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> the more I've grown to appreciate the concept of Optionals You may also enjoy a language that supports the generalization of Optionals, which are Algebraic D

by wyager 10y ago

> the more I've grown to appreciate the concept of Optionals You may also enjoy a language that supports the generalization of Optionals, which are Algebraic Data Types (ADTs). Optionals are a single, limited application of ADTs. Optionals allow you to shift null pointers (and null pointer exceptions) to the type level. So instead of having to worry about a function returning null, you just deal with Optionals whenever a function returns one. There's another ADT, usually called "Either" or "Result", that allows you to shift exceptions to the type level. You can see from the type of the function what kind of exception it might return, and you deal with exceptions like any other value instead of via a special exception handling mechanism.

austinz 10y ago
Swift supports ADTs and implements its Optional type as an ADT.
- wyager 10y ago
  > Swift supports ADTs True, but perhaps not practically relevant. The pattern matching in Swift isn't much better than what you would get with a tagged C union, which is why no one really uses it very heavily. The Optional type in Swift gets a lot of special compiler support, which is indicative of the fact that the broader language isn't very friendly towards using ADTs to structure data. But you're right, I should have clarified in my comment that Swift does have a basic degree of support for ADTs.
  - austinz 10y ago
    > The pattern matching in Swift isn't much better than what you would get with a tagged C union In what way? Pattern matching in Swift is quite powerful; definitely better than C's switch statement. This blog post is a good overview of its capabilities. https://appventure.me/2015/08/20/swift-pattern-matching-in-detail/ https://appventure.me/2015/08/20/swift-pattern-matching-in-d...
  - xenadu02 10y ago
    In what way is Swift pattern matching not "much better than what you would get with a tagged C union"? enum in Swift has pattern matching support, can have properties, adopt protocols, etc. I would disagree with your assessment that "nobody uses them".
  - eridius 10y ago
    I don't know what kind of Swift code you're writing, but you're very, very wrong when you say "no one really uses it heavily". Pattern matching is in fact quite powerful and is used heavily in any codebase that isn't simply a straight porting of Obj-C idioms. > The Optional type in Swift gets a lot of special compiler support Only the ?. optional chaining operator (and technically the ! postfix operator too, though that could have been implemented in the standard library instead of being special syntax and in fact I'm not really sure why it isn't just a stdlib operator). And probably some logic in the switch exhaustiveness checker so it knows nil is the same as Optional.none. Everything else (including the actual pattern matching against the "nil" keyword) is done in a way that can be used by any type, not just Optionals (nil for pattern matching specifically evaluates to a stdlib type called _OptionalNilComparisonType which implements the ~= operator so it can be compared against any Optional in a switch, and you could do the same for other ADTs too, assuming they have a reasonable definition for nil).
ridiculous_fish 10y ago
One of the things that makes Optional so pleasant in Swift is the syntax support. This includes optional-chaining, if-let and guard-let unwrapping, and some convenient operators. For example, in Haskell, by default you can't compare an Optional with the value it wraps: `5 == Just 5` fails. But in Swift this works like you would want. All that is to say that Options in Swift are a bit nicer than what you could get with pure ADTs. It's a similar story for Swift's syntax-level error handling vs type-level monadic error handling. (The downside of course is that the compiler has to be specially taught about these - but maybe you want that anyways, e.g. for Rust's null-pointer optimization.)
- wyager 10y ago
  > This includes optional-chaining, Haskell has bind (>>=) and do-syntax, rust has `and_then`. This is pretty standard with any ADT-supporting language. > if-let Most languages with ADT support have good pattern matching that subsumes if-let syntax and allows you to do other things as well. Swift's pattern matching, such as it is, is a bit of a mess. > guard-let unwrapping Haskell has `guard` and `MonadFail`, which address the use cases for this in a more principled way. `guard` for boolean matching (like equality) and `MonadFail` for structural matching (like pattern matching on a constructor). Rust has (?) and `try`, which are syntactic sugar around a simple match/return statement that addresses most use cases of guard-let. Obviously there are going to be small differences between this implementations, but Swift doesn't really excel here. > `5 == Just 5` fails. As it probably should. Supporting such implicit casting could lead to some obvious confusion and buggy behavior. Ideally, the fact that "a == b" typechecks should indicate that "a" and "b" are of the same type. In Haskell, you would just do `Just 5 == x` if you want to check that something is, in fact, `Just 5`. If that really wasted a lot of space somehow, you can define something like `x ==: y = Just x == y`.
  - comex 10y ago
    Rust copied 'if let' from Swift (~2 years ago) despite having decent pattern matching; community consensus today is that it's highly worthwhile as a feature. There have also been proposals to add 'guard let'. So, while I don't have enough Swift experience to judge its ADT support overall, I wouldn't cite those features as evidence that it's wanting. They might not be as natural in traditional FP languages like Haskell, but they seem to fit pretty well in the stylistic niche Rust and Swift (among other languages) have carved out. edit: FWIW, there's some overlap between '?' and 'guard let', but not that much. Using hypothetical Rust syntax, they'd overlap in this case: guard let Some(x) = foo else { return Err(GettingFoo); } better expressed as let x = foo.ok_or(GettingFoo)?; …but if you want to return something other than a Result, or the ADT being matched on is something custom rather than Option/Result, there's no good way to make '?' do the job.
kazagistar 10y ago
Swift does fully support ADTs, and can easily model result types, they just chose to have catchable exceptions as well.

