 10:00 pm, averaging 100 mph. At what time will the second train overtake the first train?

And then, halfway through the problem:

Now the first train goes 85mph! Go!

Wait, what? Which is it? Those are two different problems! π‘

π As you can see, you don't change values halfway through a math problem, and you don't change definitions halfway through a Haskell program. In technical terms, Haskell variables are immutable. They vary only based on the data we enter into a program (i.e., `speed = 80`). We can't define `speed` two times in the same code, but we could change the value by changing the definition.

Note that GHCi doesn't stop you from redefining variables or functions, but that's because of a particular way that GHCi works under the hood. This is really convenient for testing things quickly. But when writing a Haskell program, the compiler will yell at you if you dare.

OK, great! We know about functions and variables. Now let's learn about their types!

### Type signatures

Because variables and the value they contain are interchangeable, it follows that they also have the same type! Let's look at a few examples (Try to guess the type before checking):

``````name = "Lars"
``````
``name :: [Char]``
``````expr1 = 5 + (4 :: Float)
``````
``expr1 :: Float``

It has a type of Float because the value returned by the expression on the right side of `=` evaluates to a Float of value `9.0`.

``````expr2 = if True then 5 else (7 :: Int)
``````
``expr2 :: Int``

It has a type of Int because the value returned by the expression on the right side of `=` evaluates to an Int of value `5`.

How does Haskell know that 5 is an Int? Because we indicated that 7 was an Int and Haskell understood that the expression is meant to return an Int. So it correctly infers that 5 is an Int.

As you can see, It's not that hard to know the type of a variable. It's just the type of value that it returns after evaluating the expression that it contains. Easy enough. Let's step it up a notch by learning about function types!

## Function's type signatures

Functions have types, too! Although they are somewhat different:

``````simpleGreeting :: [Char] -> [Char]
simpleGreeting name = "Hi, " ++ name ++ "!"
``````

The line before the function's declaration is the function's type signature. And it represents the function's type. This is how we write signatures:

• We start with the function's name and the type operator (`simpleGreeting ::`) to indicate that we're defining the type of that specific function.
• We add the parameter types separated by the `->` symbol.
• When we run out of parameters, we add a final `->` and add the return type.

Here's how we read the last signature: "`simpleGreeeting` takes a value of type `[Char]` and returns a value of type `[Char]`."

Easy enough. But what about a function with multiple values?:

``````complexGreeting :: [Char] -> [Char] -> [Char]
complexGreeting name lastname = "We meet again, " ++ name ++ " " ++ lastname ++ "!"
``````

Here's how we read that signature: "`complexGreeting` takes a value of type `[Char]`, then another value of type `[Char]`, and returns a value of type `[Char]`."

Basically, the type after the last `->` is always the return type, and all the types before are parameter types separated by `->`. It's that simple.

When you encounter function signatures for the first time, it feels like they could've done a better job with the symbols. But it'll make perfect sense once you learn about curried functions (in the distant future, no need to overcomplicate things now).

Let's practice a little!

A function that multiplies a number by two:

``````multByTwo x = x * (2 :: Float)
``````
``multByTwo :: Float -> Float``

A function thatβgiven the radius and the heightβcalculates the volume of a cylinder:

``````volumeOfACylinder r h = pi * r^2 * (h :: Float)
``````

(`pi` is a variable that represents the number `Ο`, and comes with Haskell.)

``volumeOfACylinder :: Float -> Float -> Float``

The great thing about types (remember from lesson 2) is that they protect us from ourselves! If we say that a function takes an input of type `[Char]`, Haskell will check that we meet that requirement each time we use that function. If we pass a `Double`, the compiler will yell at us to correct that mistake!

But now we have a problem! π€¨ What if we want to use `volumeOfACylinder` with `Double`s? We know that it'll work because they're still fractional numbers, and the formula will provide the correct answer. π€

We could create a new function that does the same but specifies `(h :: Double)`. Something like:

``````volumeOfACylinder2 r h = pi * r^2 * (h :: Double)
``````

The thing is, if we do this for every function that could work with multiple types, we'll sit on LOTS AND LOTS of duplicated code! π That goes against one of the fundamental coding principles: DRY (Don't Repeat Yourself). So, what should we do? π€

Polymorphic values to the rescue! π¦Έ

## Polymorphic values

Polymorphic means something that has multiple forms. And a polymorphic value is a value that can have multiple types.

For example, the `fst` and `snd` functions that we used in the last lesson were polymorphic. If you think about it, which type should they have? We know that the input value is a pair (a tuple of two elements) and that it returns the first one.

So, if we use it for this pair: `('a', True)` we'd need a `fst` with a signature like this:

``````fst :: (Char, Bool) -> Char
``````

It takes a tuple of type `(Char, Bool)` and returns the first value (`Char`). But what if we need to use it for a different pair like: `(Double, [Char])`? Should we create a new function? No! We specify a signature with type variables, like this:

``````fst :: (a, b) -> a
``````

That signature reads: "The `fst` function takes a pair of type `(a, b)` and returns a value of type `a`."

`a` and `b` are type variables, meaning they can be of any type. And no matter the type, the value returned by `fst` has to be of the same type as the first element of the pair (because they are both of type `a`).

