Add rule to infer property types from the right-hand-side value rather than writing the type explicitly on the left-hand side #263

calda · 2024-03-13T23:28:48Z

Please react with 👍/👎 below if you agree or disagree with this proposal.

Summary

This PR proposes a new rule to prefer letting the type of a property be inferred from the right-hand-side value rather than writing the type explicitly on the left-hand side:

// WRONG
let sun: Star = .init(mass: 1.989e30)
let earth: Planet = .earth

// RIGHT
let sun = Star(mass: 1.989e30)
let earth = Planet.earth

Autocorrect support for this rule is implemented in nicklockwood/SwiftFormat#1640.

Reasoning

We already have a rule to omit redundant types. It converts let sun: Star = Star() to let sun = Star(), rather than to let sun: Star = .init().

The syntax with inferred types is more idiomatic, and is much more common, than the syntax with explicit types.

I briefly checked the frequency of these two styles in our codebase with a regex search:

Syntax with implicit types (let sun = Star() or let earth = Planet.earth, etc):
- ~20,000 matches using the regex ((let)|(var)) \w+ = [A-Z]+\w+(\.|\()
Syntax with explicit types (let sun: Star = .init(), etc)
- ~5,000 matches using the regex ((let)|(var)) \w+: \w+ = \..

This rule only applies to simple cases where the right-hand side starts with a leading dot (meaning we know that the RHS value refers to a static member of the type written on the LHS). For other cases, we don't state a preference.

