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Does the caller care? By day I work with a team in a language that sees errors ride on the exception handling system. Staying within the original example, I se

Does the caller care? By day I work with a team in a language that sees errors ride on the exception handling system. Staying within the original example, I see code like this all the time (too often, even, but that's another topic for another day): try { file = getFile() } catch(/* ... */) { fileUnavailable() } Here, the assumption of getFile that the caller wanted an error was incorrect. A Result-using language would end up in a similar place. Idiomatic Go says leave it to the caller. Like above, when only wants to know if there is "file or no file" without concern for why there is no file, then: file, _ := getFile() // The second return argument is an error. if file == nil { fileUnavailable() } I doubt either way makes much difference in this contrived example, but the difference shows up when it extends out into real code. There are plusses and minuses to each way of seeing the world. Tradeoffs, as always.

nulld3v 3y ago
But in Rust you can do the same, I do this all the time: let _ = fs::mkdir_all() // Error ignored, Rust will not complain because you explicitly assigned to _ Or if the function returns something I need but I don't care about the error: let Ok(file) = get_file() else { file_unavailable(); return } upload_file(file); Or this: let file = get_file().unwrap_or_default()
- randomdata 3y ago
  [flagged]
  - nulld3v 3y ago
    So what is the point of the distinction then? The examples I gave were idomatic Rust too. Both Go and Rust hand the caller an error and the caller then must do something with the error. In both Go and Rust, you can assign the error to underscore and ignore it. Java is different but we aren't talking about Java right now. The only difference is that in Go the function can (and must) still provide a value for "file" even when there is an error. In Rust you can kind of do something similar, by giving the File struct a default value (that's what "unwrap_or_default()" is) but it isn't exactly the same. I would argue that such a case is extremely rare though.
    - randomdata 3y ago
      > Both Go and Rust hand the caller an error and the caller then must do something with the error. This is simply not true. Go preaches that values should always be useful. Given a return signature (T, error), the value of type T can be used irrespective of what is contained in the value of type error. They are independent states. > The only difference is that Go provides a value for "file" when there is an error. Yes, that is ultimately the difference. Result takes what are independent states and tries to make them dependent. Result<T, error> assumes that if there is a value of type error then the caller would not make use of the value of type T. Often they will be dependent for all practical purposes. It is not an unreasonable assumption to assume that they are, especially if you know the caller. However, Go believes you cannot make that assumption. That you have to let the caller determine what it finds useful, not what you think it might find useful. Ease of code reusability is clearly a design goal of Go, so the idea that you can't get to know your callers and how they will use your function isn't unwarranted. Tradeoffs, as always.
      - awused 3y ago
        Result covers the very common case where there is T or error but never both. Having a decent type system doesn't prevent you from returning both. In Go I simply cannot have something as simple as "A or B, but never both or neither" as a type. >However, Go believes you cannot make that assumption. This is trying to twist a weakness of Go's type system as a virtue. In Go I always have "A or B, including neither and both". I can have that in Rust or Haskell if I want, but usually I want "A or B, never both or neither" and rarely "A or B, sometimes both but never neither" which Rust and Haskell can do but Go cannot. This comes from Go using product types as a poor replacement for sum types.
        randomdata 3y ago
        > This is trying to twist a weakness of Go's type system as a virtue. No. There is no discussion about type systems taking place here at all. The discussion is about patterns where the producer or the consumer is in control. Specifically, Result puts the producer in control. Idiomatic Go (T, error) sees the consumer in control. There is likely no language in existence that prevents you from choosing. You can write code where the producer is in control in Go, and you can write code where the consumer is in control in every other. These are not features of a language, although language idioms do often push developers one way or the other.
earthboundkid 3y ago
You mean file == nil.
simiones 3y ago
> Idiomatic Go says leave it to the caller. Like above, when only wants to know if there is "file or no file" without concern for why there is no file, then: > file, _ := getFile() // The second return argument is an error. if file == nil { fileUnavailable() } That very often doesn't work in Go. Most functions which return errors offer no guarantee whatsoever about the return value if there is an error. No one would consider it a breaking change to modify the return in case of error. And many functions return a struct, where Go offers no way to compare two arbitrary struct values for equality, or to check if an arbitrary struct value is that struct's "zero value". So no, Go does not recommend (or even endorse) this pattern.

