In Chapter 3 you saw some of the simple but powerful data types and language constructs that make up F# functional programming. The functional programming paradigm is strongly associated with “programming without side effects,” called pure functional programming. In this paradigm, programs compute the result of a mathematical expression and do not cause any side effects, except perhaps reporting the result of the computation. The formulae used in spreadsheets are often pure, as is the core of functional programming languages such as Haskell. F# is not, however, a “pure” functional language; for example, you can write programs that mutate data, perform I/O communications, start threads, and raise exceptions. Furthermore, the F# type system doesn’t enforce a strict distinction between expressions that perform these actions and expressions that don’t.
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