Comparison With Functional Programming
The principles of modularity and code reuse in practical functional languages are fundamentally the same as in procedural languages, since they both stem from structured programming. So for example:
- Procedures correspond to functions. Both allow the reuse of the same code in various parts of the programs, and at various points of its execution.
- By the same token, procedure calls correspond to function application.
- Functions and their invocations are modularly separated from each other in the same manner, by the use of function arguments, return values and variable scopes.
The main difference between the styles is that functional programming languages remove or at least deemphasize the imperative elements of procedural programming. The feature set of functional languages is therefore designed to support writing programs as much as possible in terms of pure functions:
- Whereas procedural languages model execution of the program as a sequence of imperative commands that may implicitly alter shared state, functional programming languages model execution as the evaluation of complex expressions that only depend on each other in terms of arguments and return values. For this reason, functional programs can have a freer order of code execution, and the languages may offer little control over the order in which various parts of the program are executed. (For example, the arguments to a procedure invocation in Scheme are executed in an arbitrary order.)
- Functional programming languages support (and heavily use) first-class functions, anonymous functions and closures.
- Functional programming languages tend to rely on tail call optimization and higher-order functions instead of imperative looping constructs.
Many functional languages, however, are in fact impurely functional and offer imperative/procedural constructs that allow the programmer to write programs in procedural style, or in a combination of both styles. It is common for input/output code in functional languages to be written in a procedural style.
There do exist a few esoteric functional languages (like Unlambda) that eschew structured programming precepts for the sake of being difficult to program in (and therefore challenging). These languages are the exception to the common ground between procedural and functional languages.
Read more about this topic: Procedural Programming
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