[ildjarn]

What mainstream languages have a good suite of FP features though?

If you try to write mostly pure code in Java I’m afraid you’re in for a bad time, despite the (big!) improvements of records and lambdas.

Minimum viable FP starts at OCaml, F#, Scala and Closure, yet none of these are mainstream.

[AS37]

I code in C# and use a ton of LINQ when writing business logic. It's FP-ish enough to avoid logic mistakes. The mediator design pattern, which is kind of bringing another FP paradigm to the OO world, also features heavily.

[ska]

I think this is the point really, not that mainstream, "general purpose" languages support FP well, but that FP ideas and aspects have been adapted into many of them.

[ildjarn]

C# LINQ, being a round-about implementation of do-notation, is a pretty advanced FP feature, and beyond most mainstream languages. C# is certainly a step up over Java etc.

However, most developers wouldn’t understand, say, a result monad implemented via LINQ, so you’re still fighting the ecosystem somewhat.

[anon291]

Hold on... Purity and functional programming are different things. Functional programs can be pure but not necessarily.

Functional programming means functions are first class citizens and can be constructed on the fly. Modern python, c++, rust, even java now do this.

Purity is a nice to have (and arguably rusts borrow system enforces a kind of purity).

[ildjarn]

> Functional programming means functions are first class citizens and can be constructed on the fly.

FP is much more than this one language feature.

[anon291]

Yes, the field of FP is much more, but the core of FP is that. A language doesn't become non-FP simply because it has some imperative abilities, or logic programming builtin, etc.

[ildjarn]

I’m arguing it’s not FP when you take an OOP language and bolt on lambdas. Lambdas are necessary but not sufficient.

You need expression orientation, immutability by default, persistent collections in the standard library, some way to handle monadic code, etc…

[anon291]

Immutability by default is not a requirement for functional programming (I mean... if it were, Haskell would be obviously not FP since the 'default' entrypoint is very mutable).

Neither are monads. There are entire FP languages without monads for effects (obviously, you can write a monadic interface in them, but it's not part of the idiomatic core). For example, clean uses linear types to control effects and purescript / Idris use a custom effect-system. So no, monads are not a requirement, and even if they are, modern c++ fully supports them, as does rust, javascript, etc. It's very common in javascript to use Array.map() and Array.flat().

[ildjarn]

> Immutability by default is not a requirement for functional programming (I mean... if it were, Haskell would be obviously not FP since the 'default' entrypoint is very mutable).

I mean the bindings and collections. For example, when you make an array in JS, the default syntax is a mutable list with a mutable binding:

    let xs = [ 1, 2, 3 ]

> Neither are monads. There are entire FP languages without monads for effects (obviously, you can write a monadic interface in them, but it's not part of the idiomatic core).

I should be more precise - there needs to be some way to mange effects. Monads with syntax extensions is simply the most common (Haskell, Scala, F#, Closure macros)

> and even if they are, modern c++ fully supports them, as does rust, javascript, etc. It's very common in javascript to use Array.map() and Array.flat().

JavaScript does not have good monad support. This is why async/await was added as a new language feature. Yeah, I know you can hack together something with generator functions, but it’s hardly idiomatic.

But we’re getting into the weeds here. My point is: I don’t consider a language to support FP when writing functional code in that language leads to lots of friction compared to what is idiomatic.

Have you ever tried FP in Java? It works for some toy thing but then you hit the lack of TCO, or the ridiculously long type names (not inferred) or the pyramid of doom…

[paulddraper]

Java streams.

java.time

Valhalla even