by wyager 10y ago

austinz 10y ago

Swift supports ADTs and implements its Optional type as an ADT.

wyager 10y ago
> Swift supports ADTs True, but perhaps not practically relevant. The pattern matching in Swift isn't much better than what you would get with a tagged C union, which is why no one really uses it very heavily. The Optional type in Swift gets a lot of special compiler support, which is indicative of the fact that the broader language isn't very friendly towards using ADTs to structure data. But you're right, I should have clarified in my comment that Swift does have a basic degree of support for ADTs.
- austinz 10y ago
  > The pattern matching in Swift isn't much better than what you would get with a tagged C union In what way? Pattern matching in Swift is quite powerful; definitely better than C's switch statement. This blog post is a good overview of its capabilities. https://appventure.me/2015/08/20/swift-pattern-matching-in-detail/ https://appventure.me/2015/08/20/swift-pattern-matching-in-d...
- xenadu02 10y ago
  In what way is Swift pattern matching not "much better than what you would get with a tagged C union"? enum in Swift has pattern matching support, can have properties, adopt protocols, etc. I would disagree with your assessment that "nobody uses them".
- eridius 10y ago
  I don't know what kind of Swift code you're writing, but you're very, very wrong when you say "no one really uses it heavily". Pattern matching is in fact quite powerful and is used heavily in any codebase that isn't simply a straight porting of Obj-C idioms. > The Optional type in Swift gets a lot of special compiler support Only the ?. optional chaining operator (and technically the ! postfix operator too, though that could have been implemented in the standard library instead of being special syntax and in fact I'm not really sure why it isn't just a stdlib operator). And probably some logic in the switch exhaustiveness checker so it knows nil is the same as Optional.none. Everything else (including the actual pattern matching against the "nil" keyword) is done in a way that can be used by any type, not just Optionals (nil for pattern matching specifically evaluates to a stdlib type called _OptionalNilComparisonType which implements the ~= operator so it can be compared against any Optional in a switch, and you could do the same for other ADTs too, assuming they have a reasonable definition for nil).

ridiculous_fish 10y ago

One of the things that makes Optional so pleasant in Swift is the syntax support. This includes optional-chaining, if-let and guard-let unwrapping, and some convenient operators. For example, in Haskell, by default you can't compare an Optional with the value it wraps: `5 == Just 5` fails. But in Swift this works like you would want. All that is to say that Options in Swift are a bit nicer than what you could get with pure ADTs. It's a similar story for Swift's syntax-level error handling vs type-level monadic error handling. (The downside of course is that the compiler has to be specially taught about these - but maybe you want that anyways, e.g. for Rust's null-pointer optimization.)

wyager 10y ago
> This includes optional-chaining, Haskell has bind (>>=) and do-syntax, rust has `and_then`. This is pretty standard with any ADT-supporting language. > if-let Most languages with ADT support have good pattern matching that subsumes if-let syntax and allows you to do other things as well. Swift's pattern matching, such as it is, is a bit of a mess. > guard-let unwrapping Haskell has `guard` and `MonadFail`, which address the use cases for this in a more principled way. `guard` for boolean matching (like equality) and `MonadFail` for structural matching (like pattern matching on a constructor). Rust has (?) and `try`, which are syntactic sugar around a simple match/return statement that addresses most use cases of guard-let. Obviously there are going to be small differences between this implementations, but Swift doesn't really excel here. > `5 == Just 5` fails. As it probably should. Supporting such implicit casting could lead to some obvious confusion and buggy behavior. Ideally, the fact that "a == b" typechecks should indicate that "a" and "b" are of the same type. In Haskell, you would just do `Just 5 == x` if you want to check that something is, in fact, `Just 5`. If that really wasted a lot of space somehow, you can define something like `x ==: y = Just x == y`.
- comex 10y ago
  Rust copied 'if let' from Swift (~2 years ago) despite having decent pattern matching; community consensus today is that it's highly worthwhile as a feature. There have also been proposals to add 'guard let'. So, while I don't have enough Swift experience to judge its ADT support overall, I wouldn't cite those features as evidence that it's wanting. They might not be as natural in traditional FP languages like Haskell, but they seem to fit pretty well in the stylistic niche Rust and Swift (among other languages) have carved out. edit: FWIW, there's some overlap between '?' and 'guard let', but not that much. Using hypothetical Rust syntax, they'd overlap in this case: guard let Some(x) = foo else { return Err(GettingFoo); } better expressed as let x = foo.ok_or(GettingFoo)?; …but if you want to return something other than a Result, or the ADT being matched on is something custom rather than Option/Result, there's no good way to make '?' do the job.

kazagistar 10y ago

Swift does fully support ADTs, and can easily model result types, they just chose to have catchable exceptions as well.