nulld3v 3y ago

But in Rust you can do the same, I do this all the time: let _ = fs::mkdir_all() // Error ignored, Rust will not complain because you explicitly assigned to _ Or if the function returns something I need but I don't care about the error: let Ok(file) = get_file() else { file_unavailable(); return } upload_file(file); Or this: let file = get_file().unwrap_or_default()

randomdata 3y ago
[flagged]
- nulld3v 3y ago
  So what is the point of the distinction then? The examples I gave were idomatic Rust too. Both Go and Rust hand the caller an error and the caller then must do something with the error. In both Go and Rust, you can assign the error to underscore and ignore it. Java is different but we aren't talking about Java right now. The only difference is that in Go the function can (and must) still provide a value for "file" even when there is an error. In Rust you can kind of do something similar, by giving the File struct a default value (that's what "unwrap_or_default()" is) but it isn't exactly the same. I would argue that such a case is extremely rare though.
  - randomdata 3y ago
    > Both Go and Rust hand the caller an error and the caller then must do something with the error. This is simply not true. Go preaches that values should always be useful. Given a return signature (T, error), the value of type T can be used irrespective of what is contained in the value of type error. They are independent states. > The only difference is that Go provides a value for "file" when there is an error. Yes, that is ultimately the difference. Result takes what are independent states and tries to make them dependent. Result<T, error> assumes that if there is a value of type error then the caller would not make use of the value of type T. Often they will be dependent for all practical purposes. It is not an unreasonable assumption to assume that they are, especially if you know the caller. However, Go believes you cannot make that assumption. That you have to let the caller determine what it finds useful, not what you think it might find useful. Ease of code reusability is clearly a design goal of Go, so the idea that you can't get to know your callers and how they will use your function isn't unwarranted. Tradeoffs, as always.
    - awused 3y ago
      Result covers the very common case where there is T or error but never both. Having a decent type system doesn't prevent you from returning both. In Go I simply cannot have something as simple as "A or B, but never both or neither" as a type. >However, Go believes you cannot make that assumption. This is trying to twist a weakness of Go's type system as a virtue. In Go I always have "A or B, including neither and both". I can have that in Rust or Haskell if I want, but usually I want "A or B, never both or neither" and rarely "A or B, sometimes both but never neither" which Rust and Haskell can do but Go cannot. This comes from Go using product types as a poor replacement for sum types.
      - randomdata 3y ago
        > This is trying to twist a weakness of Go's type system as a virtue. No. There is no discussion about type systems taking place here at all. The discussion is about patterns where the producer or the consumer is in control. Specifically, Result puts the producer in control. Idiomatic Go (T, error) sees the consumer in control. There is likely no language in existence that prevents you from choosing. You can write code where the producer is in control in Go, and you can write code where the consumer is in control in every other. These are not features of a language, although language idioms do often push developers one way or the other.

earthboundkid 3y ago

You mean file == nil.

simiones 3y ago

> Idiomatic Go says leave it to the caller. Like above, when only wants to know if there is "file or no file" without concern for why there is no file, then: > file, _ := getFile() // The second return argument is an error. if file == nil { fileUnavailable() } That very often doesn't work in Go. Most functions which return errors offer no guarantee whatsoever about the return value if there is an error. No one would consider it a breaking change to modify the return in case of error. And many functions return a struct, where Go offers no way to compare two arbitrary struct values for equality, or to check if an arbitrary struct value is that struct's "zero value". So no, Go does not recommend (or even endorse) this pattern.