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0 pointsSkyMarshal6y ago0 comments

You're skipping half the recommendation though:

> Array indexing must be properly tested, or the get method should be used to return an Option.

Fwiw, for anyone who’s dabbled with Haskell this is SOP. Any result that could be undefined is returned as a Maybe (Haskell’s version of Option). If this seems odd to anyone, understanding it in Haskell will probably help understand it better in Rust too:

http://learnyouahaskell.com/a-fistful-of-monads#getting-our-...

https://en.wikibooks.org/wiki/Haskell/Understanding_monads/M...

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6 comments · 2 top-level

geofft6y ago· 4 in thread

Er, don't Haskell's head and (!!) functions default to throwing an exception?

    $ ghci
    Prelude> let a = [3, 4, 5]
    Prelude> a !! 0
    3
    Prelude> a !! 3
    *** Exception: Prelude.(!!): index too large
    
    Prelude> head []
    *** Exception: Prelude.head: empty list

I don't think Haskell is really different from Rust in this respect. Both have a wrapper type for optional values with good syntax, but in both, there's still some syntax so common operations sometimes choose to skip it.

In fact, Hoogle doesn't seem to find me a function like Rust's .get() in the standard library, just a handful of third-party packages: https://hoogle.haskell.org/?hoogle=%5Ba%5D+-%3E+Int+-%3E+May...

olalonde6y ago

This was posted on /r/rust a couple days ago: https://notes.iveselov.info/cheatsheet-rust-option-vs-haskel...

EvilTerran6y ago

> Hoogle doesn't seem to find me a function like Rust's .get() in the standard library

In practice, you don't really need one - the safe alternative to "xs !! n" is pattern-matching on the result of "drop n xs", as that's [] if xs has ≤n elements:

https://www.haskell.org/onlinereport/standard-prelude.html#$...

geofft6y ago

Sure, that seems syntactically more cumbersome than 'if let Some(foo) = xs.get(n)' or 'xs.get(n).map(|foo| ...)' in Rust, but yes, you can do it. As I said, because both the Rust and Haskell versions are more cumbersome than using the version that raises an exception/panics, both Rust and Haskell's standard libraries choose to give you a syntactically-easier alternative that isn't a total function.

All I'm saying is that Haskell doesn't seem to do anything different here - Rust has incorporated the lessons from Haskell's type system. (As someone who fell in love with Haskell a long time ago but never got to use it professionally, this is basically why I like Rust.) Is there something Haskell does that Rust does not do? I'm not trying to say Haskell is insufficient - I'm just refuting the claim that Rust is insufficient and should act more like Haskell.

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masklinn6y ago

> In practice, you don't really need one - the safe alternative to "xs !! n" is pattern-matching on the result of "drop n xs", as that's [] if xs has ≤n elements:

Great so instead of `xs !! n` you're supposed to write

    case drop n xs of
        [] -> …
        x :: _ -> …

that seems… less than likely?

masklinn6y ago

> Fwiw, for anyone who’s dabbled with Haskell this is SOP. Any result that could be undefined is returned as a Maybe (Haskell’s version of Option).

It would be nice if that were true but that ain't exactly the case is it?

AFAIK exhaustive pattern matching still isn't enabled by default, and the prelude is full of partial functions, especially on lists (to such an extent that GHC has a utility function just for blowing up on empty lists: https://hackage.haskell.org/package/base-4.12.0.0/docs/GHC-L...)

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6 comments · 2 top-level

geofft6y ago· 4 in thread

Er, don't Haskell's head and (!!) functions default to throwing an exception?

    $ ghci
    Prelude> let a = [3, 4, 5]
    Prelude> a !! 0
    3
    Prelude> a !! 3
    *** Exception: Prelude.(!!): index too large
    
    Prelude> head []
    *** Exception: Prelude.head: empty list

In fact, Hoogle doesn't seem to find me a function like Rust's .get() in the standard library, just a handful of third-party packages: https://hoogle.haskell.org/?hoogle=%5Ba%5D+-%3E+Int+-%3E+May...

olalonde6y ago

This was posted on /r/rust a couple days ago: https://notes.iveselov.info/cheatsheet-rust-option-vs-haskel...

EvilTerran6y ago

> Hoogle doesn't seem to find me a function like Rust's .get() in the standard library

In practice, you don't really need one - the safe alternative to "xs !! n" is pattern-matching on the result of "drop n xs", as that's [] if xs has ≤n elements:

https://www.haskell.org/onlinereport/standard-prelude.html#$...

geofft6y ago

1 more reply

masklinn6y ago

> In practice, you don't really need one - the safe alternative to "xs !! n" is pattern-matching on the result of "drop n xs", as that's [] if xs has ≤n elements:

Great so instead of `xs !! n` you're supposed to write

    case drop n xs of
        [] -> …
        x :: _ -> …

that seems… less than likely?

masklinn6y ago

> Fwiw, for anyone who’s dabbled with Haskell this is SOP. Any result that could be undefined is returned as a Maybe (Haskell’s version of Option).

It would be nice if that were true but that ain't exactly the case is it?

j / k navigate · click thread line to collapse