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Reading through the articles on programming languages, you are bound to stumble upon statements similar to the following:
- "Python is a strongly typed, dynamic language."
- "Go is a statically typed, strong language."
- "JavaScript is a dynamic language which is weakly typed."
(Aside: Wikipedia pages of individual languages themselves have a bullet titled "Typing Discipline" in the sidebox).
Depending upon your programming background and experience, the above statements might either look deceptively simple to read or seem downright counter-intuitive, yet they are the source of some very famous warts in the respective languages.
A good understanding of the fundamentals behind such statements can help programmers develop more reasonable expectations from the code they write.
We begin by defining 4 terms central to the discussion, namely:
- Static Typing | Dynamic Typing
- Strong Typing | Weak Typing
The terms grouped are semantically antonyms to each other.
Each group defines important characteristics of a programming language independent of the other group.
Static Typing, Dynamic Typing
Whether a language is statically typed or dynamically typed depends on how the compiler/interpreter handles variables and at what point during the execution of the program.
Static Typing
In a statically typed language, the variable itself has a type; if you have a variable that's an integer, you won't be able to assign any other type of value to it later. Some statically typed languages require you to write out the types of all your variables, while others will deduce many of them for you automatically.
Moreover, the compiler can tell which the type of a variable, without executing the program.
Dynamic Typing
In a dynamically typed language, a variable is simply a value bound to a name; the value(or object) has a type -- like "integer" or "string" or "list" -- but the variable itself doesn't. We can have a variable which, right now, holds a number, and later assign a string to it.
The programmer is free to bind names to different objects with a different type. So long as you only perform operations valid for the type the interpreter doesn't care what type they are.
Why both Static and Dynamic Typing?
At present, almost all programming languages support only static or dynamic typing, or they favor one of them.
Dynamic and static typing have different costs and benefits. Picking the best one for a particular task is not always easy.
Static typing has the following main benefits:
- It allows statically (without running the program) detecting many programming errors quickly, reliably and automatically. This helps reduce the number of bugs and reduces the time spent on debugging.
- Type declarations serve as automatically-checked documentation. They make programs easier to understand and maintain.
- Static typing may improve runtime efficiency.
Dynamic typing has a different, complementary set of benefits:
- Dynamic typing is conceptually simpler and easier to understand than static typing.
- Dynamic typing is more flexible. A static type system always restricts what can be conveniently expressed. Programming with a static type system often requires more design and implementation effort.
- Dynamic typing results in more compact programs since it is more flexible and does not require types to be spelt out.
The benefits of static typing are more pronounced for large and complex programs. It offers little benefit over dynamic typing when writing short scripts and prototypes, for example. In these cases, it mainly slows down the programmer and dynamic typing is preferable.
Python
>>> age = 21
>>> age = '21'
Conclusion: The type of name age is the type of the object it refers to at that particular instant. Moreover, no further restriction is laid on it with regards to what values it can/cannot hold in the future.
Go
age := 21
age = "21"
Conclusion: A compile-time assignment error is raised. Once a type is associated with a variable, the variable cannot hold a value of other data types.
JavaScript
age = 21
age = "21"
Conclusion: Same as Python
Strong Typing and Weak Typing
The distinction between such typing comes to the fore when we write programming statements which involve operating values of different types.
Expected Behaviors
- Strongly typed language: more likely to generate an error (either at runtime or compile time depending on whether the language is interpreted or compiled).
- Weakly typed language: either produces unpredictable results or perform implicit type conversions to make sense of the expression.
To resolve the error raised in a strongly typed language, the programmer needs to explicitly convert the type of values to the desired type.
The terms "strong" and "weak" are themselves relative by nature. Thus, a person calls some programming language "X" as strongly (or weakly) typed is subjective. From the First Principles perspective, what the person wants to claim is that language "X" enforces type checks before performing any operations. Every developer will have a different measure of how rigorous these checks are.
Rule of Thumb: the more type coercions (implicit conversions) for built-in operators the language offers, the weaker the typing. (This could also be viewed as more built-in overloading of built-in operators.)
i.e a strongly typed language is restrictive of type intermingling.
Let us understand this distinction with 2 operations which every programmer must have comes across:
- Adding a number to a string
- Adding an integer to a floating-point number
Advantages and Disadvantages of Strongly Typed and Weakly Typed
Strong Typing:
- Strong/Static types provide constraints which help to catch errors during compile time.
- Strong typing provide more opportunities for performance enhancements.
- Strongly typed code is easy to understand.
- Limits the developer programming expressiveness.
- Application development goes more slowly.
Weak Typing :
- Requires less programing effort as the compiler or interpreter implicitly performs certain kinds of type conversions. So applications can be built rapidly.
- Fewer errors are caught at compile time. Many bugs are caught at run-time. Requires more discipline while coding.
Hence weakly typed languages helps in faster application development and more expressiveness of programming logic (like ad hoc-polymorphism, mix-ins) than strongly typed.
Python
- Adding a number to a string
>>> a, b = 10, 'K'
>>> a + b # Binary operation on different types
...
TypeError: unsupported operand type(s) for +: 'int' and 'str'
>>> a + ord(b) # Explicit type conversion of string to integer
85
>>> str(a) + b # Explicit type conversion of integer to string
'10K'
- Adding an integer to a floating point number
>>> a, b = 1, 2.5
>>> a + b # No error
3.5
Conclusion: Raises errors when sense cannot be made. Also, the interpreter is not restrictive enough to spoil the fun in programming.
Go
- Adding a number to a string
func AddNumStr() {
a, b := 10, "K"
fmt.Println(a + b)
}
- Adding an integer to a floating-point number
func AddNumFloat() {
a, b := 1, 2.5
fmt.Println(a + b)
}
Conclusion: In both the cases, an "invalid operation" compile-time error is raised.
JavaScript
- Adding a number to a string
> a = 10
> b = "K"
> a + b
'10K'
- Adding an integer to a floating-point number
> a = 1
> b = 2.5
> a + b
3.5
Conclusion: The compiler implicitly converts the type of values to make sense of the operation rather than raising errors.
I hope that after reading this article, you can justify the statements regarding the type systems used in different languages. In this post, I explored these principles across Go, JavaScript & Python just to cover a variety of examples.