By using type variables, we can use the `fst` function with pairs of any type (polymorphic values)!

Notice that `a` and `b` both CAN be of any type AND different types from each other. But they don't HAVE to be. You can use `fst` on a tuple with values of the same type: `('a','b') :: (Char, Char)`.

IMPORTANT: Specific types (i.e., `Char`, `Bool`, `Int`) start with capital letters. But polymorphic types start with lower case letters. We can use longer names for polymorphic types, but the usual is to use single letters (i.e., `a`, `b`, `c`).
``snd :: (a, b) -> b``

Awesome! Let's seize the opportunity to learn two more functions that are native to the Haskell language: `head` and `tail`.

### `head` and `tail`

``head :: [a] -> a``

We don't care about the specific types. We're just extracting an element. So, the parameter is a polymorphic list (a list of any type, let's call it `[a]`). And the result has to be an element of the same type as the elements on the list. That's why it has to be `a`.

We saw that the function takes a polymorphic list, and this is a function that comes with Haskell! So let's fire up GHCi and test it!

``````head [1,2,3,4] -- This will give us 1
``````
``````head "Helooo!" -- This will give us 'H'
``````
``````head [True, False, False, True] -- This will give us True
``````
``tail :: [a] -> [a]``

As you can see, `tail` returns what `head` discards and vice versa. An example of how `tail` works:

``````tail [1,2,3,4] -- Will give us [2,3,4]
``````
``````tail "Helooo!" -- This will give us "elooo!"
``````

Two more functions under our belt! πͺ We're on fire! π₯ These look like they won't be practical, but trust me, they are!

Now that you know all about type signatures, I can let you in on a little secret π you can check the type of any expression using commands! π¬

## The `:t` and `:?` commands

GHCi has commands the same as the terminal. (We learned about commands in lesson 1.) And you already know a GHCi command! It's `:q` (or `:quit`), the command to exit GHCi!

GHCi has a ton of commands, and they all start with `:` (colon) and are lower-cased. Today we'll learn two new commands: `:t` and `:?`.

### The `:t` command

The `:t` command shows the type of the expression that comes after it. Like this:

``````:t 'a'   -- 'a' :: Char
``````

It also works with native functions:

``````:t head  -- head :: [a] -> a
``````

And you can use it to find out the type of a function that you created! (Haskell can infer the types):

Create the function in GHCi:

``````complexGreeting name lastname = "We meet again, " ++ name ++ " " ++ lastname ++ "!"
``````

And check its type:

``````:t complexGreeting -- complexGreeting :: [Char] -> [Char] -> [Char]
``````

You can also write a complex expression right after the command:

``````:t if True then 5 else (7 :: Int) -- if True then 5 else (7 :: Int) :: Int
``````
This command is useful when learning new functions and libraries because it allows us to explore the types to understand them better.

As a spoiler of future lessons, check `:t 4` π.

What's that `=>` in the type? What's `Num`? And why it's a polymorphic type if it's a number? π« What is going on????"

β Overly dramatic student

π That's a class constraint. We'll learn about it shortly (maybe in the next lesson). So don't worry about it for now. π

The `:t` command is just one of many! A good way to learn about the rest is using `:?`

### The `:?` command

The `:?` command displays the list of all the commands that you have available inside GHCi. It's a great way to explore what you can do with it!

Run it and take a look at the other available commands. Right now, most won't make sense because they're used with things that we'll learn in the future, but take a look anyway and get familiarized with the ones that can be useful to you now. Like `:i` and `:show bindings`.

OK, before we go, I have one last confession to make. It's all functions!

## It's all functions

Now comes the big reveal. We've been using functions for a long time now! `==` is a function, `>=` is a function, `+` is a function, `/` is a function, `++` is a function, everything that we use so far that takes something and produces another thing has been a function! π€―

Don't believe me? Check the types for yourself! (use parenthesis):

``````:t (++)  -- (++) :: [a] -> [a] -> [a]
``````

From now on, remember that everything that we'll learn about functions applies to all the native functions too!! π

OK, that's enough new stuff for one day! π But the class is not over!

We learned a lot, and now we have to consolidate the information. Make sure to do the homework and ask if you have any questions. πͺ

## Homework

### Signature writing practice.

Figure out (without GHCi) the signatures of these functions:

``const :: String``
``fun :: [a] -> Bool``
``fun :: [a] -> a -> Bool``

DOUBLE TRICK QUESTION! π€ͺ You can't modify a variable, and you must return something!

### Functions practice

``fToC x = (x - 32)*5/9``
``func l = (head l) * 2``

(It also works without parenthesis. But this way is more explicit.)

``theSame f c = c == (fToC f)``

Use examples:

``theSame 32 0 -- True``
``theSame 0 32 -- False``
``theSame 212 100 -- True``
``theSame 100 37.77 -- False``
``theSame 100 37.77777777777778 -- True``

Play around with functions! Make something cool, and (if you want) let me know! π

I know today we covered a lot. Take your time to explore the ideas, and feel free to ask any questions! π Next lesson will either be type classes, more about functions, or we'll write our first Haskell script. I'm not sure yet. If you have any preferences, let me know on Twitter! π Else, see you next week! πͺ Have a great one!

PD: If you find this course valuable, please share it so more people can learn! π