- cross-posted to:
- programmerhumor@lemmy.ml
- cross-posted to:
- programmerhumor@lemmy.ml
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The actual reason why let … in syntax tends to not use C-style “type var” like syntax is because it’s derived from the syntax type theory uses, and type theorists know about parameterised types. Generics, in C++ parlance, excuse my Haskell:
let foo :: Map Int String = memptyWe have an empty map, and it maps integers to Strings. We call it foo. Compare:
Map Int String foo = memptyIf nothing else, that’s just awkward to read and while it may be grammatically unambiguous (a token is a name if it sits directly in front of
=) parser error messages are going to suck.Map<Int,String>is also awkward but alas that’s what we’re stuck with in Rust because they reasoned that it would be cruel to put folks coming from C++ on angle bracket withdrawal. Also Rust has ML ancestry don’t get me started on their type syntax.There is also the thing where the compiler might mistake your c++ style variable declaration for a function, e.g.
String myfunction():
String myvariable();
How do you do nested parameterized types without it becoming ambiguous though? That’s IMO the biggest advantage of the bracket syntax. For example:
Map<Tuple<Int, Int, Int> Int>Map (Int, Int) Int. Kind of a bad example because tuples have special-case infix syntax, the general case would beMap Int (Either Int Bool). Follows the same exact syntax as function application just that types (by enforced convention) start upper case. Modulo technical wibbles to ensure that type inference is possible you can consider type constructors to be functions from types to types.…function application syntax is a story in itself in Haskell because
foo a b cgets desugared to(((foo a) b) c): There’s only one-argument functions. If you want to have more arguments, accept an argument and return a function that accepts yet another argument. Then hide all that under syntactic sugar so that it looks innocent. And, of course, optimise it away when compiling. Thus you can write stuff likemap (+5) xsin Haskell while other languages need the equivalent ofmap (\x -> x + 5) xs(imagine the\is a lambda symbol).Interesting. Thanks!
Until now, I looked at
letand thought, “maybe they just felt like doing it that way”.
Makes a lot more sense now.
I’ve always wondered where all this ‘let’ business started
It’s commonly used in math to declare variables so I assume programming languages borrowed it from there.
More specifically, they’re borrowing the more mathematical meaning of variables, where if you say x equals 5, you can’t later say x is 6, and where a statement like “x = x + 1” is nonsense. Using “let” means you’re setting the value once and that’s what it’s going to remain as long as it exists, while “var” variables can be changed later. Functional languages, which are usually made by very math-y people, will often protest the way programmers use operators by saying that
=is strictly for equality and variable assignment is:=instead of==and=in most C-style languages.Unless you’re in JS.
let a: &'static strNot to short-circuit the joke, but in this case, it’s because the valid JavaScript version is…
let a…and one of TypeScript’s main design goals is to be a superset of JavaScript, that only adds syntax, and doesn’t re-write it.
Beyond that, it’s probably a case of some new language just using what the designer is familiar with.
Who says this is JS? Might be Rust.
Then the second part of my statement applies.
In the case of Rust, you can also omit the type annotation in the vast majority of cases and the compiler will infer it.
String a: new String()Can we talk about PHP functions with typehints too?
public static function foo(): string {Practically every other language with similar syntax does this instead:
public static string foo() {TIL PHP has statics.
Also, does PHP actually enforce the type declarations? I’d assume it would but knowing PHP…
It enforces scalar types (string, int, etc) at runtime if you enable strict mode. There’s also static analysis tools like PHPStan and Psalm that will flag issues at build time.
First time i used let it was to inline variable declaration with assignment . Can’t remember the language.
