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Comment by tomp | Hacker News Reader

This article is another one of those that confuses functional programming with pure programming (which is, I believe, by definition also functional).

You can do a lot of functional programming in a language where values are mutable. I use it all the time in python: the map and filter functions, list comprehensions, zip, ... For me, functional programming means having first-class functions that can be passed to/from other functions. A standard library of common functional combinators help, but you can always write your own.

Funny, but some of the most often mentioned functional languages, such as OCaml and Scala, allow liberal use of mutable variables. They just make it a bit harder/more explicit, so the programmer has to give more consideration into it.

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You must have read a different article than the one I read.

"Functional programming is a practical implementation of Alonzo Church's ideas. Not all lambda calculus ideas transform to practice because lambda calculus was not designed to work under physical limitations. Therefore, like object oriented programming, functional programming is a set of ideas, not a set of strict guidelines. There are many functional programming languages, and most of them do many things very differently. In this article I will explain the most widely used ideas from functional languages"

No where does he say that functional programming is pure programming.

You can do a lot of functional programming in a language where values are mutable.

Sure you can do whatever you want, but if the language does not support things like tail call optimization, non-imperative function definition, writing code in functional style turns out to be a lot slower (due to non-tail-optimized function call overhead) and less safe (due to potential side effects in function definition) than if you are writing it in a functional language with explicit support for these facilities.

That's true, but this functionality is perfectly compatible with mutability.

Various flavours of Lisp have been in the forefront (CMA: among 'popular' languages) in making this functionality available, but I don't know of any (CMA: 'popular') pure flavour of Lisp. (Maybe Clojure --but its preference for 'immutable by default' is quite different from not having mutable values at all!)

OK - I checked wikipedia:

In computer science, functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data.

Sounds exactly what I was thinking - map is not mathematical function - because it can interact with the global state (you can apply map twice to the same data and get different results - for example if in the function you are mapping you use 'time') - and thus using it is not 'functional programming'. If your first-class functions were mathematical functions - then we could agree - but map is not one.

I'm reading this differently than you are;

> functional programming is a programming paradigm

NOT a programming language. What I think this means is that your usage of map is such that the computation is what matters, not mutating the state. I can do that just as well in Python, JavaScript, OCaml, Scala, Clojure and Haskell. In other languages too, yes, but not nearly as easily (due to lack of closures/complicated functional types/no first-class functions/...).

If you were to insist that functional programming is just math (no mutable state), then even Haskell and Mathematica are far from functional.

I insist that what is important is how much of the purity you have - because it is a more widely used definition (as supported by my wikipedia quote) and also this is what implies the nice computational properties of the code, and what is not important is if you use first class functions (not in the mathematical sense).

"map is not mathematical function - because it can interact with the global state (you can apply map twice to the same data and get different results"

I don't think I quite understand you. I think when you say "mathematical function" you mean "pure function". And when you say "global state" I assume you mean "impure operations".

But (from my understanding) map is purely functional and doesn't do anything impure like IO. Its purely a mathematical mapping between a collection of input values to a collection of output values, based on applying a function to each input value in turn. You seem to be suggesting "map f (map f x)" (that is applying map twice to the same data) is not the same value as "map f x" and therefore its not "mathematical".

OK - I can agree with the interpretation that the 'impurity' is not inside map - but rather in the argument it takes. You can reconcile it with the mathematical definition of function by saying that some 'programming functions' take the implicit 'state' argument and that map is not one of them.

It still does not resolve the definition problem - is functional programming programming with pure mathematical functions - or is it using first-class functions (not in mathematical sense)? Which definition is more useful? Which is more widely used?

This is a single introductory sentence. You should not reason that far with it.

With that reasoning you could claim addition is impure, because time() + 1 changes over time. map is definitely a pure function. If you apply it to impure arguments, of course the result will be impure. The point in FP is to avoid using 'time', not 'map'.

That's a good argument - but it does not really resolve the definition problem. It seems that some people think that 'functional programming' means 'programming with mathematical functions' - while others think that it means 'programming with first-class functions'. I can take this or that definition - but if wikipedia uses one definition - this sounds like a good argument that more people use the 'mathematical functions' definition. I also think that 'mathematical functions' is a more useful definition - because it implies 'referential transparency' and probably other interesting things that are usually quoted as the advantage of using functional programming.

> wikipedia uses one definition

Let me repeat: you used an introductory sentence as a "definition" of FP and are overinterpreting it. If you read later you will see:

> Functional programming languages, especially purely functional ones...

So Wikipedia distinguishes functional programming and pure functional programming. In fact there is a separate article on pure functional programming, which says that destructive modifications are "excluded", not just "avoided".

Functional programming does not have clear boundaries. You can have advantages of functional programming even if you introduce _a bit_ of FP style in a very imperative program. Conversely, you can write in very imperative style in Haskell.

My point is that the definition of functional programming is this purity you mention not using first-class functions. Pure means that something has more of that thing you want and impure means it has less - so pure functional programming has more 'functional programming' then 'standard functional programming'. Sure completely pure functional programming is impractical - there are degrees to how purely functional are the languages we use in practice - but this is the point practical languages are more or less functional and how much are they functional depends on how much they depend on state manipulation not on using higher order functions or having first class functions.

Defining functional programming as programming with first-class functions is a consistent definition and could be used if not the fact that more people use the other definition (at least that is what the wikipedia definition and other online material makes me believe).

Is Prolog a functional language, as it avoids state manipulation but has no first-class functions? I think not. There are some characteristics of functional languages - including first class and higher order functions, relying on immutability, declarativity etc. Choosing just one of them to measure how much a language is "functional" is a mistake IMO.

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