Graham Hutton has a slightly different, and quite interesting, perspective on the topic of purely functional languages: Conversely, in ML or Scheme, a side effect can be buried in any function.īy this perspective, the purely functional subset of Haskell can be seen as the embedded language to deal with the behavior inside each monad (of course this is an odd perspective, as almost all the computation is happening in this “embedded” subset), and the purely functional subset of Erlang can be seen as the embedded language do deal with local behavior. In Erlang, all programs that don't use IO-inducing libraries or concurrency features are side-effect-free (this is more of a stretch than the Haskell case). For example, in Haskell, all programs whose type is not built from IO or other effect-denoting monad are side-effect-free. A language can be considered purely functional if there is a reasonably large, useful and well-characterised subset where side effects are impossible. There is a useful relaxed sense in which languages like Haskell and Erlang can be considered purely functional, but languages like ML and Scheme cannot. Thus no general purpose programming language is purely functional (but there are useful domain-specific purely functional languages: they can typically be seen as embedded languages in some way). Any language in which you can write programs that display their result, read and write files, have a GUI, and so on, is not purely functional. The term “purely functional language” is synonymous.īy this definition, Haskell is not a pure functional language. a language where functions are first-class values) where expressions have no side effects. Strictly speaking, a pure functional language is a functional language (i.e. Haskell is a functional programming language, in which (almost) all expressions are pure thus, Haskell is a purely functional programming language. Programming in functional programming languages is often done in a mostly-pure style, and it is difficult to be strictly pure without higher-order function manipulation enabled by functional programming languages. For example, being able to use a "function returning bool" as a "data structure representing a set" would be easy in a functional programming language. In other words, you can manipulate functions with exactly the same ease with which you can manipulate all other first-class values. For example, if a function mutated its arguments, set a variable somewhere, or changed behavior based on something other than its input alone, then that function call is not pure.Ī functional programming language is one in which functions are first-class. This means the expression cannot have any observable side-effects. Multi-line comments also called block comments."Pure" and "functional" are two separate concepts, although one is not very useful without the other.Ī pure expression is idempotent: it can be evaluated any number of times, with identical results each time. Main = putStrLn "Hello, World!" - inline comments example comments textĬomments can be added in any single line or inline comments - hello world program Single-line comments start with double hyphens, and spaces followed by a comment string. Documentation commands Haskell Single line comments.In Kotlin, You can write a comment in different following ways. How to write comments in Haskell language?Ĭomments are, explain single or multiple lines of a codeĪ developer can better understand the code by reading the comments.
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