20 CHAPTER 2. EXPRESSIONS, TYPES, AND VALUES
When using the Scala console our programs are evaluated as soon as they compile, which gives the appearance
that there is only one stage. It is important to understand that compile- and run-me really are disnct, as it is
this disncon that allows us to properly understand the difference between types and values.
Compilaon is a process of checking that a program makes sense. There are two ways in which a program must
“make sense”:
1. It must be syntaccally correct, meaning the parts of the program must be arranged according to the
grammar of the language. An example English sentence that is not syntaccally correct is “on cat mat sat
the”. An example syntaccally incorrect Scala program is
toUpperCase."Hello world!"
// error: identifier expected but string literal found.
// toUpperCase."Hello world!"
// ^
2. It must type check, meaning it must obey certain constraints on what a sensible program is. An example
English sentence that is syntaccally correct but fails to make sense is “the mat sat on the cat”. A simple
program that would fail to type check is trying to convert a number to uppercase.
2.toUpperCase
// error: value toUpperCase is not a member of Int
// 2.toUpperCase
// ^
The concept of upper and lowercase doesn’t make sense for numbers, and the type system will catch this error.
If a program passes the checks at compile-me it may then be run. This is the process of the computer per-
forming the instrucons in the program.
Even though a program successfully compiles it can sll fail at run-me. Dividing an integer by zero causes a
run-me error in Scala.
2 / 0
// java.lang.ArithmeticException: / by zero
The type of integers, Int, allows division so the program type checks. At run-me the program fails because
there is no Int that can represent the result of the division.
2.1.2 Expressions, Types, and Values
So what exactly are expressions, types, and values?
Expressions are part of a program’s text—what we type into a file, or the console or worksheet. They are the
main components of a Scala program. We will see other components, namely definions and statements, in due
course. Expressions exist at compile-me.
The defining characterisc of an expression is that it evaluates to a value. A value is informaon stored in the
computer’s memory. It exists at run-me. For example, the expression 2 evaluates to a parcular sequence of
bits in a parcular locaon in the computer’s memory.
We compute with values. They are enes that our programs can pass around and manipulate. For example,
to compute the minimum of two numbers we might write a program like
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