COMP1531

🐶Software Engineering

8.2 - Development - Safety & Type Checking

Author: Hayden Smith 2021

In this lecture

Why?

We need to be able to able to understand the characteristics of programming languages and approaches in terms of how they may prevent bugs

What?

Software Safety
Memory Safety
Runtime Error Safety
Type Safety

Safety

Protection from accidental misuse

Security

Protection from deliberate misuse

Software safety

Software becomes unsafe when its design or implementation allow for unexpected or unintended behaviours, particularly during runtime.
For example:
- Reading uninitalized memory
- Writing outside array bounds

Case study: spreadsheets

Around 94% of spreadsheets contain errors*
For any given spreadsheet formula, there's a 1% chance it contains an error**
Why?

* What We Know About Spreadsheet Errors (2005)

** Errors in Operational Spreadsheets (2009)

Static

Static properties can be inferred without executing the code
E.g. pylint statically checks that variables are initialised before they're used

Dynamic

Dynamic properties are checked during execution
E.g. python dynamically checks that an index is inside the bounds of a list and throws an exception if it isn't (unlike an array in C)

Removing errors statically

Rather than dynamically checking for certain errors, it is always better if errors can be detected statically
Rules out entire classes of bugs
In Python, pylint can statically detect certain errors (e.g. unknown identifier)
In C, the compiler detects a number of errors including type errors.

Examples of errors

Things that can go wrong:
- C:
  - Reading from memory that has not been initialised
  - Dereferencing a null pointer
  - "Using" memory after it has been freed
  - Writing outside the bounds of an array
  - Forgetting to free allocated memory
- Python:
  - Accessing a variable that hasn't been initialised
  - Accessing a member that an object doesn't have
  - Passing a function a type of object it doesn't expect

Memory safety

Protecting from bugs relating to memory access
Python is memory safe as it prevents access memory that hasn't been initialised or allocated
The checks are mostly dynamic (at runtime)
In python, safety is prioritised over the negligible performance cost of bounds-checking

Memory Safety

C is not memory safe
No bounds checking is performed for array accesses
Pointers can still be dereferenced even if they don't point to allocated memory
C prioritises performance over safety (and security)

Handling runtime errors

Different languages have difference conventions for handling errors
Python relies on Exceptions for the majority of error handling. E.g.

will thrown a KeyError exception if "fish" is not in the dictionary animals.
C does not support exceptions at all, so errors typically have to be included in the return value.

animals["fish"]

Easier to Ask for Forgiveness than Permission

EAFP is the python convention for handling errors.
It encourages you to assume something will work and just have an exception handler to deal with anything that might go wrong
Pros:
- Can simplify the core logic
- Multiple different sorts of errors can be handled with one except block
Cons:
- Makes code non-structured
- Harder to reason what code will be executed.

Look Before You Leap

LBYL is a convention for avoiding errors popular in languages like C
Unlike EAFP it encourages you to check that something can be done before you do it
Pros:
- Doesn't require exceptions
- Code is structured and therefore easier to reason about
Cons:
- Core logic can be obscured by error checks

Type safety

Preventing mismatches between the actual and expected type of variables, constants and functions
C is type-safe*, as types must be declared and the compiler will check that the types are correct
Python, on its own, is not type-safe. Everything has a type, but that type is not known till the program is executed

* mostly

Type-checking

Languages with a non-optional built-in static type checking
- C
- Java
- Haskell
Languages with optional but still built-in static type checking
- Typescript
- Objective C
Languages with optional external type checkers
- Python
- Ruby

Mypy

Mypy is a type checker for python
Python allows you to give variables static types, but without an external checker they are ignored
Because of python's semantics, type checking it can be complex
- Duck typing
- Objects with dynamically changing members

Example

def count(needle, haystack):
    '''
    Returns the number of copies of integer needle in the list of integers haystack.
    '''
    copies = 0
    for value in haystack:
        if needle == value:
            copies += 1
    return copies

def search(needle, haystack):
    '''
    Returns the first index of the integer needle in the list of integers haystack.
    '''
    for i in range(len(haystack)):
        if haystack[i] == needle:
            return i