I've written up a draft proposal, suggestions or improvements are welcome, especially relating to the title.
__Change subscript declarations to use a colon__
--Introduction--
Currently, subscript declarations follow the following model:
subscript(externalName internalName: ParamType) -> ElementType {
get { … }
set { … }
}
The initial keyword ‘subscript’ is followed by a parameter list, followed by an arrow to the accessed type.
--Motivation--
The arrow, borrowed from function syntax, is very much out of place in this context, and so can act as a mental stumbling block.
Subscripts act like parameterised property accessors. This means, like a property, they can appear on the left hand side of an assignment, and values accessed through subscripts can be mutated in-place. The colon has precedent in declaring this kind of construct, so it makes sense to reuse it here.
--Proposed solution--
A simple replacement of ‘->’ with ‘:’ in the declaration syntax.
--Detailed design--
This would change the above example to look like the following:
subscript(externalName internalName: ParamType) : ElementType {
get { … }
set { … }
}
--Impact on existing code--
Existing code would have to update subscripts to use a colon. This can be automated in a conversion to Swift 3 syntax.
--Potential hazards--
Use of colons could have a negative effect on readability. This largely depends on coding style, which can match either of the following:
subscript(_ example: Type) : ElementType
subscript(_ example: Type): ElementType
This issue is most apparent in the latter example, which omits the leading space before the colon, as the colon blends into the closing bracket.
However, the real-world effect of this change is hard to predict, and subscript declarations are rare enough that the consequences of this change are very limited. In addition, the current ‘->‘ syntax can already act as a mental stumbling block.
--Alternatives Considered--
We could leave the syntax as it is, or use an alternative symbol, such as ‘:->’ or ‘<->’.
We could also leave open the possibility of expanding function syntax with ‘inout ->’.