Nice! I think it's pretty widely agreed that requiring type annotations at the function level is a good thing anyway. Apparently it's considered good practice in Haskell even though Haskell doesn't require it.

I've also worked with OCaml code that didn't do it and you lose a lot of the advantages of static typing. Definitely worse.

Rust got it right.

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dragonwriter 12 minutes ago | parent | next [-]

Type annotations on functions in Haskell (or similar languages) are useful for:

1. leveraging the type checker to verify (aspects of) the correctness of your function, and

2. communicating intent to humans

I've found in my own explorations with Haskell that its useful to write with functions with them, then verify that they work, and then remove them to see what the inferred would be (since it already compiled with the annotation, the inferred type will either be identical to or more general than the previously declared type), and generally (because it is good practice to have a declared type), replace the old declared type with the inferred type (though sometimes at this point also changing the name.)

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toolslive 2 hours ago | parent | prev | next [-]

what if your IDE can show the type of any expression as a tooltip ? Would you still think the same?

	▲	ufo an hour ago \| parent \| next [-]
		In Haskell, type error messages are always like "expected types A and B to be equal, but they are not". The problem is that, without type annotations, the compiler cannot know if it is A or B that is wrong, which can result in confusing error messages. For example, suppose that you have a bug in the body of a function, but did not provide a type annotation for it. The function might still compile but not with the type you want. The compiler will only notice something is amiss when you try to call the function and it turns out that the function's inferred type doesn't fit the call site. Basically, global type inference in the absence of type annotations means that changes in one part of the file can affect inferred types very far away. In practice it's best to use type annotations to limit inference to small sections, so that type errors are reported close to what caused them.
	▲	Quekid5 an hour ago \| parent \| prev [-]
		I can't speak for the parent poster, but for global function declarations, yes, absolutely. It's infuriating when a type error can "jump" across global functions just because you weren't clear about what types those functions should have had, even if those types are very abstract. So early adopters learned to sprinkle in type annotations at certain points until they discovered that the top-level was a good place. In OCaml this pain is somewhat lessened when you use module interface files, but without that... it's pain.

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Quekid5 an hour ago | parent | prev [-]

> I think it's pretty widely agreed that requiring type annotations at the function level is a good thing anyway. Apparently it's considered good practice in Haskell even though Haskell doesn't require it.

In Haskell-land: At the global scope, yes, that's considered good practice, especially if the function is exported from a module. When you just want a local helper function for some tail-recursive fun it's a bit of extra ceremony for little benefit.

(... but for Rust specifically local functions are not really a big thing, so... In Scala it can be a bit annoying, but the ol' subtyping inference undecidability thing rears its ugly head there, so there's that...)

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ufo 25 minutes ago | parent [-]

Languages with local type inference can sometimes omit type annotations from lambdas, if that lambda is being returned or passed as an argument to another function. In those situations we know what the expected type of the argument should be and can omit it.

	▲	Quekid5 5 minutes ago \| parent [-]
		Yeah, that's true and that's a good convenience even if it's not full inference. In the case of Scala, the parameter types may often be required, but at least the return type can be omitted, so there's that.