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

[David Hart via swift-evolution]

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.jahk...@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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