calda · 2024-03-13T23:30:04Z

README.md

@@ -333,25 +333,72 @@ _You can enable the following settings in Xcode by running [this script](resourc

  ```swift
  // WRONG
-  let host: Host = Host()
+  let sun: Star = Star(mass: 1.989e30)


While I was here I expanded the list of examples used by the "Don't include types where they can be easily inferred." rule. These updates reflect the current behavior of the SwiftFormat redundantType rule.

calda · 2024-03-13T23:31:52Z

README.md


-    // RIGHT
-    return .left


This is a nice example and is good advice in general, but doesn't reflect the behavior of the SwiftFormat redundantType rule that we use. I don't believe we have autocorrect support for this specific example.

I added an asana task to our internal board for us to look at adding autocorrect for this sort of case

Sources/AirbnbSwiftFormatTool/airbnb.swiftformat

…r than writing the type explicitly on the left-hand side

calda · 2024-04-04T19:24:29Z

README.md

+  let myShape1: any ShapeStyle = .saturnOutline
+
+  // WRONG: fails with "error: static member 'saturnOutline' cannot be used on protocol metatype '(any ShapeStyle).Type'"
+  let myShape2 = (any ShapeStyle).myShape


By the way -- for this we have the ability to tell SwiftFormat a list of symbols to exclude from this rule (--preservesymbols ShapeStyle,OtherType,etc). Inside the apps repo we could exclude any type or property name where we frequently use this syntax. Two relevant examples are:

UnresolvedShapeStyle

all existing callsites use any UnresolvedShapeStyle so this is already handled properly

canonicalFake

there are about a dozen examples of let experimentsService: ExperimentsService = .canonicalFake (etc). It seems like a good idea to exclude this using --preservesymbols canonicalFake. Updating the callsites to let experimentsService: any ExperimentsService = .canonicalFake also works.

Thanks for flagging this @calda . I was wondering about this... My recollection is that eventually it's going to be required that we use any for any existential/protocol types but that this requirement is being slowly rolled out. Is your understanding also that eventually all existential/protocol type will need to be preceded by any? If so, I think it's less problematic if we need to work around certain situations like the above ones you highlighted for the time being.

I am trying to understand if there would be cases when we still need --preservesymbols if/when the language requires that all existential/protocol types are preceded by any.

I saw on the Swift Evolution forums that they decided to not make any mandatory in Swift 6 mode after all. Although after a quick search on the forums I can't actually find where they said that.

This doesn't necessarily confirm it, but if you build code like this using a nightly Swift 6 build it doesn't require any.

I am trying to understand if there would be cases when we still need --preservesymbols if/when the language requires that all existential/protocol types are preceded by any.

If all call sites used the existential any syntax, we wouldn't need to use --preservesymbols

Found the quote: https://forums.swift.org/t/progress-toward-the-swift-6-language-mode/68315

The Language Steering Group has decided that one previously-accepted upcoming feature, ExistentialAny, will not be enabled by default in Swift 6. SE-0335: Introduce existential any introduces the any keyword to identify existential types. The proposal also specifies that "bare" protocol names will no longer be permitted as types---they must either use any or some, as appropriate---under the upcoming feature flag ExistentialAny. Given the concerns about migration to consistent use of existential any in the language, and the expectation that additional language improvements will be coming that might affect the end result of that migration, the Language Steering Group is deferring the source-incompatible changes in SE-0335 to a future language revision.

It sounds like they didn't rule out this direction, but rather just deferred it for longer (probably a very long time, if we have to wait for Swift 7!)

bachand

@calda this is looking great! I was hoping to talk through some comments before approving if that's OK but I feel like we are very close 👍

bachand · 2024-04-05T01:20:11Z

README.md

+
+  // WRONG: Most literals provide a default type that can be inferred.
+  let enableGravity: Bool = true
+  let numberOfPlanets: Int = 8


@calda how do we handle/guide on the case where you want to tell the language that a literal should be interpreted in a special way? For example, one way to get a floating-point value of 100 is to write

let myFloatingPointValue: Double = 100

which will produce a different result than

let myFloatingPointValue = 100 // implicitly of type `Int`

In this example, another option is to write

let myFloatingPointValue = 100.0 // implicitly of type `Double`

but even then it's necessary to write an explicit type in some cases, like if you explicitly wanted a Float instead of Double

let myFloatingPointValue: Float = 100

I am curious how our autocorrection handles this case where removing the type can change the meaning of the code, and I also think that situations like this may warrant some discussion in the prose of this rule.

In my testing I see that the linter is smart enough to correct let numberOfPlanets: Int = 8 to let numberOfPlanets = 8 but not correct let numberOfPlanets: Float = 8. Wow! I wonder if you happen to have a code pointer to how this is implemented.

Btw I also saw as I continued to review that this is covered in more detail in the next rule, though I wonder if we should move some of the content from the next rule up to this one since this rule is the one focused on removing types.

I implemented this back in nicklockwood/SwiftFormat#921! Here's the code: https://github.com/nicklockwood/SwiftFormat/pull/921/files#diff-1f5cbb52e333eb61c8d6fef2bd765e3e8e6fa8f3101f91f98a9dfce9e1ef480dR1363 It special-cases support for Bool, String, Int, and Double literals.

I also saw as I continued to review that this is covered in more detail in the next rule, though I wonder if we should move some of the content from the next rule up to this one since this rule is the one focused on removing types.

Good idea. I think the next rule probably goes in to too much detail about literals, and that content would be a better fit for the discussion about the redundantType rule.

README.md

bachand · 2024-04-05T01:56:21Z

README.md

+  let myShape1: any ShapeStyle = .saturnOutline
+
+  // WRONG: fails with "error: static member 'saturnOutline' cannot be used on protocol metatype '(any ShapeStyle).Type'"
+  let myShape2 = (any ShapeStyle).myShape


Thanks for flagging this @calda . I was wondering about this... My recollection is that eventually it's going to be required that we use any for any existential/protocol types but that this requirement is being slowly rolled out. Is your understanding also that eventually all existential/protocol type will need to be preceded by any? If so, I think it's less problematic if we need to work around certain situations like the above ones you highlighted for the time being.

bachand · 2024-04-05T01:59:56Z

README.md

+  let myShape1: any ShapeStyle = .saturnOutline
+
+  // WRONG: fails with "error: static member 'saturnOutline' cannot be used on protocol metatype '(any ShapeStyle).Type'"
+  let myShape2 = (any ShapeStyle).myShape


I am trying to understand if there would be cases when we still need --preservesymbols if/when the language requires that all existential/protocol types are preceded by any.

calda · 2024-04-11T17:47:19Z

Thanks for the review @bachand! I updated the rule to clarify that it applies to both local variables and instance properties, improved the main example to show both cases, and simplified some of the other examples related to edge cases.

…operties, simplify examples

calda commented Mar 13, 2024

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calda force-pushed the cal--infer-types branch from 7820918 to f0bbb0b Compare March 14, 2024 01:06

Add rule to infer property types from the right-hand-side value rathe…

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…r than writing the type explicitly on the left-hand side

calda force-pushed the cal--infer-types branch from f0bbb0b to d343fb2 Compare March 22, 2024 21:25

calda requested a review from bachand March 22, 2024 21:25

calda added 2 commits March 28, 2024 14:35

Update to SwiftFormat build with bug fixes

3750e2c

Add discussion around edge cases

5a39cdc

jqsilver approved these changes Apr 4, 2024

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calda commented Apr 4, 2024

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bachand reviewed Apr 5, 2024

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Clarify that the rule applies to both local variables and instance pr…

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…operties, simplify examples

calda force-pushed the cal--infer-types branch from dd1db39 to 39f921f Compare April 11, 2024 18:08

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Add rule to infer property types from the right-hand-side value rather than writing the type explicitly on the left-hand side #263

Add rule to infer property types from the right-hand-side value rather than writing the type explicitly on the left-hand side #263

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calda commented Apr 11, 2024

Add rule to infer property types from the right-hand-side value rather than writing the type explicitly on the left-hand side #263

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Add rule to infer property types from the right-hand-side value rather than writing the type explicitly on the left-hand side #263

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calda commented Mar 13, 2024 • edited

Summary

Reasoning

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