Expressions, Values, and Types

Every expression has a value

Variables in Haskell are immutable; once defined, their values never change. For this reason, the following code gives an error

and the order of definitions does not matter so long as all variables are defined.

Thus, since variables and more generally expressions in Haskell never change in value, any expression can be replaced with its value at any point in a Haskell program to no effect.This property, known as “referential transparency,” is what makes Haskell a “pure” language.

In this sense, Haskell variables are similar to variables in mathematics where, for example, the \(x\) in \(2 x + 1 = 0\) has a single unchanging value and replacing it with that values does not change the truth of the statement.

If variables cannot change, how does Haskell do anything useful? This first answer to this question is provided by functions.

We can use these functions in ghci or elsewhere in our code.

Functions in Haskell are values like any other. While they cannot be printed in the REPL since Haskell does not know how to print function values,

error:
    • No instance for (Show (Integer -> Integer))

we can pass functions as arguments to other functions.

This lets us define higher-order functions that control the behaviour of other functions and enable higher levels of abstraction.

Since Haskell values are immutable and therefore cannot change during runtime, programs give different results only by treating the outside world as an input to a function which the program defines.Don’t worry about this too much, it will become much clearer later on.

The most basic representation of a function in Haskell is as an unnamed, anonymous function value.

Haskell has plenty of syntactic sugar for defining functions. For example, the function definitions above do nothing more than name these anonymous functions, and they are interpreted as

In Haskell functions are applied simply by writing them before their arguments. The only exception is infix functions like + which are written between their arguments, though surrounding them in parentheses makes them act like normal functions.

While function application is implicit in Haskell, there is also a function operator $. While this might seem redundant since

one use for $ is enabled by it having the lowest precedence of any operator. This allows it to replace parentheses in many expressions as follows.

Every value has a type

Every value in Haskell, and therefore every expression with that value, has a type.

Some expressions can represent different values with different types depending on contextThis is known as polymorphism.

, for example written out integers can represent any of Haskell’s number types.

This type signature says that 1 can be of any type p so long as p is part of the number type class Num.This will be explained in great detail when we cover typeclasses, but for now it is sufficient to know that Num represents a set of types for which the usual numerical operations are defined.

Here, p is a type variable. In general, everything before => in a type signature is a constraint on its type variables. When we want a polymorphic expression to have a specific type we can annotate it.

Since functions are values, they also have types.

Here (+) takes two values of some type a and returns a value of type a, so long as a is a type in the Num typeclass. Haskell is strongly typed, meaning that functions cannot be applied to values of the wrong type and such errors are caught at compilation.

error:
     • Couldn't match expected type ‘Bool’
       with actual type ‘Int’

One of Haskell’s strongest features is type inference. Every expression in Haskell has an associated type, but one rarely needs to write this out explicitely since Haskell can usually figure out the type on its own. For example, when we defined inc Haskell inferred the type for us.

Just like written integers, the inc function is polymorphic, and returns a value of its input type so long as that input type is in Num.

While Haskell’s ability to infer types is powerful, it is often a good idea to specify the type of a function by writing the type above its definition. This can be very useful in reasoning about code, and also forces the function to have the prescribed type even if Haskell could give it a more general one. Thus, we could writeNote that Haskell’s infix functions can be written as normal functions that take their arguments on the right by surrounding them in parentheses.

and this annotated function could not be passed a value of any type besides Int.

error:
    • Couldn't match expected type ‘Int’
      with actual type ‘Float’

Currying

Haskell functions are applied to only one argument, and function application is evaluated left to right, i.e. is left associative. This means that a function f of two arguments f x y is correctly interpreted as f being applied to x to give a function which is applied to y, i.e.

The type signature of f hints at this as well.

This says that f takes a value of type a and returns a value of type a -> a, which is a function that takes a value of type b and returns a value of type c. Here, the -> symbol in the type signature is applied right to left, i.e. it is right associative. Thus, the type above can be written as

Note that function application associates to the left and -> associates to the right because the first or innermost function application corresponds to the outermost ->. If you don’t see this right away, it is a good point to dwell on.

This property allows us to simplify many definitions using equational reasoning, i.e. rewriting definitions as one might in math.Equational reasoning will be a reoccuring theme. Here, we are translating a function into so-called pointfree style. More will be said about this later on.

Here (+) 1 is a partial application of (+).