[swift-evolution] Move where clause to end of declaration

[David Hart]

Hello swift-evolution,

I took the pitch originally from Developer to move the where clause out of the generic parameter list, with improvements brought up by Pyry Jahkola, and wrote a proposal for it. I opened a Pull Request, but if anybody wants to bring some modifications to it before it is merged, please let me know what you think:

Move where clause to end of declaration

Proposal: SE-XXXX <https://github.com/apple/swift-evolution/blob/master/proposals/XXXX-move-where-expression.md>
Author(s): David Hart <https://github.com/hartbit>, Developer, Pry Jahkola <https://github.com/hartbit/swift-evolution/blob/move-where-clause/proposals/pyry.jahkola@iki.fi>
Status: TBD
Review manager: TBD
 <https://github.com/hartbit/swift-evolution/blob/move-where-clause/proposals/XXXX-move-where-expression.md#introduction>Introduction

This proposal suggests moving the where clause to the end of the declaration syntax, but before the body, for readability reasons. It has been discussed at length on the following swift-evolution thread:

[Pitch] Moving where Clauses Out Of Parameter Lists <https://github.com/hartbit/swift-evolution/blob/move-where-clause/proposals/GMANE_DOWN_FOR_NOW>
 <https://github.com/hartbit/swift-evolution/blob/move-where-clause/proposals/XXXX-move-where-expression.md#motivation>Motivation

The where clause in declarations can become quite long. When that happens, it breaks the declaration syntax in two, hurting its readability. There is also no good way of formatting the declaration syntax to make it much better.

 <https://github.com/hartbit/swift-evolution/blob/move-where-clause/proposals/XXXX-move-where-expression.md#proposed-solution>Proposed solution

The proposal suggests moving the where clause at the end of the declaration, but before the body of concerned declarations. With the proposed change, where clauses do not impede the main declaration and are also more easily formattable. For example, here is the same function declaration before and after the change: 

func anyCommonElements<T : SequenceType, U : SequenceType where
    T.Generator.Element: Equatable,
    T.Generator.Element == U.Generator.Element>(lhs: T, _ rhs: U) -> Bool where
{
    ...
}

func anyCommonElements<T : SequenceType, U : SequenceType>(lhs: T, _ rhs: U) -> Bool where
    T.Generator.Element: Equatable,
    T.Generator.Element == U.Generator.Element
{
    ...
}
This proposal has no impact on extension declarations with constraints because those declarations already have the where clauses right before the body. In that regard, the proposal makes the other declarations more consistent with extension declarations.

 <https://github.com/hartbit/swift-evolution/blob/move-where-clause/proposals/XXXX-move-where-expression.md#detailed-design>Detailed design

First of all, the grammar of generic-parameter-clause is modified to loose the requirement- clause:

generic-parameter-clause → < ­generic-parameter-list­­ >­

The grammar of declarations are then amended to gain the requirement-clause: 

function-declaration → function-head­ function-name­ generic-parameter-clause­opt­ function-signature requirement-clause­opt ­function-body­opt

union-style-enum → indirect­­opt­ ­enum ­enum-name ­generic-parameter-clause­­opt ­type-inheritance-clause­­opt­ requirement-clause­opt { ­union-style-enum-members­­opt­ }

raw-value-style-enum → enum ­enum-name­ generic-parameter-clause­­opt ­type-inheritance-clause requirement-clause­opt ­{ ­raw-value-style-enum-members­ *}­*

struct-declaration → attributes­­opt ­access-level-modifier­­opt ­struct ­struct-name ­generic-parameter-clause­­opt ­type-inheritance-clause­­opt requirement-clause­opt ­struct-body­

class-declaration → attributes­­opt ­access-level-modifier­opt ­final­opt ­class ­class-name ­generic-parameter-clause­opt ­type-inheritance-clause­opt requirement-clause­opt ­class-body­

protocol-method-declaration → function-head ­function-name ­generic-parameter-clause­­opt ­function-signature­ requirement-clause­opt

protocol-initializer-declaration → initializer-head ­generic-parameter-clause­­opt ­parameter-clause ­throws­­opt­ requirement-clause­opt

protocol-initializer-declaration → initializer-head ­generic-parameter-clause­opt ­parameter-clause­ rethrows­ requirement-clause­opt

initializer-declaration → initializer-head ­generic-parameter-clause­­opt ­parameter-clause ­throws­opt requirement-clause­opt ­initializer-body­

initializer-declaration → initializer-head ­generic-parameter-clause­opt ­parameter-clause ­rethrows requirement-clause­opt ­initializer-body­

 <https://github.com/hartbit/swift-evolution/blob/move-where-clause/proposals/XXXX-move-where-expression.md#impact-on-existing-code>Impact on existing code

This proposal impacts all declarations which contain where clauses expect for extension declarations and will therefore require a Fix-It. 

 <https://github.com/hartbit/swift-evolution/blob/move-where-clause/proposals/XXXX-move-where-expression.md#alternatives-considered>Alternatives considered

The first post in the swift-evolution thread originally proposed moving the where clause just after the generic type declaration. Since then, the original author and many other participants in the thread have agreed that the current proposal is superior.

It was also proposed to remove the simple inheritance constraints from the generic parameter list, but several arguments were brought up that it would complicate declarations of simple generics which only needed inheritance constraints.
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