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Comment by dmytrish | Hacker News Reader

I don't know how can I actually do this inside a do-block:

    xs :: [Int] <- ...

does not work without `ScopedTypeVariables` (and I don't like proliferation of redundant language extensions).

In Rust, sure, it's not a problem and plays well with the language.

My point is, it is easy to abuse this feature (this example is from Haskell Prelude!), even though it is indispensable in some situations (like `mempty :: a`).

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You can either type the return value at the end of your line:

    xs <- map read <$> lines <$> getContents :: IO [Int]

Or you can force the xs type on some other line:

    let _ = xs :: Int

There is a recent idiom for that last format that lets you write on the same line as the expression. I think it's something like:

    (xs @ [Int]) <- map read <$> lines <$> getContents

But I haven't used it yet and I am not sure exactly what version of GHC allows it.

Today I learned that `:: IO [Int]` at the end of an expression is applied to the whole expression. Thanks!

> does not work without `ScopedTypeVariables` (and I don't like proliferation of redundant language extensions).

It does something useful for you! How can it be redundant?

You don't like the part where compiler authors are free to make efforts to improve the language that are clearly delineated as not part of the standard and easy to not enable if they prove to not be a good idea in practice?

What are the alternatives? Completely discarding the language standard? Never making attempts to improve the language? Requiring all experiments to go through a standardization committee before real-world testing?

None of those strike me as obviously better.

I am definitely not against languages extensions in general. I think that it's one of the unique features of Haskell that keep it stable and flexible at the same time.

If I already have `ScopedTypeVariables` in scope for other valid reasons, I am happy to use it. It's just that I'd rather not enable it for tiny cosmetic improvement like this.

If it makes you feel any better, I suspect that ScopedTypeVariables is basically how it should have worked in the first place. (The default behavior is pretty weird/surprising.)

Almost all Haskell code written these days uses at least a handful, if not a barrel-full, of extensions, and ScopedTypeVariables is one of the most common ones. Of course, you’re completely free to do whatever you’d like with your own code, but you’ll probably find it hard to avoid extensions — at the least many of them will be used by some libraries you’re depending on.

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