nique shown above; the “undefined behavior” produced in that case
was to download and run a copy of the virus. (Undefined behavior is
often sufficiently predictable in practice to build effective security
exploits from.) The same class of exploit remains in widespread use
today. While a student at the University of Utah, researcher Peng Li
modified C and C++ compilers to make the programs they trans‐
lated report when they executed certain forms of undefined behav‐
ior. He found that nearly all programs do, including those from
well-respected projects that hold their code to high standards.
In light of that example, let’s define some terms. If a program has
been written so that no possible execution can exhibit undefined
behavior, we say that program is well
dened. If a language’s type
system ensures that every program is well defined, we say that lan‐
guage is type safe.
C and C++ are not type safe: the program shown above has no type
errors, yet exhibits undefined behavior. By contrast, Python is type
safe. Python is willing to spend processor time to detect and handle
out-of-range array indices in a friendlier fashion than C:
>>> a = [0]
>>> a[3] = 0x7ffff7b36ceb
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IndexError: list assignment index out of range
>>>
Python raised an exception, which is not undefined behavior: the
Python documentation specifies that the assignment to a[3] should
raise an IndexError exception, as we saw. As a type-safe language,
Python assigns a meaning to every operation, even if that meaning is
just to raise an exception. Java, JavaScript, Ruby, and Haskell are also
type safe: every program those languages will accept at all is well
defined.
Note that being type safe is mostly independent of
whether a language checks types at compile time or at
run time: C checks at compile time, and is not type
safe; Python checks at runtime, and is type safe. Any
practical type-safe language must do at least some
checks (array bounds checks, for example) at runtime.
4 | Why Rust?
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