What's a useful programming language feature or concept that a lot of languages don't have?

Two things I think are really powerful in F# that are not found in most languages.

Union types are a way to represent one of multiple different possibilities. It is a bit like an enum type that can have extra data with it.

type Payment =
    | ChargeCard of issuer:string * pmtToken:string
    | Check of checkNumber:int
    | Cash

let payment = Check 12345

Most languages have a way you can kinda-sorta define these. One common method in OO is an abstract base class with shallow inheritance. But it is a lot of code overhead versus the concise definition above. And it is also not particularly idiomatic to OO. People probably wouldn't bat an eye at using case on an enum, but type checking for a derived class is often considered a code smell.

You can also represent these choices as nullable fields in a single class, but then you have to write a bit of code to control access and protect against the case where none or multiple of them are set at once. Doable, but annoying enough that people usually don't.

Value equality by default is very nice for testing. It means I can do things like this:

type SomeType = {
    FooInt : int
    BarString : string
}

let expected = { FooInt = 1; BarString = "One" }
...
let actual = runSomeFunction input
actual = expected

If the actual has the same values as expected (1 and "One" here), then it will return true. And it works for union types too. In most languages, these things have reference equality by default. So the last line would only return true if actual and expected were pointing to the same memory location, which is not super helpful. You can usually get value equality in any language, but it might be a pain. In C# for example, you can override the default implementations of GetHashCode and Equals to get value equality. It adds an extra 8+ lines of code to every class definition (and each field adds more lines of code potentially). I believe a lot of people use tools to add them in automatically and then code folding to hide them. Or just manually check for equality when needed.

And I could go on: no nulls, immutability by default, pattern matching, composable asyncs, sequences, etc. Then I need to go fix something in our VB code base, and it's 😬😬😬. Well, one thing VB has that most others don't is XML literals. So there's that.

Elm's claim-to-fame type inference is pretty wonderful

No Runtime Exceptions

Elm uses type inference to detect corner cases and give friendly hints. For example, what if someone provides invalid inputs? NoRedInk switched to Elm about two years ago, and 250k+ lines later, they still have not had to scramble to fix a confusing runtime exception in production.

-- TYPE MISMATCH ---------------------------- Main.elm

The 1st argument to `drop` is not what I expect:

8|   List.drop (String.toInt userInput) [1,2,3,4,5,6]
                ^^^^^^^^^^^^^^^^^^^^^^
This `toInt` call produces:

    Maybe Int

But `drop` needs the 1st argument to be:

    Int

Hint: Use Maybe.withDefault to handle possible errors.

elm-lang.org/blog/compilers-as-ass...

Pattern matching as done in ML languages.

factorial :: Int -> Int
factorial 0 = 0
factorial 1 = 1
factorial n = n * (factorial (n - 1))

I think it's a much more elegant way to handle end conditions and special cases than if statements and it makes the code much easier to reason about.

It is also awesome when used in conjunction with union types.

data Pony = Pegasus String Int
  | Unicorn String String
  | EarthPony String

applejack = EarthPony "Applejack"
twilight = Unicorn "Twilight Sparkle" "levitation"
rainbow = Pegasus "Rainbow Dash" 20


f :: Pony -> String
f (Pegasus name speed) = name <> " flies at " <> (show speed) <> "mph"
f (Unicorn "Twilight Sparkle" _) = "Twilight Sparkle is an Alicorn now, get over it!"
f (Unicorn name spell) = name <> " casts " <> spell
f (EarthPony name) = name <> " is a nice pony"

If I had ever understood lisp's macros I would probably mention them here too.

Python's list comprehensions! The syntax is super elegant, and they are really performant.

In JavaScript, to filter values, you would do something like this:

arr.filter(function(i) {
  return i > 0
})

in Python, you would use a list comprehension

li = [i for i in li if i > 0]

You can also transform all the elements in a list (similar to a map in JavaScript):

li = [x * 2 for x in li]

I don't know if I miss them all the time, but dependent types (found in Idris, among other languages) are very interesting. Learning Idris definitely had an influence on how I think about certain problems; for example, at work I needed to write some code to partition items in a user's cart into separate shipments. A post-condition of this code is that the total number of items across all of the returned shipments should be the same as the total number of items in the user's cart; in most languages, you would use rigorous testing to assert you'd done this, but in Idris, you can express this property in the type system itself!

Something I miss more often (also found in Idris, along with other ML-family languages) is type-directed search. If you don't know the name of a function, but you can describe what it does in a type, you can use that type to find the function you need. For example, let's say we don't know how to append vectors in Idris (vectors are like lists in other functional languages, only their length is part of their type). You can think of the type of append as being "take a vector of length N containing items of type A, and a vector of length M containing items of type A, and return a vector of length (N + M) containing items of type A". So we ask Idris to find such a function for us:

*Data/Vect> :search Vect n a -> Vect m a -> Vect (n + m) a

...and it gives us the answer!

