Adding type signatures

Haskell is a statically typed programming language. That means that every expression has a type, and we check that the types are valid with regard to each other before running the program. If we discover that they are not valid, an error message will be printed, and the program will not run.

An example of a type error would be if we'd pass 3 arguments to a function that takes only 2, or pass a number instead of a string.

Haskell is also type inferred, so we don't need to specify the type of expressions - Haskell can infer from the context of the expression what its type should be, and that's what we have done until now. However, specifying types is useful - it adds a layer of documentation for you or others that will look at the code later, and it helps verify to some degree that what was intended (with the type signature) is what was written (with the expression). It is generally recommended to annotate all top-level definitions with type signatures.

We use a double-colon (::) to specify the type of names. We usually write it right above the definition of the name itself.

Here are a few examples of types we can write:

Int - The type of integer numbers
String - The type of strings
Bool - The type of booleans
() - The type of the expression (), also called unit
a -> b - The type of a function from an expression of type a to an expression of type b
IO () - The type of an expression that represents an IO subroutine that returns ()

Let's specify the type of title_:

title_ :: String -> String

We can see in the code that the type of title_ is a function that takes a String and returns a String.

Let's also specify the type of makeHtml:

makeHtml :: String -> String -> String

Previously, we thought about makeHtml as a function that takes two strings and returns a string.

But actually, all functions in Haskell take exactly one argument as input and return exactly one value as output. It's just convenient to refer to functions like makeHtml as functions with multiple inputs.

In our case, makeHtml is a function that takes one string argument and returns a function. The function it returns takes a string argument as well and finally returns a string.

The magic here is that -> is right-associative. This means that when we write:

makeHtml :: String -> String -> String

Haskell parses it as:

makeHtml :: String -> (String -> String)

Consequently, the expression makeHtml "My title" is also a function! One that takes a string (the content, the second argument of makeHtml) and returns the expected HTML string with "My title" in the title.

This is called partial application.

To illustrate, let's define html_ and body_ in a different way by defining a new function, el.

el :: String -> String -> String
el tag content =
  "<" <> tag <> ">" <> content <> "</" <> tag <> ">"

el is a function that takes a tag and content, and wraps the content with the tag.

We can now implement html_ and body_ by partially applying el and only provide the tag.

html_ :: String -> String
html_ = el "html"

body_ :: String -> String
body_ = el "body"

Note that we didn't need to add the argument on the left side of equals sign because Haskell functions are "first class" - they behave exactly like values of primitive types like Int or String. We can name a function like any other value, put it in data structures, pass it to functions, and so on!

The way Haskell treats names is very similar to copy-paste. Anywhere you see html_ in the code, you can replace it with el "html". They are the same (this is what the equals signs say, right? That the two sides are the same). This property of being able to substitute the two sides of the equals sign with one another is called referential transparency. And it is pretty unique to Haskell (and a few similar languages such as PureScript and Elm)! We'll talk more about referential transparency in a later chapter.

Anonymous/lambda functions

To further drive the point that Haskell functions are first class and all functions take exactly one argument, I'll mention that the syntax we've been using up until now to define function is just syntactic sugar! We can also define anonymous functions - functions without a name, anywhere we'd like. Anonymous functions are also known as lambda functions as a tribute to the formal mathematical system which is at the heart of all functional programming languages - the lambda calculus.

We can create an anonymous function anywhere we'd expect an expression, such as "hello", using the following syntax:

\<argument> -> <expression>

This little \ (which bears some resemblance to the lowercase Greek letter lambda 'λ') marks the head of the lambda function, and the arrow (->) marks the beginning of the function's body. We can even chain lambda functions, making them "multiple argument functions" by defining another lambda in the body of another, like this:

three = (\num1 -> \num2 -> num1 + num2) 1 2

As before, we evaluate functions by substituting the function argument with the applied value. In the example above, we substitute num1 with 1 and get (\num2 -> 1 + num2) 2. Then substitute num2 with 2 and get 1 + 2. We'll talk more about substitution later.

So, when we write:

el :: String -> String -> String
el tag content =
  "<" <> tag <> ">" <> content <> "</" <> tag <> ">"

Haskell actually translates this under the hood to:

el :: String -> (String -> String)
el = \tag -> \content ->
  "<" <> tag <> ">" <> content <> "</" <> tag <> ">"

Hopefully, this form makes it a bit clearer why Haskell functions always take one argument, even when we have syntactic sugar that might suggest otherwise.

I'll mention one more syntactic sugar for anonymous functions: We don't actually have to write multiple argument anonymous functions this way, we can write:

\<arg1> <arg2> ... <argN> -> <expression>

to save us some trouble. For example:

three = (\num1 num2 -> num1 + num2) 1 2

But it's worth remembering what they are under the hood.

We won't be needing anonymous/lambda functions at this point, but we'll discuss them later and see where they can be useful.

Exercises:

Add types for all of the functions we created until now
Change the implementation of the HTML functions we built to use el instead
Add a couple more functions for defining paragraphs and headings:
1. p_ which uses the tag <p> for paragraphs
2. h1_ which uses the tag <h1> for headings
Replace our Hello, world! string with richer content, use h1_ and p_. We can append HTML strings created by h1_ and p_ using the append operator <>.

Bonus: rewrite a couple of functions using lambda functions, just for fun!

Solutions:

Solution for exercise #1

myhtml :: String
myhtml = makeHtml "Hello title" "Hello, world!"

makeHtml :: String -> String -> String
makeHtml title content = html_ (head_ (title_ title) <> body_ content)

html_ :: String -> String
html_ content = "<html>" <> content <> "</html>"

body_ :: String -> String
body_ content = "<body>" <> content <> "</body>"

head_ :: String -> String
head_ content = "<head>" <> content <> "</head>"

title_ :: String -> String
title_ content = "<title>" <> content <> "</title>"

Solution for exercise #2

html_ :: String -> String
html_ = el "html"

body_ :: String -> String
body_ = el "body"

head_ :: String -> String
head_ = el "head"

title_ :: String -> String
title_ = el "title"

Solution for exercise #3

p_ :: String -> String
p_ = el "p"

h1_ :: String -> String
h1_ = el "h1"

Solution for exercise #4

myhtml :: String
myhtml =
  makeHtml
    "Hello title"
    (h1_ "Hello, world!" <> p_ "Let's learn about Haskell!")

Our final program

-- hello.hs

main :: IO ()
main = putStrLn myhtml

myhtml :: String
myhtml =
  makeHtml
    "Hello title"
    (h1_ "Hello, world!" <> p_ "Let's learn about Haskell!")


makeHtml :: String -> String -> String
makeHtml title content = html_ (head_ (title_ title) <> body_ content)

html_ :: String -> String
html_ = el "html"

body_ :: String -> String
body_ = el "body"

head_ :: String -> String
head_ = el "head"

title_ :: String -> String
title_ = el "title"

p_ :: String -> String
p_ = el "p"

h1_ :: String -> String
h1_ = el "h1"

el :: String -> String -> String
el tag content =
  "<" <> tag <> ">" <> content <> "</" <> tag <> ">"

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Adding type signatures

Anonymous/lambda functions