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In a language that actually tries to help you creating error-free code, a type check would prevent this. This is really just a wonderful example how javascript
by choeger 6y ago
In a language that actually tries to help you creating error-free code, a type check would prevent this.
This is really just a wonderful example how javascript is a mental burden to the programmers instead of a useful tool.
- dsego 6y agoHow would a type check prevent this?
- choeger 6y agoA type check would give map a type, say [a] -> (int -> a -> b) -> [b]. If you try to apply it to any function that does not have the type (int -> a -> b), the check would fail.
- username90 6y agoSo basically any language that does any form of type checking at all.
- MaxBarraclough 6y agoNot including TypeScript.
- xsmasher 6y agoJust to clarify, if you update toReadableNumber to take a second parameter and that parameter is not a number, typescript will complain.* TypeScript won't catch the original landmine because it ignores the extra parameters; maybe some linter would? Is there a rule that enforces "functions used by map must spell out all the parameters?" * Example of typescript giving an error on toReadableNumber_v2: https://www.typescriptlang.org/play?#code/FAYw9gdgzgLgBAQwE5IQTzgXjgbQIwA0ATAQMwC6A3MDQGYCuEIMAlpHDGAEoCmCAJggBGAGx4A5egFshPJAAoI0gFxKZcgJQBvYHD1wkPGPSQQ4agHScAYiwAePfvKIbqAXxrAGTVu068BYTFJdSQAfQA3IkUVNVkkAjgAB0Nae2VYJBYIAHNtXX1DY1Nk1Ps4AGpzaSswWwcnF3cacGgwMQsRMBz5RBR0CykEJN7-PkFRCWl4uA1Z6hbIKHaeTu7e5FQ0QeHR7nGgqdDIoln5miA https://www.typescriptlang.org/play?#code/FAYw9gdgzgLgBAQwE5...
- wruza 6y agoSee also magicalhippo’s comment on overloading. We could introduce real types instead of primitives, so that ‘number_formatting_base_t’ would conflict with ‘array_index_t’, but nobody in the world bothers beyond bare ‘int’.
- kuschku 6y agoActually, that's what I use inline types in kotlin for. So username, password are distinct from string and objectid might be backed by an int, but is distinct from int. Obviously, once compiled, there's no overhead, it's a zero cost abstraction. But an incredibly useful one.
- username90 6y agoYeah, I see the point, in for example C# if you have overloaded methods it tries it best to map it to the accepted type. However you'd have to work really hard to invent a case where a library updates a signature to accept another type, and then you in your code have 2 overloads of a method that you pass and it now choses the wrong version, all while the library change doesn't break any other code. Scenario: Library changes signature so you always pass functions with more parameters. Result: This will break almost every codebase, they probably wouldn't do this in a minor patch. Scenario: Library adds a new signature where you can pass functions with more parameters and keep them overloaded between each other. Result: Compiler can't identify which of the two signatures to pass your overloaded function and throws a compilation error.
- Akronymus 6y agoOn my personal projects with F#, never got a chance to use it professionally, I actually use them. Makes modeling the data much easier.
- btinker 6y agoIt would check the arguments and return type of the function. Map takes a function with three arguments, toReadableNumber only takes one, therefore the functions are of a different type. So someNumbers.map(toReadableNumber) would be an error and not execute at all, instead of being a "bad practice" / potential mistake.
- dsego 6y agoWhat if toReadable also takes 3 args of the same type but different meaning?
- flohofwoe 6y agoYou can create custom types to communicate semantics to the type checker. Whether that's always a good idea is arguable though, but the tools are there (e.g. there's a wide field between weak and strong typing, and an overly strong type checker can be quite a hassle to work with while an overly weak type checker isn't much better than duck typing).
- magicalhippo 6y agoWell the issue mentioned is that map calls the callback function with up to three arguments[1]. A type check could prevent this because it would require map to take a reference to a function with three parameters, or the compiler would complain inside the map implementation. Similarly, passing it a function with only one parameter would be a type violation and the compiler would complain. Now in a language with type checking, you could still potentially run afoul. Say the map function was overloaded with one variant for one-parameter callbacks, one variant for two-parameter callbacks etc. Then the compiler might figure out it could use the second overload if the "toReadableNumber" function got changed to take the extra "base" parameter. So again you end up with the numbers getting converted with a variable base. Though, IMHO, having such an overloaded map function is inviting trouble and is a very poor design. [1]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...
- mr_toad 6y ago> or the compiler Functions can be assigned at runtime. It would have to be a runtime error.
- dirkt 6y agoA proper typesystem for functional types will check at compile-time that in all places where you would assign such a function, the function will always have the right type. So it's NOT a runtime error, it's a compile-time error, even though you can assign different functions at runtime.
- mr_toad 6y agoYou can assign a reference to a function and pass that to a function that expects a callback, and then change that assignment based on incoming data. And if that’s not enough you can create and modify functions at runtime. And you’d have to content with JavaScript’s spread syntax and rest parameters. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...
- schwartzworld 6y agoIf your code has tests, an API change like the one described in the article would get caught immediately. And it only works because `Array.prototype.map` callback has one required arguments and two optional ones. What language with optional function arguments protects you from this sort of behavior? Do people in that language not test this sort of behavior? Or at least run through the app? More than that, the whole hurrdurr-javascript-bad thing is so tired. Lots of work is being done in JavaScript. Sure, it has its quirks, but those quirks come with expressiveness. You can use functional or imperative style, throw lambdas around, and it runs on pretty much all the phones and computers in the world.
- sfvisser 6y ago> What language with optional function arguments protects you from this sort of behavior? Languages with a sane type system. TypeScript has no issues catching such mistakes. Writing tests to catch simple type errors is such an incredible waste of time.
- tom_mellior 6y agoThe article has a whole section called "TypeScript doesn't solve this", with examples and stuff. Is it mistaken?
- sfvisser 6y agoPartially. TypeScript doesn't complain when you pass in a function that ignores some of its arguments. Which is totally fine and safe. If you upgrade your function from no second argument to a numeric second argument TypeScript will not complain and your program might break. It will not crash, because it still perfectly type-safe, but it might not behave like you want to. So in that sense the article has a point. However, this is just one instance of a larger issue with changing the behavior of a function, while keeping the types compatible. - Using a function as a callback to .map() with two numeric arguments and then swapping the arguments. - Returning a tuple of two of the same type and swapping the order. - Returning a string in a new encoding. - ... Basic rule: if it's a type error and the program might crash, TypeScript will complain. If the types are fine and only the behavior changes, TypeScript will (obviously) not complain. Callback functions and optional arguments are not special in this regard.