Re: [swift-users] Swift 4 Strings and flatMap

2017-10-21 Thread Martin R via swift-users 
Compare  https://bugs.swift.org/browse/SR-1570 : Swift can infer the return 
type from single-statement closures, but not from multi-statement closures. 
Therefore in 

let result = strArr.flatMap { x in
return x
}

the closure type is inferred as (String)-> String, and therefore the flatMap 
call matches exactly the 

   func flatMap(_ transform: (Self.Element) throws -> 
SegmentOfResult) rethrows -> [SegmentOfResult.Element]
   where SegmentOfResult : Sequence

Sequence method, with Self.Element == String and SegmentOfResult == String, and 
flatMap returns [SegmentOfResult.Element] == [Character].

In

let result = strArr.flatMap { x in
print(x)
return x
}

the closure return type is not inferred from the closure itself, because it is 
a multi-statement closure. Here the compiler (apparently) chooses the

public func flatMap(_ transform: (Element) throws -> String?) rethrows -> 
[String]

Array method, and promotes `String` to `String?`. You would get the same result 
with an explicit return type in the single-statement closure:

let result = strArr.flatMap { x -> String? in
return x
}

But why do you use flatMap at all? If the intention is to map [String] to 
[String] then map() would be the right method:

let strArr = ["Hi", "hello"]
let result: [String] = strArr.map { x in
return x
}
print(result) // ["Hi", "hello"]

Regards, Martin


> On 21. Oct 2017, at 02:50, Santiago Gil via swift-users 
>  wrote:
> 
> Hi Swift users,
> 
> I just ran into a bug after a Swift 4 migration where we expected [String] 
> and got [Character] from a flatMap since flatMap will flatten the String 
> since it's now a collection.  While I understand why flatMap behaves this way 
> now that string are collections, in testing this I found some weird 
> behaviour...
> 
> var strArr = ["Hi", "hello"]
> 
> let result = strArr.flatMap { x in
> 
> return x
> 
> }
> 
> 
> The type of results ends up being [Character] in the above case. However, 
> adding a print statement changes things.
> 
> 
> var strArr = ["Hi", "hello"]
> 
> let result = strArr.flatMap { x in
> 
> print(x)
> 
> return x
> 
> }
> 
> In this case, result is of type [String]
> 
> This seems like a bug, or is this expected Swift behaviour?
> 
> Thanks,
> 
> Santiago
> 
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Re: [swift-users] Swift 4 Strings and flatMap

2017-10-21 Thread Ole Begemann via swift-users 

On 21.10.17 02:50, Santiago Gil via swift-users wrote:
I just ran into a bug after a Swift 4 migration where we expected 
[String] and got [Character] from a flatMap since flatMap will flatten 
the String since it's now a collection.  While I understand why 
flatMap behaves this way now that string are collections, in testing 
this I found some weird behaviour...


var strArr = ["Hi", "hello"]

let result = strArr.flatMap { x in

returnx

}


The type of results ends up being [Character] in the above case. 
However, adding a print statement changes things.


var strArr = ["Hi", "hello"]

let result = strArr.flatMap { x in

print(x)

return x

}

In this case, result is of type [String]

This seems like a bug, or is this expected Swift behaviour?



It looks like the compiler infers the type of the closure in the second 
example to be (String) -> Optional instead of String -> String.


I don't know why and I definitely find it surprising.
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Re: [swift-users] Re-initializing lazy vars

2017-10-21 Thread Joanna Carter via swift-users 
Oops! forgot to add example code for resetting with non-optional public var :

public struct TestStruct
{
  private var _description: String?
  
  public var description: String
  {
get
{
  return _description ?? ""
}
set
{
  _description = newValue
}
  }
  
  mutating func resetDescription()
  {
_description = nil
  }
}

Joanna

--
Joanna Carter
Carter Consulting

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Re: [swift-users] Re-initializing lazy vars

2017-10-21 Thread Joanna Carter via swift-users 
> It just seems so obvious to create-if-nil, and makes it nice to reset stuff 
> that should be re-created. In my case, after my NSURLSession gets 
> invalidated, I wanted to set the property to nil, and next time a session was 
> needed, it would just get created.

The only thing I would query with the whole concept of resetting an implicitly 
unwrapped optional var is, with a simple example struct :

public struct TestStruct
{
  private var _description: String?
  
  public var description: String!
  {
get
{
  return _description ?? ""
}
set
{
  _description = newValue
}
  }
}

… test code would be :

  {
var test = TestStruct()

test.description = nil

let str: String = test.description
  }

The idea of setting a var to nil and then getting a valid object back seems a 
bit anachronistic.

I think I would rather add an explicit "reset" method.

Joanna

--
Joanna Carter
Carter Consulting

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