*Data/Vect> :search Vect n a -> Vect m a -> Vect (n + m) a
= Data.Vect.(++) : Vect m elem -> Vect n elem -> Vect (m + n) elem
Append two vectors

> Data.Vect.merge : Ord elem => Vect n elem -> Vect m elem -> Vect (n + m) elem

Smalltalk has something similar in its method finder; we can ask it "which messages can I send to 'foo' that return 'FOO', and it'll tell us asUppercase will do this for us.

A random wacky feature in Scheme that I can't say I think about a lot (other than being just plain interesting and mind-bending) is continuations; they could merit their own post, but continuations are a way of encapsulating the "next step" a program would take. I recommend reading the Wikipedia article if that piqued your curiosity!

Deterministic object destruction is probably my favorite feature of C++ that is found in only a select few other languages. When execution leaves a function or any other scope, be it regularly or through an exception, all local objects get destroyed and their destructors executed. Not eventually when a garbage connector feels like cleaning up some leftovers but immediately. This includes the automatic destruction of that class' member objects.
That means there's also no need for finally: cleaning up all those file handles, memory allocations and network sockets and painting the sky blue again is done automagically by the destructors of properly written filestreams, smart pointers, socket classes and temporary sky blackeners.

Switches and cases. I love them. But when I tried out Python, I was really surprised that it didn’t have them, especially since the C# syntax looks a lot like it could be Python. For example, the Python equivelant of

switch(word):
    case(“Hello”):
        otherWord = “World”;
        break;
    case(“World”):
        otherWord = “Hello”;
        break;
    default:
        otherWord = “FooBar”;
        break;

is currently

if word == “Hello”:
    other_word = “World”
elif word == “World”:
    other_word = “Hello”
else:
    other_word = “FooBar”

I know that both PEP 275 and PEP 3103 have been rejected, but I have high hopes that a new one is created and accepted.

In Clojure, I really like the threading macro.

Imagine we have a string " I will be a url slug ", and we want to transform it into a valid "url". Let's build a slugify function to do this transformation.

(defn slugify
  [string]
  (clojure.string/replace
    (clojure.string/lower-case
      (clojure.string/trim string)) #" " "-"))

(slugify " I will be a url slug   ") ;; "i-will-be-a-url-slug"

Here we have:

• trim: removes whitespace from both ends of string
• lower-case: converts string to all lower-case
• replace: replaces all instance of match with replacement in a given string

Another solution is to use the threading macro. Basically we can compose functions using the -> operator.

(defn slugify
  [string]
  (-> string
      (clojure.string/trim)
      (clojure.string/lower-case)
      (clojure.string/replace #" " "-")))

(slugify " I will be a url slug   ") ;; "i-will-be-a-url-slug"

Beautiful!

PS. another solution is to compose all function using comp function :)
PS2. I wrote a post about Functional Programming with Clojure

Having worked mostly with Elm, Python and Go lately, there's a LOT of things I miss when working with other languages.

1. Good and clear ecosystems - Elm's automatic versioning is ingenious, Python has a pretty solid selection of libraries and with Pipenv it's even a bit better than standard, and Go is getting pretty solid as well. There's also no excessive use of microlibraries that add one basic function that everyone depends on multiple times in their dependency tree.

2. Joy of development - constantly cursing why the language and it's standard libraries are missing very basic things, debugging being a pain, the runtime environments or compilers not catching super obvious errors (ever done a setTimeout(callback(), 1000) and realized that JS engines don't throw exceptions when setTimeout callback is undefined?), are just not things that happen a lot with these languages

3. Clear standards - gofmt, PEP-8, and similar just save so much headaches. IDEs are configured by default to have one shortcut to reformat the whole file to the language standard, or even just reformat on save, etc. .. saves so much hassle overall

4. Good concurrency model - Go's channels and goroutines are such an awesome thing to work with, and it always pisses me off when I have to work with any other style because they take so much more work to make reliable

5. Clear frontend application model - with Elm you get so much certainty over how things are going to work due to how the language is designed. You avoid whole classes of problems because you just always build your applications in a specific way, and it's the right way for at least most cases.

6. The flexibility - there's almost never a situation where I think "I wonder if I can do this in Python" and the answer is "no". With the whole language being built with well done duck typing instead of e.g. unpredictable type juggling, it's a breeze to do so many things. Similarly with Go, the interface system allows me to customize things a lot more than I would ever have expected from such a low level language.

These are just the things that come to mind in the first few minutes.

I miss Swift's strongly typed enums with associated values a lot in other languages (Go, Python etc.).

From the textbook:

enum Barcode {
    case upc(Int, Int, Int, Int)
    case qrCode(String)
}