Next we want to give you some more types to play with, so we can start to do things that are perhaps more interesting than arithmetic.
First we should take another look at the
case keyword. Let’s go back to that conditional statement that we wrote in the last section.
We have already noticed that this branches on the result of an expression (
x < 10) that evaluates to a
Bool result. We also saw when we wrote the
spell function how a function involving a
case ... of can match columns of inputs to outputs.
else form is great when what you have is a
Bool, but it can’t take you any farther – it’s only useful for
Bool. What comes after the
if always has to be true or false. So we need to make sure we understand how to generalize this notion of branching and write expressions that can branch over anything.
What we’ll do now, just to get a little more
case practice, is rewrite the
idk function so that it uses a
case expression instead of
else. We’ll call it
idk2 just to give it a different name.
The type of the function will remain the same, so we can just copy the type declaration and change the name.
It’s the syntax of the function description itself that changes. Instead of saying
if (x < 10), we’ll say
case (x < 10) of.
Instead of the
else words, we’ll give a list of the possible Boolean result values and map them to the desired outputs, in a sort of tabular form like we did with
Please take a moment to figure out what should go in each of the two blanks.
What we had previously written after the
then keyword goes after the arrow for
True, and the
else corresponds to
False. So it should end up looking like this:There are three expressions here we have parenthesized for clarity; in all three cases, the parentheses are optional. It never hurts to add them if you are unsure.
This is called pattern matching. The patterns here are
False.These probably don’t look like “patterns”, because they are extremely simple patterns. But once we see more complex examples, it may become more clear why what’s written on the left side of an
-> arrow is called a pattern. And the result is given by what’s on the right side of the arrow, based on which pattern matches the result of
x < 10 for a given
Let’s look next at another type that is very useful in Haskell and can be thought of as encoding a kind of two-valued logic. The type is called
Maybe and, instead of choosing between false and true, it gives us a choice between nothing and something.
data Maybe a = Nothing | Just a
The biggest difference between
Maybe is that
Maybe has a type parameter.
Maybe part of this, because it has a parameter, is described most precisely not as a type, but as a type constructor.
a is the type parameter in this definition.Sometimes we use more descriptive names for type variables. But in this case, since this variable represents any type at all, and
Maybe does not ascribe any particular meaning to it, there is really nothing to describe, no information that we might want to communicate via the choice of name. In cases like these, we conventionally use single letters
c as parameter names. This is similar to the function parameters we’ve seen: it’s a variable that can stand for many different things. Whereas our
idk function needed to be applied to a value of a numeric type, though,
Maybe has to be applied to another type. Importantly, since there is no constraint on the
a (that is, nothing like
Num a =>) this
a could be any type.
Maybe can take any type and turn it into a new type
Maybe a that has the possibility of being
Just one value of that type or of being
Just like what we saw in a function definition, a variable appears on both the left and right side of the
- The place where the variable appears to the left of the equals sign is called the binding; this assigns the name
ato the variable.
aappears on the right is where we are using what we bound on the left.
a in the
Just a has to be a value of the type that we applied
Just has a parameter, we can use it like a function. For example, we can apply the
Just constructor to the string
"Julie". Because we’ve given it a
String, the type of the overall expression is
Just "Julie" :: Maybe String
For another example, suppose we apply the
Just constructor to the value
3. Just as we saw before, what we end up here is a polymorphic expression, because we haven’t specified exactly what type of number the
3 is. So the type of this expression is written as
Maybe a – but it definitely does have to be a number, so there is also a
Num constraint on the
Just 3 :: Num a => Maybe a
So why does this
Maybe type exist – why would we want to take a perfectly good type and add a
Nothing to it?
One thing that
Maybe is useful for is giving us a way to “fail” functions by returning a result of
Nothing which might mean we didn’t get an input that we consider valid. For instance, we might have wanted to use that in our
spell function, to return
Nothing instead of “I don’t know this number!” as the result for numbers that we don’t know how to write as words. We’ll be using it in this way a few lessons later when we write a small program.
We’re also going to need lists for our forthcoming program. So, without dwelling on all the details, we are going to next take a brief look at the list datatype now.Because
 is a built-in type with special syntax in Haskell, this definition is not actually valid code that you could put in your
main.hs source file. But this is what the definition would look like.
data  a =  | a : [a]
Relative to what we’ve seen so far, this looks pretty weird! For one thing the name of this type is
 not a pronounceable word like
Maybe. However, it is a type constructor like
We’ll write it this way with some extra spacing and add some annotations to make this easier to read:
When we write it down this way, we can see that it’s actually not so different from the definition of
- The first thing,
, is the name the thing being defined here, and
ais the name of its parameter. For example, applied
Integeris the type list of integer, written as either
 Integeror, more commonly,
- There are exactly two possibilities for the form a list value can take, separately by the
|that signifies or.
- On one side, the
constructor signifies an empty list, which is somewhat analogous to the
- On the right side of the pipe, the
(:)constructor means “an
aand a list of more
as.” That represents a list that has at least one item. That
(:)operator is often pronounced as “cons”, and we might pronounce
a : [a]as “
aconsed onto a list of
- On one side, the
Notice that these are what we call homogenous lists; if
a has type
Integer, then all of the items the list have type
Integer. This is because in
a : [a], the
a appears twice, representing the same type in each place.
The first item in the list has the same type
a as the items in the rest of the list.
We’ll see more about how to use lists in later lessons. What we want you to keep in mind for now is the similarity between these types:
data Bool = False | True
data Maybe a = Nothing | Just a
data  a =  | a : [a]
These are all examples of sum types, meaning they have two possibilities – false or true, nothing or just, empty or cons. When working with any of those types, we’re going to be using
case expressions to pattern match over the two possibilities.
This is the end of the first of four parts of the beginner crash course. That was a lot to take in, especially if you’re new to programming! You may want to take a break here. In the upcoming lessons, we’ll give an extended example to see how the ideas we’ve presented so far start to make sense in the context of an actual working program.