In computer science, functional programming is a programming paradigm—a style of building the structure and elements of computer programs—that treats computation as the evaluation of mathematical functions and avoids changing-state and mutable data. It is a declarative programming paradigm, which means programming is done with expressions[1] or declarations[2] instead of statements. In functional code, the output value of a function depends only on the arguments that are passed to the function, so calling a function f twice with the same value for an argument x will produce the same result f(x) each time
No. It only means that other languages support multiple paradigms.
Haskell doesn't strictly conform either, because it allows side effects (event though it's "only through escape hatches"). There's no such thing as a "pure functional programming language".
Let's take it from the top:
"In computer science, functional programming is a programming paradigm—a style of building the structure and elements of computer programs—that treats computation as the evaluation of mathematical functions and avoids changing-state and mutable data."
All of the languages I listed support this style. The moment you write an IO monad inside a function in Haskell, you break the illusion of Haskell's strict conformation to the definition.
Now you have the condescending tone! However, it appears you don't understand the IO monad. Programming using the IO monad is pure functional programming with full referential transparency. Please read Phil Wadler's paper, you will not understand this from that JavaScript snippet. The only backdoors are escape hatches like unsafePerformIO which are used for low-level libraries and FFI, they can be disabled with pragmas and/or compiler switches.
It is much harder to stay true to that definition using the other languages, multi-paradigm or not. That is why Haskell is so often mentioned in the context of FP.
As soon as you have side-effects (an IO is a side-effect) you can through your assumptions about "strict conformation to definition" out of the window:
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treats computation as the evaluation of mathematical functions and avoids changing-state and mutable data.
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> It is much harder to stay true to that definition using the other languages, multi-paradigm or not.
I doesn't matter if it's "hard". Your argument was that "Only Haskell strictly conforms to that definition."
No. Haskell conforms stricter. It doesn't mean that none of the other languages are not FP, or cannot be used to program in functional style.
> That is why Haskell is so often mentioned in the context of FP.
Yes. And the problem is that people treat the features that Haskell has as requirements to be considered a functional programming language.
- Typeclasses/Liquid types/Dependent types are not FP
> As soon as you have side-effects (an IO is a side-effect) you can through your assumptions about "strict conformation to definition" out of the window
Again, you are unfortunately quite wrong. You do not understand the IO Monad.
No IO is ever performed in any code written inside the IO monad (unless using unsafePerformIO).
Please take some time to fully understand Haskell before criticising it so openly on a public forum. Perhaps then it might not all seem rather pointless.
You're making some good points here. Neither category theory informed typeclasses nor laziness are required for pure functional programming (henceforth PFP). Up until recently Haskell was the only PFP language in common use, so all those things got conflated in a lot of people's minds. Now we have PureScript (which doesn't have laziness) and Elm (which doesn't have laziness or typeclasses).
That said, you really should listen to @willtim about the `IO` type. Purity really is a big deal, and `IO` doesn't break it at all.
> As soon as you have side-effects (an IO is a side-effect) you can through your assumptions about "strict conformation to definition" out of the window
You misunderstand what an IO action is. An IO action in Haskell is a value which describes a side effect. For example, an IO Int is a value that describes a side effect which, at runtime will produce a value of type Int inside the IO monad.
The difference is that the IO Int is a value, it’s a constant which describes how to perform something at runtime (also called promises in some languages). It’s like a callback function, which takes a value as an argument that will be available when it’s called (at runtime).
Let's take Wikipedia:
-- start quote --
In computer science, functional programming is a programming paradigm—a style of building the structure and elements of computer programs—that treats computation as the evaluation of mathematical functions and avoids changing-state and mutable data. It is a declarative programming paradigm, which means programming is done with expressions[1] or declarations[2] instead of statements. In functional code, the output value of a function depends only on the arguments that are passed to the function, so calling a function f twice with the same value for an argument x will produce the same result f(x) each time
-- end quote --